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

Tue Sep 8 15:04:49 PDT 2015

On Tue, Sep 8, 2015 at 3:03 PM, Sean Silva <chisophugis at gmail.com> wrote:

>
>
> On Fri, Sep 4, 2015 at 11:44 PM, Xinliang David Li <davidxl at google.com>
> wrote:
>
>> On Fri, Sep 4, 2015 at 11:03 PM, Sean Silva <chisophugis at gmail.com>
>> wrote:
>> >
>> >
>> > On Fri, Sep 4, 2015 at 10:11 PM, Xinliang David Li <davidxl at google.com>
>> > wrote:
>> >>
>> >> On Fri, Sep 4, 2015 at 9:11 PM, Sean Silva <chisophugis at gmail.com>
>> wrote:
>> >> >
>> >> >
>> >> > 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?
>> >>
>> >> yes -- it is fixed length (8byte) blob which may include null byte in
>> >> the middle.
>> >
>> >
>> > For reference, MD5 sum is 16 bytes (128-bit):
>> > https://en.wikipedia.org/wiki/MD5
>>
>> yes, LLVM's MD5 hash only takes the lower 64bit.
>>
>>
>> >
>> >>
>> >>
>> >> >
>> >> > 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.
>> >>
>> >> The key type before the change is StringRef, while the after the key
>> >> type is uint64_t. Are you suggesting treating uint64_t md5 sum key as
>> >> a string of 8 bytes or storing md5 has in text form which will double
>> >> the size?
>> >
>> >
>> > How much does this change the benefit? If most of the benefit is
>> avoiding
>> > extraordinarily long mangled names then it may be sufficient.
>> >
>> > With IR-level instrumentation like Rong is pursuing the size may be
>> reduced
>> > sufficiently that we do not need the optimization proposed in this
>> thread.
>> > For example, Rong found >2x size reduction on Google's C++ benchmarks,
>> which
>> > I assume are representative of the extremely large Google binaries that
>> are
>> > causing the problems addressed by your proposal in this thread. The
>> > measurements you mention for Clang in this thread provide similar size
>> > reductions, so Rong's approach may be sufficient (especially because
>> > functions with extremely large mangled names tend to be small inline
>> > functions in header-only template libraries).
>>
>> Late instrumentation helps many cases. In some cases (as shown in
>> SPEC), the reduction in size is not as large. Reducing PGO overhead
>> will lower the bar for its adoption.
>>
>> >
>> > Of the points you mention in "Large size of overhead can limit the
>> usability
>> > of PGO greatly", many of the issues are hard limits that prevent the
>> use of
>> > PGO. Do you have a lower bound on how much the size of the PGO data
>> must be
>> > reduced in order to overcome the hard limits?
>>
>> This is a static view:  Think about the situation where application
>> size is ever increasing; also think about situation where we want to
>
> collect more types of profile data. Think about situation where user
>> want to run pgo binaries on small devices with tiny memory/storage ..
>
>
> If we want to reduce memory overhead at runtime and reduce the size of the
> raw profile data extracted from the target, there are clear solutions.
> Consider that debug info does not need to be loaded into the memory image
> of the target; why should information identifying each counter need to be?
> A file containing raw profile counters is a subset of a core dump; in most
> environments, a core dump does not need to have debug info or symbol names
> in it, but can be still be read in full detail in conjunction with the
> original binary.
>
> Thus, as we require
>

this should read "if we ant to require".

-- Sean Silva

> that the binary be passed to llvm-profdata, there is no fundamental reason
> that the memory image of the program, or the raw data extracted from the
> program, must have any size overhead besides the raw values of the counters
> themselves and any text size increase for incrementing them. If we are
> willing to impose this requirement on users, then as far as reducing memory
> overhead at runtime and reducing the size of the raw profile data extracted
> from the target, using hashed function names is clearly the wrong direction.
>
> *Without* imposing the requirement of passing the binary to llvm-profdata,
> I do like the ability to use hashed function names like you are proposing.
> It is a simple solution for reducing size overhead of function name strings
> with little complexity, as it is just swapping one string for another.
>
>
>>
>> >
>> > Obviously LLVM must be able to support the extremely large binaries in
>> your
>> > configuration (otherwise what use is LLVM as a compiler ;) My questions
>> are
>> > primarily aimed at establishing which tradeoffs are acceptable for
>> > supporting this (both for LLVM and for you guys).
>>
>> As I said, with the modified proposal (after getting your feedback),
>> no PGO users in LLVM land is going to lose anything/functionality. The
>> end result will be net win for general users of LLVM (even though your
>> customers don't care about it), not just 'us' as you have mentioned
>> many times.
>>
>> >
>> > Btw, for us, the issue of PGO data size is not completely immaterial
>> but is
>> > very different from your use case. For us, the primary issue is the
>> > additional memory use at run time, since PS4 games usually use "all"
>> > available memory. We had a problem with UBSan where the large amount of
>> > memory required for storing the UBSan diagnostic data at runtime
>> required
>> > the game programmers to manually change their memory map to make room.
>> > +Filipe, do you remember how much memory UBSan was using that caused a
>> > problem?
>> >
>>
>> My proposal does help reducing rodata size significantly.
>>
>
> Yes, that is why I think that this is a useful thing to do. I just want to
> be careful about existing use cases and the relevant workflow issues.
>
> -- Sean Silva
>
>
>>
>> David
>>
>>
>> > -- Sean Silva
>> >
>> >>
>> >>
>> >> In the raw format, md5 sum key can be an embedded field in the
>> >> prf_data variable instead of as different var referenced by prf_data.
>> >>
>> >> >
>> >> > If this is not the case, you should show your current patch so that
>> we
>> >> > can
>> >> > discuss things concretely.
>> >>
>> >> It is not. See above about the difference.
>> >>
>> >> >
>> >> >>
>> >> >> >
>> >> >> >
>> >> >> >>
>> >> >> >> 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.
>> >>
>> >>  Your concern is acknowledged.
>> >>
>> >> >
>> >> > 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).
>> >>
>> >> ok.
>> >>
>> >> >
>> >> >
>> >> >>
>> >> >>   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.
>> >>
>> >> Glad to hear that :)
>> >>
>> >> thanks,
>> >>
>> >> David
>> >>
>> >> >
>> >> > -- 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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