From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from elasmtp-kukur.atl.sa.earthlink.net ([209.86.89.65]:39942 "EHLO elasmtp-kukur.atl.sa.earthlink.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728134AbeHIB4a (ORCPT ); Wed, 8 Aug 2018 21:56:30 -0400 From: "Frank Filz" To: "'Jeff Layton'" , "'J. Bruce Fields'" , "'NeilBrown'" Cc: "'Alexander Viro'" , , , "'Martin Wilck'" References: <153369219467.12605.13472423449508444601.stgit@noble> <20180808195445.GD23873@fieldses.org> <20180808200912.GE23873@fieldses.org> <20180808212832.GF23873@fieldses.org> <04ffa27c29d2bff8bd9cb9b6d4ea6b6fd3969b6c.camel@kernel.org> In-Reply-To: <04ffa27c29d2bff8bd9cb9b6d4ea6b6fd3969b6c.camel@kernel.org> Subject: RE: [PATCH 0/4] locks: avoid thundering-herd wake-ups Date: Wed, 8 Aug 2018 16:34:31 -0700 Message-ID: <01c401d42f70$5c034db0$1409e910$@mindspring.com> MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable Content-Language: en-us Sender: linux-fsdevel-owner@vger.kernel.org List-ID: > On Wed, 2018-08-08 at 17:28 -0400, J. Bruce Fields wrote: > > On Wed, Aug 08, 2018 at 04:09:12PM -0400, J. Bruce Fields wrote: > > > On Wed, Aug 08, 2018 at 03:54:45PM -0400, J. Bruce Fields wrote: > > > > 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.... > > > > > > In other words, is there the possibility of a tree of, say, > > > exclusive locks with (offset, length) like: > > > > > > (0, 2) waiting on (1, 2) waiting on (2, 2) waiting on (0, 4) > > > > > > and when waking (0, 4) you could wake up (2, 2) but not (0, 2), > > > leaving a process waiting without there being an actual conflict. > > > > After batting it back and forth with Jeff on IRC.... So do I > > understand right that when we wake a waiter, we leave its own tree = of > > waiters intact, and when it wakes if it finds a conflict it just = adds > > it lock (with tree of waiters) in to the tree of the conflicting = lock? > > > > If so then yes I think that depends on the transitivity > > assumption--you're assuming that finding a conflict between the root > > of the tree and a lock proves that all the other members of the tree > > also conflict. > > > > So maybe this example works. (All locks are exclusive and written > > (offset, length), X->Y means X is waiting on Y.) > > > > process acquires (0,3) > > 2nd process requests (1,2), is put to sleep. > > 3rd process requests (0,2), is put to sleep. > > > > The tree of waiters now looks like (0,2)->(1,2)->(0,3) > > > > (0,3) is unlocked. > > A 4th process races in and locks (2,2). > > The 2nd process wakes up, sees this new conflict, and waits on > > (2,2). Now the tree looks like (0,2)->(1,2)->(2,2), and (0,2) > > is waiting for no reason. > > >=20 > That seems like a legit problem. >=20 > One possible fix might be to have the waiter on (1,2) walk down the = entire > subtree and wake up any waiter that is waiting on a lock that doesn't = conflict > with the lock on which it's waiting. >=20 > So, before the task waiting on 1,2 goes back to sleep to wait on 2,2, = it could > walk down its entire fl_blocked subtree and wake up anything waiting = on a lock > that doesn't conflict with (2,2). >=20 > That's potentially an expensive operation, but: >=20 > a) the task is going back to sleep anyway, so letting it do a little = extra work > before that should be no big deal >=20 > b) it's probably still cheaper than waking up the whole herd Yea, I think so. Now here's another question... How does this new logic play with Open = File Description Locks? Should still be ok since there's a thread = waiting on each of those. Frank