Re: [PATCH v9] pgo: add clang's Profile Guided Optimization infrastructure

[Fangrui Song]

On 2021-06-12, Peter Zijlstra wrote:
> On Sat, Jun 12, 2021 at 10:25:57AM -0700, Bill Wendling wrote:
> > On Sat, Jun 12, 2021 at 9:59 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > > Also, and I don't see this answered *anywhere*, why are you not using
> > > perf for this? Your link even mentions Sampling Profilers (and I happen
> > > to know there's been significant effort to make perf output work as
> > > input for the PGO passes of the various compilers).
> > >
> > Instruction-based (non-sampling) profiling gives us a better
> > context-sensitive profile, making PGO more impactful. It's also useful
> > for coverage whereas sampling profiles cannot.
>
> We've got KCOV and GCOV support already. Coverage is also not an
> argument mentioned anywhere else. Coverage can go pound sand, we really
> don't need a third means of getting that.
>
> Do you have actual numbers that back up the sampling vs instrumented
> argument? Having the instrumentation will affect performance which can
> scew the profile just the same.
>
> Also, sampling tends to capture the hot spots very well.

[I don't do kernel development. My experience is user-space toolchain.]

For applications, I think instrumentation based PGO can be 1%~4% faster
than sample-based PGO (e.g. AutoFDO) on x86.

Sample-based PGO has CPU requirement (e.g. Performance Monitoring Unit).
(my gut feeling is that there may be larger gap between instrumentation
based PGO and sample-based PGO for aarch64/ppc64, even though they can
use sample-based PGO.)
Instrumentation based PGO can be ported to more architectures.

In addition, having an infrastructure for instrumentation based PGO
makes it easy to deploy newer techniques like context-sensitive PGO
(just changed compile options; it doesn't need new source level
annotation).