[PROF-7307] Wire up allocation sampling into CpuAndWallTimeWorker
#3103
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**What does this PR do?** In #2657 we added support for the `Collectors::ThreadContext` to sample a given object allocation. But until now, triggering these kinds of allocations was only done in our test suite. This PR adds the missing part: actually sampling objects as Ruby allocates them. In the `Collectors::CpuAndWallTimeWorker`, we already had a `TracePoint` that is called on every new object allocation (`RUBY_INTERNAL_EVENT_NEWOBJ`), but we were only using it to count allocations. This PR changes the `TracePoint` code to actually call into the `Collectors::ThreadContext` and thus allow allocation sampling to happen. **Motivation:** This PR is another step towards the allocation profiling feature. **Additional Notes:** It's not yet possible to turn on allocation profiling via configuration. I am reserving that change for later. But it's easy to do: change the `profiling/component.rb` to use `alloc_samples_enabled: true` and `allocation_sample_every: 1`. This will result in a big overhead impact to the application. I'll be working on this in later PRs. In particular, the sampling approach we're using (`allocation_sample_every`), is only a placeholder; it will get replaced before we get to beta. This is why I don't want to add it as a real user-available setting -- because it's temporary. **How to test the change?** This change includes test coverage. You can also manually check it out, by following the instructions in the "Additional Notes". (There's a feature flag needed for this data to show up in the Datadog UX).
Today I learned that sometimes Ruby allocates objects without a class! See notes in diff for more details.
These show up from time to time and will be user-visible, so it's nice to have a name for them. We could consider hiding them, but my thinking is: what if there's something in the user's code that causes a lot of them to be created? That seems relevant to report on, even if they are VM objects, since the user will probably want to change their code to avoid this.
ekump
reviewed
Sep 5, 2023
| if (name_length > 0) { | ||
| class_name = (ddog_CharSlice) {.ptr = name, .len = name_length}; | ||
| } else { | ||
| // @ivoanjo: I'm not sure this can ever happen, but just-in-case |
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Is it possible for this to happen with anonymous classes?
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Good question! It shouldn't happen -- we get the Ruby autogenerated name for them:
[1] pry(main)> Class.new.to_s
=> "#<Class:0x00005b721ef2a558>"Here's the output of an actual profile with such a class:
allocation class:[#<Class:0x00005dd2c7da98a8>] ruby vm type:[T_OBJECT] thread id:[123612321531712 (2560)] thread name:[main]
ekump
approved these changes
Sep 5, 2023
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What does this PR do?
In #2657 we added support for the
Collectors::ThreadContextto sample a given object allocation.But until now, triggering these kinds of allocations was only done in our test suite.
This PR adds the missing part: actually sampling objects as Ruby allocates them.
In the
Collectors::CpuAndWallTimeWorker, we already had aTracePointthat is called on every new object allocation (RUBY_INTERNAL_EVENT_NEWOBJ), but we were only using it to count allocations.This PR changes the
TracePointcode to actually call into theCollectors::ThreadContextand thus allow allocation sampling to happen.(Also included is a crash fix + another tiny tweak. I suggest reviewing commit-by-commit as it makes more sense!)
Motivation:
This PR is another step towards the allocation profiling feature.
Additional Notes:
It's not yet possible to turn on allocation profiling via configuration.
I am reserving that change for later. But it's easy to do: change the
profiling/component.rbto usealloc_samples_enabled: trueandallocation_sample_every: 1.This will result in a big overhead impact to the application. I'll be working on this in later PRs.
In particular, the sampling approach we're using (
allocation_sample_every), is only a placeholder; it will get replaced before we get to beta. This is why I don't want to add it as a real user-available setting -- because it's temporary.How to test the change?
This change includes test coverage. You can also manually check it out, by following the instructions in the "Additional Notes".
(There's a feature flag needed for this data to show up in the Datadog UX).