On this page
Profiling
Perf profiling: Jump to heading
Tools that can be used to generate/ visualise perf results:
- flamegraph-rs (https://github.com/flamegraph-rs/flamegraph)
- flamescope (https://github.com/Netflix/flamescope)
Example using perf on micro_bench_ops and visualising using flamescope:
# build `examples/micro_bench_ops`
cargo build --release --example micro_bench_ops
# run `examples/micro_bench_ops` using perf
sudo perf record -F 49 -a -g -- ./target/release/examples/micro_bench_ops
sudo perf script --header > micro_bench_ops_perf
# now open the file using flamescope
Example running deno_tcp.ts in combination with flamegraph (script.sh):
sudo flamegraph -o flamegraph.svg target/debug/deno run --allow-net cli/bench/deno_tcp.ts &
sleep 1
./third_party/prebuilt/linux64/wrk http://localhost:4500/
sleep 1
kill `pgrep perf`
v8 profiling: Jump to heading
Example using v8 profiling on micro_bench_ops:
# build `examples/micro_bench_ops`
cargo build --release --example micro_bench_ops
# run `examples/micro_bench_ops`
./target/release/examples/micro_bench_ops --prof
Example using v8 profiling on deno_tcp.ts:
# build `deno`
cargo build --release
# run `deno_tcp.ts`
./target/release/deno --v8-flags=--prof --allow-net cli/bench/deno_tcp.ts &
sleep 1
./third_party/prebuilt/linux64/wrk http://localhost:4500/
sleep 1
kill `pgrep deno`
V8 will write a file in the current directory that looks like this:
isolate-0x7fad98242400-v8.log. To examine this file:
node --prof-process isolate-0x7fad98242400-v8.log > prof.log
prof.log will contain information about tick distribution of different calls.
To view the log with Web UI, generate JSON file of the log:
Open rusty_v8/v8/tools/profview/index.html in your browser, and select
prof.json to view the distribution graphically.
Useful V8 flags during profiling:
- --prof
- --log-internal-timer-events
- --log-timer-events
- --track-gc
- --log-source-code
- --track-gc-object-stats
To learn more about profiling, check out the following links:
Debugging with LLDB Jump to heading
To debug the deno binary, we can use rust-lldb. It should come with rustc
and is a wrapper around LLDB.
$ rust-lldb -- ./target/debug/deno run --allow-net tests/http_bench.ts
# On macOS, you might get warnings like
# `ImportError: cannot import name _remove_dead_weakref`
# In that case, use system python by setting PATH, e.g.
# PATH=/System/Library/Frameworks/Python.framework/Versions/2.7/bin:$PATH
(lldb) command script import "/Users/kevinqian/.rustup/toolchains/1.36.0-x86_64-apple-darwin/lib/rustlib/etc/lldb_rust_formatters.py"
(lldb) type summary add --no-value --python-function lldb_rust_formatters.print_val -x ".*" --category Rust
(lldb) type category enable Rust
(lldb) target create "../deno/target/debug/deno"
Current executable set to '../deno/target/debug/deno' (x86_64).
(lldb) settings set -- target.run-args "tests/http_bench.ts" "--allow-net"
(lldb) b op_start
(lldb) r
V8 flags Jump to heading
V8 has many many internal command-line flags:
$ deno run --v8-flags=--help _
SSE3=1 SSSE3=1 SSE4_1=1 SSE4_2=1 SAHF=1 AVX=1 FMA3=1 BMI1=1 BMI2=1 LZCNT=1 POPCNT=1 ATOM=0
Synopsis:
shell [options] [--shell] [<file>...]
d8 [options] [-e <string>] [--shell] [[--module] <file>...]
-e execute a string in V8
--shell run an interactive JavaScript shell
--module execute a file as a JavaScript module
Note: the --module option is implicitly enabled for *.mjs files.
The following syntax for options is accepted (both '-' and '--' are ok):
--flag (bool flags only)
--no-flag (bool flags only)
--flag=value (non-bool flags only, no spaces around '=')
--flag value (non-bool flags only)
-- (captures all remaining args in JavaScript)
Options:
--use-strict (enforce strict mode)
type: bool default: false
--es-staging (enable test-worthy harmony features (for internal use only))
type: bool default: false
--harmony (enable all completed harmony features)
type: bool default: false
--harmony-shipping (enable all shipped harmony features)
type: bool default: true
--harmony-regexp-sequence (enable "RegExp Unicode sequence properties" (in progress))
type: bool default: false
--harmony-weak-refs-with-cleanup-some (enable "harmony weak references with FinalizationRegistry.prototype.cleanupSome" (in progress))
type: bool default: false
--harmony-regexp-match-indices (enable "harmony regexp match indices" (in progress))
type: bool default: false
--harmony-top-level-await (enable "harmony top level await")
type: bool default: false
--harmony-namespace-exports (enable "harmony namespace exports (export * as foo from 'bar')")
type: bool default: true
--harmony-sharedarraybuffer (enable "harmony sharedarraybuffer")
type: bool default: true
--harmony-import-meta (enable "harmony import.meta property")
type: bool default: true
--harmony-dynamic-import (enable "harmony dynamic import")
type: bool default: true
--harmony-promise-all-settled (enable "harmony Promise.allSettled")
type: bool default: true
--harmony-promise-any (enable "harmony Promise.any")
type: bool default: true
--harmony-private-methods (enable "harmony private methods in class literals")
type: bool default: true
--harmony-weak-refs (enable "harmony weak references")
type: bool default: true
--harmony-string-replaceall (enable "harmony String.prototype.replaceAll")
type: bool default: true
--harmony-logical-assignment (enable "harmony logical assignment")
type: bool default: true
--lite-mode (enables trade-off of performance for memory savings)
type: bool default: false
--future (Implies all staged features that we want to ship in the not-too-far future)
type: bool default: false
--assert-types (generate runtime type assertions to test the typer)
type: bool default: false
--allocation-site-pretenuring (pretenure with allocation sites)
type: bool default: true
--page-promotion (promote pages based on utilization)
type: bool default: true
--always-promote-young-mc (always promote young objects during mark-compact)
type: bool default: true
--page-promotion-threshold (min percentage of live bytes on a page to enable fast evacuation)
type: int default: 70
--trace-pretenuring (trace pretenuring decisions of HAllocate instructions)
type: bool default: false
--trace-pretenuring-statistics (trace allocation site pretenuring statistics)
type: bool default: false
--track-fields (track fields with only smi values)
type: bool default: true
--track-double-fields (track fields with double values)
type: bool default: true
--track-heap-object-fields (track fields with heap values)
type: bool default: true
--track-computed-fields (track computed boilerplate fields)
type: bool default: true
--track-field-types (track field types)
type: bool default: true
--trace-block-coverage (trace collected block coverage information)
type: bool default: false
--trace-protector-invalidation (trace protector cell invalidations)
type: bool default: false
--feedback-normalization (feed back normalization to constructors)
type: bool default: false
--enable-one-shot-optimization (Enable size optimizations for the code that will only be executed once)
type: bool default: false
--unbox-double-arrays (automatically unbox arrays of doubles)
type: bool default: true
--interrupt-budget (interrupt budget which should be used for the profiler counter)
type: int default: 147456
--jitless (Disable runtime allocation of executable memory.)
type: bool default: false
--use-ic (use inline caching)
type: bool default: true
--budget-for-feedback-vector-allocation (The budget in amount of bytecode executed by a function before we decide to allocate feedback vectors)
type: int default: 1024
--lazy-feedback-allocation (Allocate feedback vectors lazily)
type: bool default: true
--ignition-elide-noneffectful-bytecodes (elide bytecodes which won't have any external effect)
type: bool default: true
--ignition-reo (use ignition register equivalence optimizer)
type: bool default: true
--ignition-filter-expression-positions (filter expression positions before the bytecode pipeline)
type: bool default: true
--ignition-share-named-property-feedback (share feedback slots when loading the same named property from the same object)
type: bool default: true
--print-bytecode (print bytecode generated by ignition interpreter)
type: bool default: false
--enable-lazy-source-positions (skip generating source positions during initial compile but regenerate when actually required)
type: bool default: true
--stress-lazy-source-positions (collect lazy source positions immediately after lazy compile)
type: bool default: false
--print-bytecode-filter (filter for selecting which functions to print bytecode)
type: string default: *
--trace-ignition-codegen (trace the codegen of ignition interpreter bytecode handlers)
type: bool default: false
--trace-ignition-dispatches (traces the dispatches to bytecode handlers by the ignition interpreter)
type: bool default: false
--trace-ignition-dispatches-output-file (the file to which the bytecode handler dispatch table is written (by default, the table is not written to a file))
type: string default: nullptr
--fast-math (faster (but maybe less accurate) math functions)
type: bool default: true
--trace-track-allocation-sites (trace the tracking of allocation sites)
type: bool default: false
--trace-migration (trace object migration)
type: bool default: false
--trace-generalization (trace map generalization)
type: bool default: false
--turboprop (enable experimental turboprop mid-tier compiler.)
type: bool default: false
--concurrent-recompilation (optimizing hot functions asynchronously on a separate thread)
type: bool default: true
--trace-concurrent-recompilation (track concurrent recompilation)
type: bool default: false
--concurrent-recompilation-queue-length (the length of the concurrent compilation queue)
type: int default: 8
--concurrent-recompilation-delay (artificial compilation delay in ms)
type: int default: 0
--block-concurrent-recompilation (block queued jobs until released)
type: bool default: false
--concurrent-inlining (run optimizing compiler's inlining phase on a separate thread)
type: bool default: false
--max-serializer-nesting (maximum levels for nesting child serializers)
type: int default: 25
--trace-heap-broker-verbose (trace the heap broker verbosely (all reports))
type: bool default: false
--trace-heap-broker-memory (trace the heap broker memory (refs analysis and zone numbers))
type: bool default: false
--trace-heap-broker (trace the heap broker (reports on missing data only))
type: bool default: false
--stress-runs (number of stress runs)
type: int default: 0
--deopt-every-n-times (deoptimize every n times a deopt point is passed)
type: int default: 0
--print-deopt-stress (print number of possible deopt points)
type: bool default: false
--opt (use adaptive optimizations)
type: bool default: true
--turbo-sp-frame-access (use stack pointer-relative access to frame wherever possible)
type: bool default: false
--turbo-control-flow-aware-allocation (consider control flow while allocating registers)
type: bool default: true
--turbo-filter (optimization filter for TurboFan compiler)
type: string default: *
--trace-turbo (trace generated TurboFan IR)
type: bool default: false
--trace-turbo-path (directory to dump generated TurboFan IR to)
type: string default: nullptr
--trace-turbo-filter (filter for tracing turbofan compilation)
type: string default: *
--trace-turbo-graph (trace generated TurboFan graphs)
type: bool default: false
--trace-turbo-scheduled (trace TurboFan IR with schedule)
type: bool default: false
--trace-turbo-cfg-file (trace turbo cfg graph (for C1 visualizer) to a given file name)
type: string default: nullptr
--trace-turbo-types (trace TurboFan's types)
type: bool default: true
--trace-turbo-scheduler (trace TurboFan's scheduler)
type: bool default: false
--trace-turbo-reduction (trace TurboFan's various reducers)
type: bool default: false
--trace-turbo-trimming (trace TurboFan's graph trimmer)
type: bool default: false
--trace-turbo-jt (trace TurboFan's jump threading)
type: bool default: false
--trace-turbo-ceq (trace TurboFan's control equivalence)
type: bool default: false
--trace-turbo-loop (trace TurboFan's loop optimizations)
type: bool default: false
--trace-turbo-alloc (trace TurboFan's register allocator)
type: bool default: false
--trace-all-uses (trace all use positions)
type: bool default: false
--trace-representation (trace representation types)
type: bool default: false
--turbo-verify (verify TurboFan graphs at each phase)
type: bool default: false
--turbo-verify-machine-graph (verify TurboFan machine graph before instruction selection)
type: string default: nullptr
--trace-verify-csa (trace code stubs verification)
type: bool default: false
--csa-trap-on-node (trigger break point when a Node.js with given id is created in given stub. The format is: StubName,NodeId)
type: string default: nullptr
--turbo-stats (print TurboFan statistics)
type: bool default: false
--turbo-stats-nvp (print TurboFan statistics in machine-readable format)
type: bool default: false
--turbo-stats-wasm (print TurboFan statistics of wasm compilations)
type: bool default: false
--turbo-splitting (split nodes during scheduling in TurboFan)
type: bool default: true
--function-context-specialization (enable function context specialization in TurboFan)
type: bool default: false
--turbo-inlining (enable inlining in TurboFan)
type: bool default: true
--max-inlined-bytecode-size (maximum size of bytecode for a single inlining)
type: int default: 500
--max-inlined-bytecode-size-cumulative (maximum cumulative size of bytecode considered for inlining)
type: int default: 1000
--max-inlined-bytecode-size-absolute (maximum cumulative size of bytecode considered for inlining)
type: int default: 5000
--reserve-inline-budget-scale-factor (maximum cumulative size of bytecode considered for inlining)
type: float default: 1.2
--max-inlined-bytecode-size-small (maximum size of bytecode considered for small function inlining)
type: int default: 30
--max-optimized-bytecode-size (maximum bytecode size to be considered for optimization; too high values may cause the compiler to hit (release) assertions)
type: int default: 61440
--min-inlining-frequency (minimum frequency for inlining)
type: float default: 0.15
--polymorphic-inlining (polymorphic inlining)
type: bool default: true
--stress-inline (set high thresholds for inlining to inline as much as possible)
type: bool default: false
--trace-turbo-inlining (trace TurboFan inlining)
type: bool default: false
--turbo-inline-array-builtins (inline array builtins in TurboFan code)
type: bool default: true
--use-osr (use on-stack replacement)
type: bool default: true
--trace-osr (trace on-stack replacement)
type: bool default: false
--analyze-environment-liveness (analyze liveness of environment slots and zap dead values)
type: bool default: true
--trace-environment-liveness (trace liveness of local variable slots)
type: bool default: false
--turbo-load-elimination (enable load elimination in TurboFan)
type: bool default: true
--trace-turbo-load-elimination (trace TurboFan load elimination)
type: bool default: false
--turbo-profiling (enable basic block profiling in TurboFan)
type: bool default: false
--turbo-profiling-verbose (enable basic block profiling in TurboFan, and include each function's schedule and disassembly in the output)
type: bool default: false
--turbo-verify-allocation (verify register allocation in TurboFan)
type: bool default: false
--turbo-move-optimization (optimize gap moves in TurboFan)
type: bool default: true
--turbo-jt (enable jump threading in TurboFan)
type: bool default: true
--turbo-loop-peeling (Turbofan loop peeling)
type: bool default: true
--turbo-loop-variable (Turbofan loop variable optimization)
type: bool default: true
--turbo-loop-rotation (Turbofan loop rotation)
type: bool default: true
--turbo-cf-optimization (optimize control flow in TurboFan)
type: bool default: true
--turbo-escape (enable escape analysis)
type: bool default: true
--turbo-allocation-folding (Turbofan allocation folding)
type: bool default: true
--turbo-instruction-scheduling (enable instruction scheduling in TurboFan)
type: bool default: false
--turbo-stress-instruction-scheduling (randomly schedule instructions to stress dependency tracking)
type: bool default: false
--turbo-store-elimination (enable store-store elimination in TurboFan)
type: bool default: true
--trace-store-elimination (trace store elimination)
type: bool default: false
--turbo-rewrite-far-jumps (rewrite far to near jumps (ia32,x64))
type: bool default: true
--stress-gc-during-compilation (simulate GC/compiler thread race related to https://crbug.com/v8/8520)
type: bool default: false
--turbo-fast-api-calls (enable fast API calls from TurboFan)
type: bool default: false
--reuse-opt-code-count (don't discard optimized code for the specified number of deopts.)
type: int default: 0
--turbo-nci (enable experimental native context independent code.)
type: bool default: false
--turbo-nci-as-highest-tier (replace default TF with NCI code as the highest tier for testing purposes.)
type: bool default: false
--print-nci-code (print native context independent code.)
type: bool default: false
--trace-turbo-nci (trace native context independent code.)
type: bool default: false
--turbo-collect-feedback-in-generic-lowering (enable experimental feedback collection in generic lowering.)
type: bool default: false
--optimize-for-size (Enables optimizations which favor memory size over execution speed)
type: bool default: false
--untrusted-code-mitigations (Enable mitigations for executing untrusted code)
type: bool default: false
--expose-wasm (expose wasm interface to JavaScript)
type: bool default: true
--assume-asmjs-origin (force wasm decoder to assume input is internal asm-wasm format)
type: bool default: false
--wasm-num-compilation-tasks (maximum number of parallel compilation tasks for wasm)
type: int default: 128
--wasm-write-protect-code-memory (write protect code memory on the wasm native heap)
type: bool default: false
--wasm-async-compilation (enable actual asynchronous compilation for WebAssembly.compile)
type: bool default: true
--wasm-test-streaming (use streaming compilation instead of async compilation for tests)
type: bool default: false
--wasm-max-mem-pages (maximum initial number of 64KiB memory pages of a wasm instance)
type: uint default: 32767
--wasm-max-mem-pages-growth (maximum number of 64KiB pages a Wasm memory can grow to)
type: uint default: 65536
--wasm-max-table-size (maximum table size of a wasm instance)
type: uint default: 10000000
--wasm-max-code-space (maximum committed code space for wasm (in MB))
type: uint default: 1024
--wasm-tier-up (enable tier up to the optimizing compiler (requires --liftoff to have an effect))
type: bool default: true
--trace-wasm-ast-start (start function for wasm AST trace (inclusive))
type: int default: 0
--trace-wasm-ast-end (end function for wasm AST trace (exclusive))
type: int default: 0
--liftoff (enable Liftoff, the baseline compiler for WebAssembly)
type: bool default: true
--trace-wasm-memory (print all memory updates performed in wasm code)
type: bool default: false
--wasm-tier-mask-for-testing (bitmask of functions to compile with TurboFan instead of Liftoff)
type: int default: 0
--wasm-expose-debug-eval (Expose wasm evaluator support on the CDP)
type: bool default: false
--validate-asm (validate asm.js modules before compiling)
type: bool default: true
--suppress-asm-messages (don't emit asm.js related messages (for golden file testing))
type: bool default: false
--trace-asm-time (log asm.js timing info to the console)
type: bool default: false
--trace-asm-scanner (log tokens encountered by asm.js scanner)
type: bool default: false
--trace-asm-parser (verbose logging of asm.js parse failures)
type: bool default: false
--stress-validate-asm (try to validate everything as asm.js)
type: bool default: false
--dump-wasm-module-path (directory to dump wasm modules to)
type: string default: nullptr
--experimental-wasm-eh (enable prototype exception handling opcodes for wasm)
type: bool default: false
--experimental-wasm-simd (enable prototype SIMD opcodes for wasm)
type: bool default: false
--experimental-wasm-return-call (enable prototype return call opcodes for wasm)
type: bool default: false
--experimental-wasm-compilation-hints (enable prototype compilation hints section for wasm)
type: bool default: false
--experimental-wasm-gc (enable prototype garbage collection for wasm)
type: bool default: false
--experimental-wasm-typed-funcref (enable prototype typed function references for wasm)
type: bool default: false
--experimental-wasm-reftypes (enable prototype reference type opcodes for wasm)
type: bool default: false
--experimental-wasm-threads (enable prototype thread opcodes for wasm)
type: bool default: false
--experimental-wasm-type-reflection (enable prototype wasm type reflection in JS for wasm)
type: bool default: false
--experimental-wasm-bigint (enable prototype JS BigInt support for wasm)
type: bool default: true
--experimental-wasm-bulk-memory (enable prototype bulk memory opcodes for wasm)
type: bool default: true
--experimental-wasm-mv (enable prototype multi-value support for wasm)
type: bool default: true
--wasm-staging (enable staged wasm features)
type: bool default: false
--wasm-opt (enable wasm optimization)
type: bool default: false
--wasm-bounds-checks (enable bounds checks (disable for performance testing only))
type: bool default: true
--wasm-stack-checks (enable stack checks (disable for performance testing only))
type: bool default: true
--wasm-math-intrinsics (intrinsify some Math imports into wasm)
type: bool default: true
--wasm-trap-handler (use signal handlers to catch out of bounds memory access in wasm (currently Linux x86_64 only))
type: bool default: true
--wasm-fuzzer-gen-test (generate a test case when running a wasm fuzzer)
type: bool default: false
--print-wasm-code (Print WebAssembly code)
type: bool default: false
--print-wasm-stub-code (Print WebAssembly stub code)
type: bool default: false
--asm-wasm-lazy-compilation (enable lazy compilation for asm-wasm modules)
type: bool default: false
--wasm-lazy-compilation (enable lazy compilation for all wasm modules)
type: bool default: false
--wasm-lazy-validation (enable lazy validation for lazily compiled wasm functions)
type: bool default: false
--wasm-atomics-on-non-shared-memory (allow atomic operations on non-shared WebAssembly memory)
type: bool default: true
--wasm-grow-shared-memory (allow growing shared WebAssembly memory objects)
type: bool default: true
--wasm-simd-post-mvp (allow experimental SIMD operations for prototyping that are not included in the current proposal)
type: bool default: false
--wasm-code-gc (enable garbage collection of wasm code)
type: bool default: true
--trace-wasm-code-gc (trace garbage collection of wasm code)
type: bool default: false
--stress-wasm-code-gc (stress test garbage collection of wasm code)
type: bool default: false
--wasm-max-initial-code-space-reservation (maximum size of the initial wasm code space reservation (in MB))
type: int default: 0
--frame-count (number of stack frames inspected by the profiler)
type: int default: 1
--stress-sampling-allocation-profiler (Enables sampling allocation profiler with X as a sample interval)
type: int default: 0
--lazy-new-space-shrinking (Enables the lazy new space shrinking strategy)
type: bool default: false
--min-semi-space-size (min size of a semi-space (in MBytes), the new space consists of two semi-spaces)
type: size_t default: 0
--max-semi-space-size (max size of a semi-space (in MBytes), the new space consists of two semi-spaces)
type: size_t default: 0
--semi-space-growth-factor (factor by which to grow the new space)
type: int default: 2
--max-old-space-size (max size of the old space (in Mbytes))
type: size_t default: 0
--max-heap-size (max size of the heap (in Mbytes) both max_semi_space_size and max_old_space_size take precedence. All three flags cannot be specified at the same time.)
type: size_t default: 0
--initial-heap-size (initial size of the heap (in Mbytes))
type: size_t default: 0
--huge-max-old-generation-size (Increase max size of the old space to 4 GB for x64 systems with the physical memory bigger than 16 GB)
type: bool default: true
--initial-old-space-size (initial old space size (in Mbytes))
type: size_t default: 0
--global-gc-scheduling (enable GC scheduling based on global memory)
type: bool default: true
--gc-global (always perform global GCs)
type: bool default: false
--random-gc-interval (Collect garbage after random(0, X) allocations. It overrides gc_interval.)
type: int default: 0
--gc-interval (garbage collect after <n> allocations)
type: int default: -1
--retain-maps-for-n-gc (keeps maps alive for <n> old space garbage collections)
type: int default: 2
--trace-gc (print one trace line following each garbage collection)
type: bool default: false
--trace-gc-nvp (print one detailed trace line in name=value format after each garbage collection)
type: bool default: false
--trace-gc-ignore-scavenger (do not print trace line after scavenger collection)
type: bool default: false
--trace-idle-notification (print one trace line following each idle notification)
type: bool default: false
--trace-idle-notification-verbose (prints the heap state used by the idle notification)
type: bool default: false
--trace-gc-verbose (print more details following each garbage collection)
type: bool default: false
--trace-gc-freelists (prints details of each freelist before and after each major garbage collection)
type: bool default: false
--trace-gc-freelists-verbose (prints details of freelists of each page before and after each major garbage collection)
type: bool default: false
--trace-evacuation-candidates (Show statistics about the pages evacuation by the compaction)
type: bool default: false
--trace-allocations-origins (Show statistics about the origins of allocations. Combine with --no-inline-new to track allocations from generated code)
type: bool default: false
--trace-allocation-stack-interval (print stack trace after <n> free-list allocations)
type: int default: -1
--trace-duplicate-threshold-kb (print duplicate objects in the heap if their size is more than given threshold)
type: int default: 0
--trace-fragmentation (report fragmentation for old space)
type: bool default: false
--trace-fragmentation-verbose (report fragmentation for old space (detailed))
type: bool default: false
--minor-mc-trace-fragmentation (trace fragmentation after marking)
type: bool default: false
--trace-evacuation (report evacuation statistics)
type: bool default: false
--trace-mutator-utilization (print mutator utilization, allocation speed, gc speed)
type: bool default: false
--incremental-marking (use incremental marking)
type: bool default: true
--incremental-marking-wrappers (use incremental marking for marking wrappers)
type: bool default: true
--incremental-marking-task (use tasks for incremental marking)
type: bool default: true
--incremental-marking-soft-trigger (threshold for starting incremental marking via a task in percent of available space: limit - size)
type: int default: 0
--incremental-marking-hard-trigger (threshold for starting incremental marking immediately in percent of available space: limit - size)
type: int default: 0
--trace-unmapper (Trace the unmapping)
type: bool default: false
--parallel-scavenge (parallel scavenge)
type: bool default: true
--scavenge-task (schedule scavenge tasks)
type: bool default: true
--scavenge-task-trigger (scavenge task trigger in percent of the current heap limit)
type: int default: 80
--scavenge-separate-stack-scanning (use a separate phase for stack scanning in scavenge)
type: bool default: false
--trace-parallel-scavenge (trace parallel scavenge)
type: bool default: false
--write-protect-code-memory (write protect code memory)
type: bool default: true
--concurrent-marking (use concurrent marking)
type: bool default: true
--concurrent-array-buffer-sweeping (concurrently sweep array buffers)
type: bool default: true
--concurrent-allocation (concurrently allocate in old space)
type: bool default: false
--local-heaps (allow heap access from background tasks)
type: bool default: false
--stress-concurrent-allocation (start background threads that allocate memory)
type: bool default: false
--parallel-marking (use parallel marking in atomic pause)
type: bool default: true
--ephemeron-fixpoint-iterations (number of fixpoint iterations it takes to switch to linear ephemeron algorithm)
type: int default: 10
--trace-concurrent-marking (trace concurrent marking)
type: bool default: false
--concurrent-store-buffer (use concurrent store buffer processing)
type: bool default: true
--concurrent-sweeping (use concurrent sweeping)
type: bool default: true
--parallel-compaction (use parallel compaction)
type: bool default: true
--parallel-pointer-update (use parallel pointer update during compaction)
type: bool default: true
--detect-ineffective-gcs-near-heap-limit (trigger out-of-memory failure to avoid GC storm near heap limit)
type: bool default: true
--trace-incremental-marking (trace progress of the incremental marking)
type: bool default: false
--trace-stress-marking (trace stress marking progress)
type: bool default: false
--trace-stress-scavenge (trace stress scavenge progress)
type: bool default: false
--track-gc-object-stats (track object counts and memory usage)
type: bool default: false
--trace-gc-object-stats (trace object counts and memory usage)
type: bool default: false
--trace-zone-stats (trace zone memory usage)
type: bool default: false
--zone-stats-tolerance (report a tick only when allocated zone memory changes by this amount)
type: size_t default: 1048576
--track-retaining-path (enable support for tracking retaining path)
type: bool default: false
--concurrent-array-buffer-freeing (free array buffer allocations on a background thread)
type: bool default: true
--gc-stats (Used by tracing internally to enable gc statistics)
type: int default: 0
--track-detached-contexts (track native contexts that are expected to be garbage collected)
type: bool default: true
--trace-detached-contexts (trace native contexts that are expected to be garbage collected)
type: bool default: false
--move-object-start (enable moving of object starts)
type: bool default: true
--memory-reducer (use memory reducer)
type: bool default: true
--memory-reducer-for-small-heaps (use memory reducer for small heaps)
type: bool default: true
--heap-growing-percent (specifies heap growing factor as (1 + heap_growing_percent/100))
type: int default: 0
--v8-os-page-size (override OS page size (in KBytes))
type: int default: 0
--always-compact (Perform compaction on every full GC)
type: bool default: false
--never-compact (Never perform compaction on full GC - testing only)
type: bool default: false
--compact-code-space (Compact code space on full collections)
type: bool default: true
--flush-bytecode (flush of bytecode when it has not been executed recently)
type: bool default: true
--stress-flush-bytecode (stress bytecode flushing)
type: bool default: false
--use-marking-progress-bar (Use a progress bar to scan large objects in increments when incremental marking is active.)
type: bool default: true
--stress-per-context-marking-worklist (Use per-context worklist for marking)
type: bool default: false
--force-marking-deque-overflows (force overflows of marking deque by reducing it's size to 64 words)
type: bool default: false
--stress-compaction (stress the GC compactor to flush out bugs (implies --force_marking_deque_overflows))
type: bool default: false
--stress-compaction-random (Stress GC compaction by selecting random percent of pages as evacuation candidates. It overrides stress_compaction.)
type: bool default: false
--stress-incremental-marking (force incremental marking for small heaps and run it more often)
type: bool default: false
--fuzzer-gc-analysis (prints number of allocations and enables analysis mode for gc fuzz testing, e.g. --stress-marking, --stress-scavenge)
type: bool default: false
--stress-marking (force marking at random points between 0 and X (inclusive) percent of the regular marking start limit)
type: int default: 0
--stress-scavenge (force scavenge at random points between 0 and X (inclusive) percent of the new space capacity)
type: int default: 0
--gc-experiment-background-schedule (new background GC schedule heuristics)
type: bool default: false
--gc-experiment-less-compaction (less compaction in non-memory reducing mode)
type: bool default: false
--disable-abortjs (disables AbortJS runtime function)
type: bool default: false
--randomize-all-allocations (randomize virtual memory reservations by ignoring any hints passed when allocating pages)
type: bool default: false
--manual-evacuation-candidates-selection (Test mode only flag. It allows an unit test to select evacuation candidates pages (requires --stress_compaction).)
type: bool default: false
--fast-promotion-new-space (fast promote new space on high survival rates)
type: bool default: false
--clear-free-memory (initialize free memory with 0)
type: bool default: false
--young-generation-large-objects (allocates large objects by default in the young generation large object space)
type: bool default: true
--debug-code (generate extra code (assertions) for debugging)
type: bool default: false
--code-comments (emit comments in code disassembly; for more readable source positions you should add --no-concurrent_recompilation)
type: bool default: false
--enable-sse3 (enable use of SSE3 instructions if available)
type: bool default: true
--enable-ssse3 (enable use of SSSE3 instructions if available)
type: bool default: true
--enable-sse4-1 (enable use of SSE4.1 instructions if available)
type: bool default: true
--enable-sse4-2 (enable use of SSE4.2 instructions if available)
type: bool default: true
--enable-sahf (enable use of SAHF instruction if available (X64 only))
type: bool default: true
--enable-avx (enable use of AVX instructions if available)
type: bool default: true
--enable-fma3 (enable use of FMA3 instructions if available)
type: bool default: true
--enable-bmi1 (enable use of BMI1 instructions if available)
type: bool default: true
--enable-bmi2 (enable use of BMI2 instructions if available)
type: bool default: true
--enable-lzcnt (enable use of LZCNT instruction if available)
type: bool default: true
--enable-popcnt (enable use of POPCNT instruction if available)
type: bool default: true
--arm-arch (generate instructions for the selected ARM architecture if available: armv6, armv7, armv7+sudiv or armv8)
type: string default: armv8
--force-long-branches (force all emitted branches to be in long mode (MIPS/PPC only))
type: bool default: false
--mcpu (enable optimization for specific cpu)
type: string default: auto
--partial-constant-pool (enable use of partial constant pools (X64 only))
type: bool default: true
--sim-arm64-optional-features (enable optional features on the simulator for testing: none or all)
type: string default: none
--enable-source-at-csa-bind (Include source information in the binary at CSA bind locations.)
type: bool default: false
--enable-armv7 (deprecated (use --arm_arch instead))
type: maybe_bool default: unset
--enable-vfp3 (deprecated (use --arm_arch instead))
type: maybe_bool default: unset
--enable-32dregs (deprecated (use --arm_arch instead))
type: maybe_bool default: unset
--enable-neon (deprecated (use --arm_arch instead))
type: maybe_bool default: unset
--enable-sudiv (deprecated (use --arm_arch instead))
type: maybe_bool default: unset
--enable-armv8 (deprecated (use --arm_arch instead))
type: maybe_bool default: unset
--enable-regexp-unaligned-accesses (enable unaligned accesses for the regexp engine)
type: bool default: true
--script-streaming (enable parsing on background)
type: bool default: true
--stress-background-compile (stress test parsing on background)
type: bool default: false
--finalize-streaming-on-background (perform the script streaming finalization on the background thread)
type: bool default: false
--disable-old-api-accessors (Disable old-style API accessors whose setters trigger through the prototype chain)
type: bool default: false
--expose-gc (expose gc extension)
type: bool default: false
--expose-gc-as (expose gc extension under the specified name)
type: string default: nullptr
--expose-externalize-string (expose externalize string extension)
type: bool default: false
--expose-trigger-failure (expose trigger-failure extension)
type: bool default: false
--stack-trace-limit (number of stack frames to capture)
type: int default: 10
--builtins-in-stack-traces (show built-in functions in stack traces)
type: bool default: false
--experimental-stack-trace-frames (enable experimental frames (API/Builtins) and stack trace layout)
type: bool default: false
--disallow-code-generation-from-strings (disallow eval and friends)
type: bool default: false
--expose-async-hooks (expose async_hooks object)
type: bool default: false
--expose-cputracemark-as (expose cputracemark extension under the specified name)
type: string default: nullptr
--allow-unsafe-function-constructor (allow invoking the function constructor without security checks)
type: bool default: false
--force-slow-path (always take the slow path for builtins)
type: bool default: false
--test-small-max-function-context-stub-size (enable testing the function context size overflow path by making the maximum size smaller)
type: bool default: false
--inline-new (use fast inline allocation)
type: bool default: true
--trace (trace javascript function calls)
type: bool default: false
--trace-wasm (trace wasm function calls)
type: bool default: false
--lazy (use lazy compilation)
type: bool default: true
--max-lazy (ignore eager compilation hints)
type: bool default: false
--trace-opt (trace lazy optimization)
type: bool default: false
--trace-opt-verbose (extra verbose compilation tracing)
type: bool default: false
--trace-opt-stats (trace lazy optimization statistics)
type: bool default: false
--trace-deopt (trace optimize function deoptimization)
type: bool default: false
--trace-file-names (include file names in trace-opt/trace-deopt output)
type: bool default: false
--always-opt (always try to optimize functions)
type: bool default: false
--always-osr (always try to OSR functions)
type: bool default: false
--prepare-always-opt (prepare for turning on always opt)
type: bool default: false
--trace-serializer (print code serializer trace)
type: bool default: false
--compilation-cache (enable compilation cache)
type: bool default: true
--cache-prototype-transitions (cache prototype transitions)
type: bool default: true
--parallel-compile-tasks (enable parallel compile tasks)
type: bool default: false
--compiler-dispatcher (enable compiler dispatcher)
type: bool default: false
--trace-compiler-dispatcher (trace compiler dispatcher activity)
type: bool default: false
--cpu-profiler-sampling-interval (CPU profiler sampling interval in microseconds)
type: int default: 1000
--trace-side-effect-free-debug-evaluate (print debug messages for side-effect-free debug-evaluate for testing)
type: bool default: false
--hard-abort (abort by crashing)
type: bool default: true
--expose-inspector-scripts (expose injected-script-source.js for debugging)
type: bool default: false
--stack-size (default size of stack region v8 is allowed to use (in kBytes))
type: int default: 984
--max-stack-trace-source-length (maximum length of function source code printed in a stack trace.)
type: int default: 300
--clear-exceptions-on-js-entry (clear pending exceptions when entering JavaScript)
type: bool default: false
--histogram-interval (time interval in ms for aggregating memory histograms)
type: int default: 600000
--heap-profiler-trace-objects (Dump heap object allocations/movements/size_updates)
type: bool default: false
--heap-profiler-use-embedder-graph (Use the new EmbedderGraph API to get embedder nodes)
type: bool default: true
--heap-snapshot-string-limit (truncate strings to this length in the heap snapshot)
type: int default: 1024
--sampling-heap-profiler-suppress-randomness (Use constant sample intervals to eliminate test flakiness)
type: bool default: false
--use-idle-notification (Use idle notification to reduce memory footprint.)
type: bool default: true
--trace-ic (trace inline cache state transitions for tools/ic-processor)
type: bool default: false
--modify-field-representation-inplace (enable in-place field representation updates)
type: bool default: true
--max-polymorphic-map-count (maximum number of maps to track in POLYMORPHIC state)
type: int default: 4
--native-code-counters (generate extra code for manipulating stats counters)
type: bool default: false
--thin-strings (Enable ThinString support)
type: bool default: true
--trace-prototype-users (Trace updates to prototype user tracking)
type: bool default: false
--trace-for-in-enumerate (Trace for-in enumerate slow-paths)
type: bool default: false
--trace-maps (trace map creation)
type: bool default: false
--trace-maps-details (also log map details)
type: bool default: true
--allow-natives-syntax (allow natives syntax)
type: bool default: false
--allow-natives-for-differential-fuzzing (allow only natives explicitly allowlisted for differential fuzzers)
type: bool default: false
--parse-only (only parse the sources)
type: bool default: false
--trace-sim (Trace simulator execution)
type: bool default: false
--debug-sim (Enable debugging the simulator)
type: bool default: false
--check-icache (Check icache flushes in ARM and MIPS simulator)
type: bool default: false
--stop-sim-at (Simulator stop after x number of instructions)
type: int default: 0
--sim-stack-alignment (Stack alignment in bytes in simulator (4 or 8, 8 is default))
type: int default: 8
--sim-stack-size (Stack size of the ARM64, MIPS64 and PPC64 simulator in kBytes (default is 2 MB))
type: int default: 2048
--log-colour (When logging, try to use coloured output.)
type: bool default: true
--trace-sim-messages (Trace simulator debug messages. Implied by --trace-sim.)
type: bool default: false
--async-stack-traces (include async stack traces in Error.stack)
type: bool default: true
--stack-trace-on-illegal (print stack trace when an illegal exception is thrown)
type: bool default: false
--abort-on-uncaught-exception (abort program (dump core) when an uncaught exception is thrown)
type: bool default: false
--correctness-fuzzer-suppressions (Suppress certain unspecified behaviors to ease correctness fuzzing: Abort program when the stack overflows or a string exceeds maximum length (as opposed to throwing RangeError). Use a fixed suppression string for error messages.)
type: bool default: false
--randomize-hashes (randomize hashes to avoid predictable hash collisions (with snapshots this option cannot override the baked-in seed))
type: bool default: true
--rehash-snapshot (rehash strings from the snapshot to override the baked-in seed)
type: bool default: true
--hash-seed (Fixed seed to use to hash property keys (0 means random)(with snapshots this option cannot override the baked-in seed))
type: uint64 default: 0
--random-seed (Default seed for initializing random generator (0, the default, means to use system random).)
type: int default: 0
--fuzzer-random-seed (Default seed for initializing fuzzer random generator (0, the default, means to use v8's random number generator seed).)
type: int default: 0
--trace-rail (trace RAIL mode)
type: bool default: false
--print-all-exceptions (print exception object and stack trace on each thrown exception)
type: bool default: false
--detailed-error-stack-trace (includes arguments for each function call in the error stack frames array)
type: bool default: false
--adjust-os-scheduling-parameters (adjust OS specific scheduling params for the isolate)
type: bool default: true
--runtime-call-stats (report runtime call counts and times)
type: bool default: false
--rcs (report runtime call counts and times)
type: bool default: false
--rcs-cpu-time (report runtime times in cpu time (the default is wall time))
type: bool default: false
--profile-deserialization (Print the time it takes to deserialize the snapshot.)
type: bool default: false
--serialization-statistics (Collect statistics on serialized objects.)
type: bool default: false
--serialization-chunk-size (Custom size for serialization chunks)
type: uint default: 4096
--regexp-optimization (generate optimized regexp code)
type: bool default: true
--regexp-mode-modifiers (enable inline flags in regexp.)
type: bool default: false
--regexp-interpret-all (interpret all regexp code)
type: bool default: false
--regexp-tier-up (enable regexp interpreter and tier up to the compiler after the number of executions set by the tier up ticks flag)
type: bool default: true
--regexp-tier-up-ticks (set the number of executions for the regexp interpreter before tiering-up to the compiler)
type: int default: 1
--regexp-peephole-optimization (enable peephole optimization for regexp bytecode)
type: bool default: true
--trace-regexp-peephole-optimization (trace regexp bytecode peephole optimization)
type: bool default: false
--trace-regexp-bytecodes (trace regexp bytecode execution)
type: bool default: false
--trace-regexp-assembler (trace regexp macro assembler calls.)
type: bool default: false
--trace-regexp-parser (trace regexp parsing)
type: bool default: false
--trace-regexp-tier-up (trace regexp tiering up execution)
type: bool default: false
--testing-bool-flag (testing_bool_flag)
type: bool default: true
--testing-maybe-bool-flag (testing_maybe_bool_flag)
type: maybe_bool default: unset
--testing-int-flag (testing_int_flag)
type: int default: 13
--testing-float-flag (float-flag)
type: float default: 2.5
--testing-string-flag (string-flag)
type: string default: Hello, world!
--testing-prng-seed (Seed used for threading test randomness)
type: int default: 42
--testing-d8-test-runner (test runner turns on this flag to enable a check that the function was prepared for optimization before marking it for optimization)
type: bool default: false
--fuzzing (Fuzzers use this flag to signal that they are ... fuzzing. This causes intrinsics to fail silently (e.g. return undefined) on invalid usage.)
type: bool default: false
--embedded-src (Path for the generated embedded data file. (mksnapshot only))
type: string default: nullptr
--embedded-variant (Label to disambiguate symbols in embedded data file. (mksnapshot only))
type: string default: nullptr
--startup-src (Write V8 startup as C++ src. (mksnapshot only))
type: string default: nullptr
--startup-blob (Write V8 startup blob file. (mksnapshot only))
type: string default: nullptr
--target-arch (The mksnapshot target arch. (mksnapshot only))
type: string default: nullptr
--target-os (The mksnapshot target os. (mksnapshot only))
type: string default: nullptr
--target-is-simulator (Instruct mksnapshot that the target is meant to run in the simulator and it can generate simulator-specific instructions. (mksnapshot only))
type: bool default: false
--minor-mc-parallel-marking (use parallel marking for the young generation)
type: bool default: true
--trace-minor-mc-parallel-marking (trace parallel marking for the young generation)
type: bool default: false
--minor-mc (perform young generation mark compact GCs)
type: bool default: false
--help (Print usage message, including flags, on console)
type: bool default: true
--dump-counters (Dump counters on exit)
type: bool default: false
--dump-counters-nvp (Dump counters as name-value pairs on exit)
type: bool default: false
--use-external-strings (Use external strings for source code)
type: bool default: false
--map-counters (Map counters to a file)
type: string default:
--mock-arraybuffer-allocator (Use a mock ArrayBuffer allocator for testing.)
type: bool default: false
--mock-arraybuffer-allocator-limit (Memory limit for mock ArrayBuffer allocator used to simulate OOM for testing.)
type: size_t default: 0
--gdbjit (enable GDBJIT interface)
type: bool default: false
--gdbjit-full (enable GDBJIT interface for all code objects)
type: bool default: false
--gdbjit-dump (dump elf objects with debug info to disk)
type: bool default: false
--gdbjit-dump-filter (dump only objects containing this substring)
type: string default:
--log (Minimal logging (no API, code, GC, suspect, or handles samples).)
type: bool default: false
--log-all (Log all events to the log file.)
type: bool default: false
--log-api (Log API events to the log file.)
type: bool default: false
--log-code (Log code events to the log file without profiling.)
type: bool default: false
--log-handles (Log global handle events.)
type: bool default: false
--log-suspect (Log suspect operations.)
type: bool default: false
--log-source-code (Log source code.)
type: bool default: false
--log-function-events (Log function events (parse, compile, execute) separately.)
type: bool default: false
--prof (Log statistical profiling information (implies --log-code).)
type: bool default: false
--detailed-line-info (Always generate detailed line information for CPU profiling.)
type: bool default: false
--prof-sampling-interval (Interval for --prof samples (in microseconds).)
type: int default: 1000
--prof-cpp (Like --prof, but ignore generated code.)
type: bool default: false
--prof-browser-mode (Used with --prof, turns on browser-compatible mode for profiling.)
type: bool default: true
--logfile (Specify the name of the log file.)
type: string default: v8.log
--logfile-per-isolate (Separate log files for each isolate.)
type: bool default: true
--ll-prof (Enable low-level linux profiler.)
type: bool default: false
--gc-fake-mmap (Specify the name of the file for fake gc mmap used in ll_prof)
type: string default: /tmp/__v8_gc__
--log-internal-timer-events (Time internal events.)
type: bool default: false
--redirect-code-traces (output deopt information and disassembly into file code-<pid>-<isolate id>.asm)
type: bool default: false
--redirect-code-traces-to (output deopt information and disassembly into the given file)
type: string default: nullptr
--print-opt-source (print source code of optimized and inlined functions)
type: bool default: false
--vtune-prof-annotate-wasm (Used when v8_enable_vtunejit is enabled, load wasm source map and provide annotate support (experimental).)
type: bool default: false
--win64-unwinding-info (Enable unwinding info for Windows/x64)
type: bool default: true
--interpreted-frames-native-stack (Show interpreted frames on the native stack (useful for external profilers).)
type: bool default: false
--predictable (enable predictable mode)
type: bool default: false
--predictable-gc-schedule (Predictable garbage collection schedule. Fixes heap growing, idle, and memory reducing behavior.)
type: bool default: false
--single-threaded (disable the use of background tasks)
type: bool default: false
--single-threaded-gc (disable the use of background gc tasks)
type: bool default: false
Particularly useful ones:
--async-stack-traces