From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 20214 invoked by alias); 17 Dec 2018 11:06:32 -0000 Mailing-List: contact gcc-help@gcc.gnu.org; run by ezmlm Precedence: bulk List-Id: List-Archive: List-Post: List-Help: Sender: gcc-owner@gcc.gnu.org Received: (qmail 19783 invoked by uid 89); 17 Dec 2018 11:06:31 -0000 Authentication-Results: sourceware.org; auth=none X-Spam-SWARE-Status: No, score=-1.9 required=5.0 tests=BAYES_00,FREEMAIL_FROM,KAM_SHORT,RCVD_IN_DNSWL_NONE,SPF_PASS autolearn=ham version=3.3.2 spammy=strategy, transformations, phase, worried X-HELO: mail-lf1-f54.google.com Received: from mail-lf1-f54.google.com (HELO mail-lf1-f54.google.com) (209.85.167.54) by sourceware.org (qpsmtpd/0.93/v0.84-503-g423c35a) with ESMTP; Mon, 17 Dec 2018 11:06:23 +0000 Received: by mail-lf1-f54.google.com with SMTP id f23so9089279lfc.13 for ; Mon, 17 Dec 2018 03:06:22 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc:content-transfer-encoding; bh=2IlUbgsBOXB/fLW12uGba+JGD2esQ1pKk1PEUnRkJdE=; b=aiT4CCeh/WPVkF9/KDuJcxPJqR1D1reWjXQGW5Us8peivRMB/NP3KU0Xmpy8G7kOJm LGccWcWtESpKQCL1cBAwBnXV++fpl2puxB+IQW9Aw1wW91v6/sEPjVoPcPAPyj1gW3nI D6SXjMy6qAL54iujd7OPf9Rq8l24mrJolonNKRW1abRoJy7NgQ3ivchCormAetjM9wgi ulbExk3EcYxXQs2dW60YkHe7LKCxIqevOlwQErbAcbxPLfEsFxv4J1FADBcerVhyd8lJ UIt/HnnQhahlr4adfD0jw+28MeK5Vcu3L6JIYJD+wrM1aGQ1ze3FUCIlH5bpqb0/dJYj I0lA== MIME-Version: 1.0 References: In-Reply-To: From: Richard Biener Date: Mon, 17 Dec 2018 11:06:00 -0000 Message-ID: Subject: Re: Parallelize the compilation using Threads To: Giuliano Augusto Faulin Belinassi Cc: GCC Development , kernel-usp@googlegroups.com, gold@ime.usp.br, Alfredo Goldman Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-IsSubscribed: yes X-SW-Source: 2018-12/txt/msg00097.txt.bz2 On Wed, Dec 12, 2018 at 4:46 PM Giuliano Augusto Faulin Belinassi wrote: > > Hi, I have some news. :-) > > I replicated the Martin Li=C5=A1ka experiment [1] on a 64-cores machine f= or > 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 Li=C5=A1ka 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=3D43440 > [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 > wrote: > > > > On Fri, Nov 16, 2018 at 8:00 PM Giuliano Augusto Faulin Belinassi > > wrote: > > > > > > Hi! Sorry for the late reply again :P > > > > > > On Thu, Nov 15, 2018 at 8:29 AM Richard Biener > > > wrote: > > > > > > > > On Wed, Nov 14, 2018 at 10:47 PM Giuliano Augusto Faulin Belinassi > > > > wrote: > > > > > > > > > > As a brief introduction, I am a graduate student that got interes= ted > > > > > > > > > > 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 plac= e to > > > > > discuss this topic. > > > > > > > > > > From my point of view, parallelizing GCC itself will only speed u= p 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 PR8= 4402). > > > > > > > > > > > One way to improve parallelism is to use link-time optimization whe= re > > > > even single source files can be split up into multiple link-time un= its. But > > > > then there's the serial whole-program analysis part. > > > > > > Did you mean this: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D844= 02 ? > > > 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 pers= pective, > > > > > this may be a bad smell that indicates that the file should be br= oken > > > > > 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 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 bui= ld > > > >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=3D43440 > > > > > > > > My questions are: > > > > > > > > > > 1. Is there any project compilation that will significantly be i= mproved > > > > > if GCC runs in parallel? Do someone has data about something rela= ted > > > > > to that? How about the Linux Kernel? If not, I can try to bring s= ome. > > > > > > > > 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? C= an > > > > > 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 discussi= on. > > > > > > > > In my view (I proposed the thing) the most interesting parts are > > > > getting GCCs global state documented and reduced. The parallelizat= ion > > > > itself is an interesting experiment but whether there will be any > > > > substantial improvement for builds that can already benefit from ma= ke > > > > 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 = 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 paralleliz= ation > > > > > 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 parale= llism. > > 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 wrot= e: > > > > > > > > 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 discus= sion. > > > > > > > > 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