[llvm-dev] Memory utilization problems in profile reader

Sean Silva via llvm-dev llvm-dev at lists.llvm.org
Fri Dec 11 16:48:08 PST 2015

On Wed, Dec 9, 2015 at 12:14 PM, Xinliang David Li via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Can you extract the relevant part of the heap profile data?   How large is
> the sample profile data fed to the compiler?
> The indexed format profile size for clang is <100MB.  The InstrProfRecord
> for each function is read, used and discarded one at a time, so there
> should not be problem  as described.

If I'm reading the code right, we are also doing O(keys of the hash table)
memory allocation in the indexed reader here:
That seems unnecessary. (it seems to be used for value profiling stuff for
some reason?)

-- Sean Silva

> David
> On Wed, Dec 9, 2015 at 7:52 AM, Diego Novillo via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>> I've been experimenting with profiled bootstraps using sample profiles.
>> Initially, I made stage2 build stage3 while running under Perf.  This
>> produced a 20Gb profile which took too long to convert to LLVM, and used
>> ~30Gb of RAM.  So, I decided that this was not going to be very useful for
>> general usage.
>> I then changed the bootstrap to instead run each individual compile under
>> Perf.  This produced ~2,200 profiles, each of which took up to 1 minute to
>> convert, and then they all have to be merged into a single profile.  Also
>> didn't like it.
>> Since all compiles are more or less the same in terms of what the
>> compiler does, I decided to take the top 10 biggest profiles and merge
>> those.  That seemed to work.  This resulted in a 21Mb profile that I could
>> use as input to -fprofile-sample-use.
>> I started stage 3 of the bootstrap and left it to work.  I noticed it was
>> slow, so I thought "we'll need to speed things up".  The build never
>> finished.  Instead, ninja crashed my machine.
>> It turns out that each clang invocation was growing to 4Gb of RSS.  All
>> that memory is being allocated by the profile reader (
>> https://drive.google.com/file/d/0B9lq1VKvmXKFQVp1cGtZM2RSdWc/view?usp=sharing
>> ).
>> So, heads up, we need to trim it down.  Perhaps by only loading one
>> function profile at a time, use it and actively discard it.  Or simply be
>> better at flushing the reader data structures as they're used during
>> annotations.  I'll be sending patches about this in the coming days.
>> It's likely that the sample reader is doing something silly here.
>> Duncan, Justin, do you have memories of issues like this one with
>> instrumentation?  I'll be trying a similar experiment with it after I'm
>> done with the biggest issues in the sampler.
>> Thanks.  Diego.
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