From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from fieldses.org ([173.255.197.46]:47032 "EHLO fieldses.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728359AbeHHWP5 (ORCPT ); Wed, 8 Aug 2018 18:15:57 -0400 Date: Wed, 8 Aug 2018 15:54:45 -0400 From: "J. Bruce Fields" To: NeilBrown Cc: Jeff Layton , Alexander Viro , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, Martin Wilck Subject: Re: [PATCH 0/4] locks: avoid thundering-herd wake-ups Message-ID: <20180808195445.GD23873@fieldses.org> References: <153369219467.12605.13472423449508444601.stgit@noble> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <153369219467.12605.13472423449508444601.stgit@noble> Sender: linux-fsdevel-owner@vger.kernel.org List-ID: On Wed, Aug 08, 2018 at 11:51:07AM +1000, NeilBrown wrote: > If you have a many-core machine, and have many threads all wanting to > briefly lock a give file (udev is known to do this), you can get quite > poor performance. > > When one thread releases a lock, it wakes up all other threads that > are waiting (classic thundering-herd) - one will get the lock and the > others go to sleep. > When you have few cores, this is not very noticeable: by the time the > 4th or 5th thread gets enough CPU time to try to claim the lock, the > earlier threads have claimed it, done what was needed, and released. > With 50+ cores, the contention can easily be measured. > > This patchset creates a tree of pending lock request in which siblings > don't conflict and each lock request does conflict with its parent. > When a lock is released, only requests which don't conflict with each > other a woken. Are you sure you aren't depending on the (incorrect) assumption that "X blocks Y" is a transitive relation? OK I should be able to answer that question myself, my patience for code-reading is at a real low this afternoon.... --b. > > Testing shows that lock-acquisitions-per-second is now fairly stable even > as number of contending process goes to 1000. Without this patch, > locks-per-second drops off steeply after a few 10s of processes. > > There is a small cost to this extra complexity. > At 20 processes running a particular test on 72 cores, the lock > acquisitions per second drops from 1.8 million to 1.4 million with > this patch. For 100 processes, this patch still provides 1.4 million > while without this patch there are about 700,000. > > NeilBrown > > --- > > NeilBrown (4): > fs/locks: rename some lists and pointers. > fs/locks: allow a lock request to block other requests. > fs/locks: change all *_conflict() functions to return bool. > fs/locks: create a tree of dependent requests. > > > fs/cifs/file.c | 2 - > fs/locks.c | 142 +++++++++++++++++++++++++-------------- > include/linux/fs.h | 5 + > include/trace/events/filelock.h | 16 ++-- > 4 files changed, 103 insertions(+), 62 deletions(-) > > -- > Signature