linux-kernel - Re: [PATCH 0/4] locks: avoid thundering-herd wake-ups

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Date:   Wed, 8 Aug 2018 16:09:12 -0400
From:   "J. Bruce Fields" <bfields@...ldses.org>
To:     NeilBrown <neilb@...e.com>
Cc:     Jeff Layton <jlayton@...nel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Martin Wilck <mwilck@...e.de>
Subject: Re: [PATCH 0/4] locks: avoid thundering-herd wake-ups

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.