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

Wed Oct 7 23:12:32 PDT 2015

There is no further response to this, so I will assume general
direction of solution-3 is acceptable ;)

Solution-3 can be further improved. Instead of using static symbol
table (with zero size __llvm_prf_nm symbols) to store function names
for profile display and coverage mapping, the function names can be
stored compressed in a non-allocatable section. The compression ratio
for function name strings can be very high (~5x).  The covmapping data
can also be made non-allocatable.

thanks,

David

On Tue, Sep 29, 2015 at 9:52 AM, Xinliang David Li <davidxl at google.com> wrote:
> Sorry for the late update. I finally found time to implement a solution
> (Solution-3) that has the best size savings (for both PGO and coverage
> testing) with symbolic information available.  Here is a re-cap of what we
> have discussed so far:
>
> Solution-1:
>
> This is the original solution proposed. In this solution, A function name's
> MD5 hash is used as the primary key (combined with IR hash) for function
> lookup. __llvm_prf_names section won't be emitted into the binary nor dumped
> into the profile data unless -fcoverage-mapping is also specified.
>
>     Pros:
>         1. maximal reduction of instrumentation binary process image size
>         2. maximal reduction of object and unstripped binary size
>         3. maximal reduction of raw profile data size
>         4. maximal reduction of indexed profile data size
>
>    Cons:
>         1. -fcoverage-mapping use case is not handled -- the size problem
> still exist
>         2. profile dump with llvm-profdata no longer have function names
> associated -- user needs to use postprocessing tool to get the functionality
>         3. function filtering with partial function name is not supported
>         4. Requires incompatible format change
>
>
> Solution-2: (http://reviews.llvm.org/D12715)
>
> In this solution, the MD5 hash string is used to replace the raw name string
>       Pros:
>           1. Very simple to implement
>           2. have good reduction of all sizes for typical large C++
> applications
>           3. No profile data format change is required.
>
>       Cons:
>           1. Still requires 16byte overhead per-function -- can actually
> hurt C programs
>           2. -fcoverage-mapping use case is still not handled
>           3. The problem with llvm-profdata still exists (no symbolic info,
> partial filtering support)
>
>
> Solution-3:
>
> This is the new solution I am proposing. It is basically an enhancement of
> Solution-1 with most of the weakness resolved.  The difference with
> Solution-1 is
>       1. Function name symbols are emitted into the symbol table as weak
> externs. They don't occupy any space at runtime and can be easily stripped.
>       2. -fcoverage-mapping does not need special handling -- it
> automatically benefit from the same size saving.
>       3. llvm-cov is changed to read symbol info from the symtab instead of
> reading them from the name section data
>       4. llvm-profdata is enhanced to take a binary as input and dump
> profile with names attached. Function filtering is fully supported (option
> can also be introduced to force dumping names into binary and profile data,
> so that llvm-profdata use case is not changed at all).
>
> Pros:
>        1. All the pros from Solution-1
>        2. Size savings for coverage-mapping case
> Cons:
>        Format change is required for profile data and coverage mapping.
>
> The initial patch is here: http://reviews.llvm.org/D13251
>
> With this patch, the size of a release clang binary with coverage mapping is
> reduced from 986M to 569M.
>
> If there are no major concerns,  I will carve out the patch into smaller
> ones for review.
>
> thanks,
>
> David
>
>
>
>
>
>
>
> On Tue, Sep 8, 2015 at 3:47 PM, Xinliang David Li <davidxl at google.com>
> wrote:
>>
>> >> >>
>> >> >> 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.
>>
>> Yes -- there are many alternatives:
>> 1) emit the name key mapping as a side data at compile time, or
>> 2) emit them into nonloadable sections of the object file.
>>
>> Compared with the above, LLVM's existing design does have its own
>> advantage -- making it easier for tool to access 'debug' info for
>> counters.
>>
>> LLVM's coverage testing, on the other hand, take a hybrid approach: It
>> emits the coverage map as rodata, but does not pass it to the profile
>> dumper. I think it is better to emit covmap as a side data not
>> attached to target binary.
>>
>>
>> >
>> > Thus, as we require 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.
>>
>> Agree. The good news is that the overhead of hashed function names is
>> small enough that makes this approach attractive.
>>
>> thanks,
>>
>> David
>> >
>> >>
>> >>
>> >> >
>> >> > 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
>> >> >> >> >> >
>> >> >> >> >> >
>> >> >> >> >
>> >> >> >> >
>> >> >> >
>> >> >> >
>> >> >
>> >> >
>> >
>> >
>
>