From: NeilBrown <neilb@suse.com>
To: "J. Bruce Fields" <bfields@fieldses.org>
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: Thu, 09 Aug 2018 08:39:28 +1000 [thread overview]
Message-ID: <87bmactrdr.fsf@notabene.neil.brown.name> (raw)
In-Reply-To: <20180808212832.GF23873@fieldses.org>
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On Wed, Aug 08 2018, 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.
Ahhh... I see what you are getting at. When lock requests are added
individually, they will always be blocked by all ancestors in the tree.
But when they are added as a group, that might not be the case.
So we might need to re-add every request individually.
In the (common) case where a lock request is blocked across its whole
range, we can just attach the whole tree beneath the blocker. In other
cases we need a finer grained approach.
I'll have a look and see how to make the code work for this case.
Thanks for the thorough review!
NeilBrown
>
> 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.
>
> ?
>
> --b.
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next prev parent reply other threads:[~2018-08-08 22:39 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
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 [this message]
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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