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

Xinliang David Li via llvm-dev
Tue, 29 Sep 2015 16:52:29 -0000

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
> >> >> >> >> >
> >> >> >> >> >
> >> >> >> >
> >> >> >> >
> >> >> >
> >> >> >
> >> >
> >> >
> >
> >
>
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