[PATCH] D68351: [profile] Add a mode to continuously sync counter updates to a file

[Sajjad Mirza via Phabricator via llvm-commits]

sajjadm added a comment.

In D68351#1691915 <https://reviews.llvm.org/D68351#1691915>, @vsk wrote:

> In D68351#1691883 <https://reviews.llvm.org/D68351#1691883>, @sajjadm wrote:
>
> > Not sure it belongs in this CL specifically, but would it be possible to later add a mode that defers the initialization of the mmap'd file until specifically requested by the process?
>
>
> This sounds tricky to me, as the initialization requires some work to be done in each copy of the profile runtime linked into the process.
>
> Could you clarify whether you're referring to processes which use the profiling runtime's static initializer, or not? My understanding is that for processes which //do// use the runtime's static initializer, it's always desirable to set up the mmap'd file in that initializer. If that's not true, I'd appreciate a counterexample. For processes that //don't// use the runtime's static initializer, the situation (at Apple, at least) is typically that it's a kernel extension or bare-metal piece of firmware, and the continuous mode isn't really applicable anyway.


We do use the static initializer, but in our use case the process doesn't necessarily know where to write coverage data until some other application-specific code has run. Ideally, I would like to be able to have the instrumented process receive a shared memory buffer from another process, do continuous profile sync into that memory region, and not care whether it is file-backed or anonymous mapping. So it would be great if application code could give the runtime some sort of handle to that object [1] and tell the runtime to use that space for profiling.

[1] This could be just a pointer to a buffer that the application has already mmap'd, or it could be a file descriptor that the runtime is responsible for calling mmap on. Not sure which one would make more sense.


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