[llvm-dev] Add support for in-process profile merging in profile-runtime

Xinliang David Li via llvm-dev llvm-dev at lists.llvm.org
Sun Feb 28 00:13:43 PST 2016

On Sat, Feb 27, 2016 at 6:50 PM, Sean Silva <chisophugis at gmail.com> wrote:

> I have thought about this issue too, in the context of games. We may want
> to turn profiling only for certain frames (essentially, this is many small
> profile runs).
> However, I have not seen it demonstrated that this kind of refined data
> collection will actually improve PGO results in practice.
> The evidence I do have though is that IIRC Apple have found that almost
> all of the benefits of PGO for the Clang binary can be gotten with a
> handful of training runs of Clang. Are your findings different?

We have a very wide customer base so we can not claim one use model is
sufficient for all users. For instance, we have users using fine grained
profile dumping control (programatically) as you described above. There are
also other possible use cases such as dump profiles for different
periodical phases into files associated with phases. Later different
phase's profile data can be merged with different weights.

> Also, in general, I am very wary of file locking. This can cause huge
> amounts of slowdown for a build and has potential portability problems.

I don't see much slow down with a clang build using instrumented clang as
the build compiler. With file locking and profile merging enabled, the
build time on my local machine looks like:

real    18m22.737s
user    293m18.924s
sys     9m55.532s

If profile merging/locking is disabled (i.e, let the profile dumper to
clobber/write over each other),  the real time is about 14m.

> I don't see it as a substantially better solution than wrapping clang in a
> script that runs clang and then just calls llvm-profdata to do the merging.
> Running llvm-profdata is cheap compared to doing locking in a highly
> parallel situation like a build.

That would require synchronization for merging too.

>From Justin's email, it looks like there is a key point I have not made
clear: the on-line profile merge is a very simple raw profile to raw
profile merging which is super fast. The end result of the profile run is
still in raw format. The raw to indexed merging is still needed -- but
instead of merging thousands of raw profiles which can be very slow, with
this model, only one raw profile input is needed.



> -- Sean Silva
> On Sat, Feb 27, 2016 at 6:02 PM, Xinliang David Li via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>> One of the main missing features in Clang/LLVM profile runtime is the
>> lack of support for online/in-process profile merging support. Profile data
>> collected for different workloads for the same executable binary need to be
>> collected and merged later by the offline post-processing tool.  This
>> limitation makes it hard to handle cases where the instrumented binary
>> needs to be run with large number of small workloads, possibly in
>> parallel.  For instance, to do PGO for clang, we may choose to  build  a
>> large project with the instrumented Clang binary. This is because
>>  1) to avoid profile from different runs from overriding others, %p
>> substitution needs to be specified in either the command line or an
>> environment variable so that different process can dump profile data into
>> its own file named using pid. This will create huge requirement on the disk
>> storage. For instance, clang's raw profile size is typically 80M -- if the
>> instrumented clang is used to build a medium to large size project (such as
>> clang itself), profile data can easily use up hundreds of Gig bytes of
>> local storage.
>> 2) pid can also be recycled. This means that some of the profile data may
>> be overridden without being noticed.
>> The way to solve this problem is to allow profile data to be merged in
>> process.  I have a prototype implementation and plan to send it out for
>> review soon after some clean ups. By default, the profiling merging is off
>> and it can be turned on with an user option or via an environment variable.
>> The following summarizes the issues involved in adding this feature:
>>  1. the target platform needs to have file locking support
>>  2. there needs an efficient way to identify the profile data and
>> associate it with the binary using binary/profdata signature;
>>  3. Currently without merging, profile data from shared libraries
>> (including dlopen/dlcose ones) are concatenated into the primary profile
>> file. This can complicate matters, as the merger also needs to find the
>> matching shared libs, and the merger also needs to avoid unnecessary data
>> movement/copy;
>>  4. value profile data is variable in length even for the same binary.
>> All the above issues are resolved and clang self build with instrumented
>> binary passes (with both j1 and high parallelism).
>> If you have any concerns, please let me know.
>> 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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