[llvm-dev] RFC: Reducing Instr PGO size overhead

Fri Sep 4 21:11:22 PDT 2015

On Fri, Sep 4, 2015 at 5:42 PM, Xinliang David Li <davidxl at google.com>
wrote:

> On Fri, Sep 4, 2015 at 5:21 PM, Sean Silva <chisophugis at gmail.com> wrote:
> >
> >
> > On Fri, Sep 4, 2015 at 3:57 PM, Xinliang David Li <davidxl at google.com>
> > wrote:
> >>
> >> >
> >> > I think it is reasonable to simply replace the key we currently use
> with
> >> > MD5(key) for getting a size reduction.  In practice for my use cases,
> I
> >> > have
> >> > not observed any of the issues you mentioned under "Large size of
> >> > overhead
> >> > can limit the usability of PGO greatly", but I can understand that
> some
> >> > of
> >> > these issues could become problems in Google's use case. I would
> >> > personally
> >> > prefer to keep the existing behavior as the default (see below), and
> >> > have
> >> > MD5(key) as an option.
> >>
> >> The problem is that this requires profile format changes.
> >
> >
> > Why? AFAIK the "function name" is just an arbitrary string. Using s or
> > MD5(s) shouldn't matter. Of course, the user will need to pass consistent
> > flags to clang.
>
> The raw format for 64bit target can be made 'unchanged', but not for
> the 32bit raw format -- the nameptr field is only 32bit.
>
> The indexed format can not be made the same --  The ondisk profile
> record layout totally changes. The key field changes from a blob of
> chars into an 64bit integer.
>

An MD5 sum cannot be represented as a blob of chars?

Or to say it another way, suppose that Itanium mangling required as a final
step to replace the string with its md5 sum in hex. Therefore all symbol
names are "small". My understanding is that this is effectively all your
patch is doing.

If this is not the case, you should show your current patch so that we can
discuss things concretely.

> >
> >
> >>
> >> It will be
> >> very messy to support multiple formats in instr-codegen and
> >> instr-runtime.  For compatibility concerns, the reader is taught to
> >> support previous format, but the changes there are isolated (also
> >> expected to be removed in the future).
> >>
> >> >
> >> > My primary concern is that if the function name are not kept at all
> >> > stages,
> >> > then it becomes difficult to analyze the profile data in a standalone
> >> > way.
> >> > Many times, I have used `llvm-profdata show -all-functions
> foo.profdata`
> >> > on
> >> > the resulting profile data and then imported that data into
> Mathematica
> >> > for
> >> > analysis.
> >>
> >> This is certainly a very valid use case.
> >>
> >> >My understanding of your proposal is that `llvm-profdata show
> >> > -all-functions foo.profdata` will not show the actual function names
> but
> >> > instead MD5 hashes,
> >>
> >> Yes.
> >>
> >> To support your use case, there are two solutions:
> >>
> >> 1) user can add -fcoverage-mapping option in the build
> >> 2) introduce a new option -fprofile-instr-names that force the
> >> emission of the name sections in the .o file. This is similar to 1),
> >> but no covmap section is needed.
> >>
> >> llvm-profdata tool  will be taught to read the name section and attach
> >> function names to the profile records.
> >
> >
> > Needing to pass the executable to llvm-profdata would cause deployment
> > issues for my customers in practice.
>
> Why? The deployment needs to pass the profile data anyway right?

Yes, but not the executable.

The PGO training run is likely being run by a gameplay tester
(non-programmer). In general the binary will not be lying around as a loose
file anywhere, it will be part of a full package of the binary+assets
(think like what will end up on a bluray disc). A game's binary *completely
useless* without the assets, so except locally on a programmer's machine
while they iterate/debug, there is no reason for a binary to ever exist as
a standalone file.

I'm not saying that needing the binary is insurmountable in any particular
scenario. Just that it will cause a strict increase in the number of issues
to deploying PGO.

These are much bigger "compatibility concerns" for me than for newer
toolchains to accept the old format. For a change in format I can easily
tell my users to replace an exe with a newer one and that is all they need
to do and it takes 10 seconds, guaranteed. A workflow change is potentially
a massive disruption and guaranteed to take more than 10 seconds to fix
(perhaps hours or days).

>   This
> is no different from llvm-cov usage model.
>

In practice, getting the performance of PGO is a higher priority for my
users, so we should not assume that llvm-cov is being used.

-- Sean Silva

>
> David
>
>
>
> >
> >>
> >>
> >> Note that with 1) or 2), the user can still benefit from the reduced
> >> profile size.
> >
> >
> > Let me reiterate that the size of the profile is not a problem I have
> > observed in practice (nor have I heard of this being a problem in
> practice
> > until this thread). Therefore I'm skeptical of any changes to our default
> > behavior or any new requirements that are not opt-in.
> >
> > -- Sean Silva
> >
> >>
> >>
> >> thanks,
> >>
> >> David
> >>
> >>
> >>
> >>
> >> >which will make it more difficult for me to do this kind
> >> > of analysis (would require using nm on the original binary, hashing
> >> > everything, etc.).
> >> >
> >> > btw, feel free to attach the patch even if it in a rough state. It can
> >> > still
> >> > help to clarify the proposal and be a good talking point. Fine-grained
> >> > patch
> >> > review for caring about the rough parts will happen on llvm-commits;
> the
> >> > rough parts will not distract the discussion here on llvm-dev.
> >> >
> >> > -- Sean Silva
> >> >
> >> >>
> >> >>
> >> >> thanks,
> >> >>
> >> >> David
> >> >> _______________________________________________
> >> >> LLVM Developers mailing list
> >> >> llvm-dev at lists.llvm.org
> >> >> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> >> >
> >> >
> >
> >
>
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