From: "J. Bruce Fields" <bfields@fieldses.org>
To: NeilBrown <neilb@suse.com>
Cc: Jeff Layton <jlayton@kernel.org>,
Alexander Viro <viro@zeniv.linux.org.uk>,
linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
Martin Wilck <mwilck@suse.de>
Subject: Re: [PATCH 0/4] locks: avoid thundering-herd wake-ups
Date: Wed, 8 Aug 2018 16:09:12 -0400 [thread overview]
Message-ID: <20180808200912.GE23873@fieldses.org> (raw)
In-Reply-To: <20180808195445.GD23873@fieldses.org>
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.
--b.
next prev parent reply other threads:[~2018-08-08 20:09 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-08-08 1:51 [PATCH 0/4] locks: avoid thundering-herd wake-ups NeilBrown
2018-08-08 1:51 ` [PATCH 1/4] fs/locks: rename some lists and pointers NeilBrown
2018-08-08 10:47 ` Jeff Layton
2018-08-08 19:07 ` J. Bruce Fields
2018-08-08 1:51 ` [PATCH 3/4] fs/locks: change all *_conflict() functions to return bool NeilBrown
2018-08-08 1:51 ` [PATCH 2/4] fs/locks: allow a lock request to block other requests NeilBrown
2018-08-08 1:51 ` [PATCH 4/4] fs/locks: create a tree of dependent requests NeilBrown
2018-08-08 16:47 ` [PATCH 0/4] locks: avoid thundering-herd wake-ups Jeff Layton
2018-08-08 18:29 ` J. Bruce Fields
2018-08-09 0:58 ` NeilBrown
2018-08-20 11:02 ` Martin Wilck
2018-08-20 20:02 ` J. Bruce Fields
2018-08-20 20:06 ` Martin Wilck
2018-08-08 19:54 ` J. Bruce Fields
2018-08-08 20:09 ` J. Bruce Fields [this message]
2018-08-08 21:15 ` Frank Filz
2018-08-08 22:34 ` NeilBrown
2018-08-08 21:28 ` J. Bruce Fields
2018-08-08 22:39 ` NeilBrown
2018-08-08 22:50 ` Jeff Layton
2018-08-08 23:34 ` Frank Filz
2018-08-09 2:52 ` NeilBrown
2018-08-09 13:00 ` J. Bruce Fields
2018-08-09 14:49 ` Jeff Layton
2018-08-09 23:56 ` NeilBrown
2018-08-10 1:05 ` J. Bruce Fields
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