public inbox for gcc-bugs@sourceware.org help / color / mirror / Atom feed
From: "dennis.jespersen at nasa dot gov" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug libgomp/49490] New: suboptimal load balancing in loops Date: Tue, 21 Jun 2011 16:48:00 -0000 [thread overview] Message-ID: <bug-49490-4@http.gcc.gnu.org/bugzilla/> (raw) http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49490 Summary: suboptimal load balancing in loops Product: gcc Version: unknown Status: UNCONFIRMED Severity: minor Priority: P3 Component: libgomp AssignedTo: unassigned@gcc.gnu.org ReportedBy: dennis.jespersen@nasa.gov Created attachment 24573 --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=24573 test code to show how a compiler/runtime splits an OpenMP loop The OpenMP runtime library produces a correct but suboptimal load balance in parallel loops. For example, a loop of length 33 with 8 OpenMP threads will give the threads work of lengths 5, 5, 5, 5, 5, 5, 3, 0 respectively. This is logically correct, but imagine a dual-socket 4 core + 4 core configuration; then the "left" socket has 20 units of work while the "right" socket has 13 units of work. This could put undue pressure on the left cache(s) and/or memory connection. It would be better to spread out the work as much as possible, so in the example in question the threads would get work of lengths 5, 4, 4, 4, 4, 4, 4, 4. It should be fairly easy to modify libgomp/iter.c to produce the better load balancing (at least I think that's where the modification would go). The attached Fortran code will show the load balance; the Portland Group and Intel products give the desired even balance.
next reply other threads:[~2011-06-21 16:48 UTC|newest] Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top 2011-06-21 16:48 dennis.jespersen at nasa dot gov [this message] 2011-06-22 14:42 ` [Bug libgomp/49490] " jakub at gcc dot gnu.org 2011-06-22 20:39 ` jakub at gcc dot gnu.org
Reply instructions: You may reply publicly to this message via plain-text email using any one of the following methods: * Save the following mbox file, import it into your mail client, and reply-to-all from there: mbox Avoid top-posting and favor interleaved quoting: https://en.wikipedia.org/wiki/Posting_style#Interleaved_style * Reply using the --to, --cc, and --in-reply-to switches of git-send-email(1): git send-email \ --in-reply-to=bug-49490-4@http.gcc.gnu.org/bugzilla/ \ --to=gcc-bugzilla@gcc.gnu.org \ --cc=gcc-bugs@gcc.gnu.org \ /path/to/YOUR_REPLY https://kernel.org/pub/software/scm/git/docs/git-send-email.html * If your mail client supports setting the In-Reply-To header via mailto: links, try the mailto: linkBe sure your reply has a Subject: header at the top and a blank line before the message body.
This is a public inbox, see mirroring instructions for how to clone and mirror all data and code used for this inbox; as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).