Re: Parallelize the compilation using Threads

[Richard Biener <richard dot guenther at gmail dot com>]

On Wed, Dec 12, 2018 at 4:46 PM Giuliano Augusto Faulin Belinassi
<giuliano.belinassi@usp.br> wrote:
>
> Hi, I have some news. :-)
>
> I replicated the Martin Liška experiment [1] on a 64-cores machine for
> gcc [2] and Linux kernel [3] (Linux kernel was fully parallelized),
> and I am excited to dive into this problem. As a result, I want to
> propose GSoC project on this issue, starting with something like:
>     1- Systematically create a benchmark for easily information
> gathering. Martin Liška already made the first version of it, but I
> need to improve it.
>     2- Find and document the global states (Try to reduce the gcc's
> global states as well).
>     3- Define the parallelization strategy.
>     4- First parallelization attempt.
>
> I also proposed this issue as a research project to my advisor and he
> supported me on this idea. So I can work for at least one year on
> this, and other things related to it.
>
> Would anyone be willing to mentor me on this?

As the one who initially suggested the project I'm certainly willing
to mentor you on this.

Richard.

> [1] https://gcc.gnu.org/bugzilla/attachment.cgi?id=43440
> [2] https://www.ime.usp.br/~belinass/64cores-experiment.svg
> [3] https://www.ime.usp.br/~belinass/64cores-kernel-experiment.svg
> On Mon, Nov 19, 2018 at 8:53 AM Richard Biener
> <richard.guenther@gmail.com> wrote:
> >
> > On Fri, Nov 16, 2018 at 8:00 PM Giuliano Augusto Faulin Belinassi
> > <giuliano.belinassi@usp.br> wrote:
> > >
> > > Hi! Sorry for the late reply again :P
> > >
> > > On Thu, Nov 15, 2018 at 8:29 AM Richard Biener
> > > <richard.guenther@gmail.com> wrote:
> > > >
> > > > On Wed, Nov 14, 2018 at 10:47 PM Giuliano Augusto Faulin Belinassi
> > > > <giuliano.belinassi@usp.br> wrote:
> > > > >
> > > > > As a brief introduction, I am a graduate student that got interested
> > > > >
> > > > > in the "Parallelize the compilation using threads"(GSoC 2018 [1]). I
> > > > > am a newcommer in GCC, but already have sent some patches, some of
> > > > > them have already been accepted [2].
> > > > >
> > > > > I brought this subject up in IRC, but maybe here is a proper place to
> > > > > discuss this topic.
> > > > >
> > > > > From my point of view, parallelizing GCC itself will only speed up the
> > > > > compilation of projects which have a big file that creates a
> > > > > bottleneck in the whole project compilation (note: by big, I mean the
> > > > > amount of code to generate).
> > > >
> > > > That's true.  During GCC bootstrap there are some of those (see PR84402).
> > > >
> > >
> > > > One way to improve parallelism is to use link-time optimization where
> > > > even single source files can be split up into multiple link-time units.  But
> > > > then there's the serial whole-program analysis part.
> > >
> > > Did you mean this: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84402 ?
> > > That is a lot of data :-)
> > >
> > > It seems that 'phase opt and generate' is the most time-consuming
> > > part. Is that the 'GIMPLE optimization pipeline' you were talking
> > > about in this thread:
> > > https://gcc.gnu.org/ml/gcc/2018-03/msg00202.html
> >
> > It's everything that comes after the frontend parsing bits, thus this
> > includes in particular RTL optimization and early GIMPLE optimizations.
> >
> > > > > Additionally, I know that GCC must not
> > > > > change the project layout, but from the software engineering perspective,
> > > > > this may be a bad smell that indicates that the file should be broken
> > > > > into smaller files. Finally, the Makefiles will take care of the
> > > > > parallelization task.
> > > >
> > > > What do you mean by GCC must not change the project layout?  GCC
> > > > happily re-orders functions and link-time optimization will reorder
> > > > TUs (well, linking may as well).
> > > >
> > >
> > > That was a response to a comment made on IRC:
> > >
> > > On Thu, Nov 15, 2018 at 9:44 AM Jonathan Wakely <jwakely.gcc@gmail.com> wrote:
> > > >I think this is in response to a comment I made on IRC. Giuliano said
> > > >that if a project has a very large file that dominates the total build
> > > >time, the file should be split up into smaller pieces. I said  "GCC
> > > >can't restructure people's code. it can only try to compile it
> > > >faster". We weren't referring to code transformations in the compiler
> > > >like re-ordering functions, but physically refactoring the source
> > > >code.
> > >
> > > Yes. But from one of the attachments from PR84402, it seems that such
> > > files exist on GCC,
> > > https://gcc.gnu.org/bugzilla/attachment.cgi?id=43440
> > >
> > > > > My questions are:
> > > > >
> > > > >  1. Is there any project compilation that will significantly be improved
> > > > > if GCC runs in parallel? Do someone has data about something related
> > > > > to that? How about the Linux Kernel? If not, I can try to bring some.
> > > >
> > > > We do not have any data about this apart from experiments with
> > > > splitting up source files for PR84402.
> > > >
> > > > >  2. Did I correctly understand the goal of the parallelization? Can
> > > > > anyone provide extra details to me?
> > > >
> > > > You may want to search the mailing list archives since we had a
> > > > student application (later revoked) for the task with some discussion.
> > > >
> > > > In my view (I proposed the thing) the most interesting parts are
> > > > getting GCCs global state documented and reduced.  The parallelization
> > > > itself is an interesting experiment but whether there will be any
> > > > substantial improvement for builds that can already benefit from make
> > > > parallelism remains a question.
> > >
> > > As I agree that documenting GCC's global states is good for the
> > > community and the development of GCC, I really don't think this a good
> > > motivation for parallelizing a compiler from a research standpoint.
> >
> > True ;)  Note that my suggestions to the other GSoC student were
> > purely based on where it's easiest to experiment with paralellization
> > and not where it would be most beneficial.
> >
> > > There must be something or someone that could take advantage of the
> > > fine-grained parallelism. But that data from PR84402 seems to have the
> > > answer to it. :-)
> > >
> > > On Thu, Nov 15, 2018 at 4:07 PM Szabolcs Nagy <Szabolcs.Nagy@arm.com> wrote:
> > > >
> > > > On 15/11/18 10:29, Richard Biener wrote:
> > > > > In my view (I proposed the thing) the most interesting parts are
> > > > > getting GCCs global state documented and reduced.  The parallelization
> > > > > itself is an interesting experiment but whether there will be any
> > > > > substantial improvement for builds that can already benefit from make
> > > > > parallelism remains a question.
> > > >
> > > > in the common case (project with many small files, much more than
> > > > core count) i'd expect a regression:
> > > >
> > > > if gcc itself tries to parallelize that introduces inter thread
> > > > synchronization and potential false sharing in gcc (e.g. malloc
> > > > locks) that does not exist with make parallelism (glibc can avoid
> > > > some atomic instructions when a process is single threaded).
> > >
> > > That is what I am mostly worried about. Or the most costly part is not
> > > parallelizable at all. Also, I would expect a regression on very small
> > > files, which probably could be avoided implementing this feature as a
> > > flag?
> >
> > I think the the issue should be avoided by avoiding fine-grained paralellism.
> > Which might be somewhat hard given there are core data structures that
> > are shared (the memory allocator for a start).
> >
> > The other issue I am more worried about is that we probably have to
> > interact with make somehow so that we do not end up with 64 threads
> > when one does -j8 on a 8 core machine.  That's basically the same
> > issue we run into with -flto and it's threaded WPA writeout or recursive
> > invocation of make.
> >
> > >
> > > On Fri, Nov 16, 2018 at 11:05 AM Martin Jambor <mjambor@suse.cz> wrote:
> > > >
> > > > Hi Giuliano,
> > > >
> > > > On Thu, Nov 15 2018, Richard Biener wrote:
> > > > > You may want to search the mailing list archives since we had a
> > > > > student application (later revoked) for the task with some discussion.
> > > >
> > > > Specifically, the whole thread beginning with
> > > > https://gcc.gnu.org/ml/gcc/2018-03/msg00179.html
> > > >
> > > > Martin
> > > >
> > >
> > > Yes, I will research this carefully ;-)
> > >
> > > Thank you