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diff --git a/tools/profiler/docs/memory.rst b/tools/profiler/docs/memory.rst new file mode 100644 index 0000000000..347a91f9e7 --- /dev/null +++ b/tools/profiler/docs/memory.rst @@ -0,0 +1,46 @@ +Profiling Memory +================ + +Sampling stacks from native allocations +--------------------------------------- + +The profiler can sample allocations and de-allocations from malloc using the +"Native Allocations" feature. This can be enabled by going to `about:profiling` and +enabling the "Native Allocations" checkbox. It is only available in Nightly, as it +uses a technique of hooking into malloc that could be a little more risky to apply to +the broader population of Firefox users. + +This implementation is located in: `tools/profiler/core/memory_hooks.cpp +<https://searchfox.org/mozilla-central/source/tools/profiler/core/memory_hooks.cpp>`_ + +It works by hooking into all of the malloc calls. When the profiler is running, it +performs a `Bernoulli trial`_, that will pass for a given probability of per-byte +allocated. What this means is that larger allocations have a higher chance of being +recorded compared to smaller allocations. Currently, there is no way to configure +the per-byte probability. This means that sampled allocation sizes will be closer +to the actual allocated bytes. + +This infrastructure is quite similar to DMD, but with the additional motiviations of +making it easy to turn on and use with the profiler. The overhead is quite high, +especially on systems with more expensive stack walking, like Linux. Turning off +thee "Native Stacks" feature can help lower overhead, but will give less information. + +For more information on analyzing these profiles, see the `Firefox Profiler docs`_. + +Memory counters +--------------- + +Similar to the Native Allocations feature, memory counters use the malloc memory hook +that is only available in Nightly. When it's available, the memory counters are always +turned on. This is a lightweight way to count in a very granular fashion how much +memory is being allocated and deallocated during the profiling session. + +This information is then visualized in the `Firefox Profiler memory track`_. + +This feature uses the `Profiler Counters`_, which can be used to create other types +of cheap counting instrumentation. + +.. _Bernoulli trial: https://en.wikipedia.org/wiki/Bernoulli_trial +.. _Firefox Profiler docs: https://profiler.firefox.com/docs/#/./memory-allocations +.. _Firefox Profiler memory track: https://profiler.firefox.com/docs/#/./memory-allocations?id=memory-track +.. _Profiler Counters: https://searchfox.org/mozilla-central/source/tools/profiler/public/ProfilerCounts.h |