[llvm-dev] Memory utilization problems in profile reader

[Diego Novillo via llvm-dev]

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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