On Wed, Dec 12, 2018 at 11:46 PM Giuliano Augusto Faulin Belinassi <giuliano.belina...@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. Hi Giuliano,
Thanks very much for working on this. It could be very useful, for example, one bottleneck we have is slow compilation of big single source file after intensively using distribution compilation. Of course, a good parallelization strategy is needed. Thanks, bin > > 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? > > [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.guent...@gmail.com> wrote: > > > > On Fri, Nov 16, 2018 at 8:00 PM Giuliano Augusto Faulin Belinassi > > <giuliano.belina...@usp.br> wrote: > > > > > > Hi! Sorry for the late reply again :P > > > > > > On Thu, Nov 15, 2018 at 8:29 AM Richard Biener > > > <richard.guent...@gmail.com> wrote: > > > > > > > > On Wed, Nov 14, 2018 at 10:47 PM Giuliano Augusto Faulin Belinassi > > > > <giuliano.belina...@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....@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.n...@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 <mjam...@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
