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Date:   Thu, 09 Aug 2018 08:39:28 +1000
From:   NeilBrown <neilb@...e.com>
To:     "J. Bruce Fields" <bfields@...ldses.org>
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, 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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