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Date:   Thu, 09 Aug 2018 10:58:38 +1000
From:   NeilBrown <neilb@...e.com>
To:     "J. Bruce Fields" <bfields@...ldses.org>,
        Jeff Layton <jlayton@...nel.org>
Cc:     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 12:47:22PM -0400, Jeff Layton wrote:
>> On Wed, 2018-08-08 at 11:51 +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.
>> > 
>> > Testing shows that lock-acquisitions-per-second is now fairly stable even
>> > as number of contending process goes to 1000.  Without this patch,
>> > locks-per-second drops off steeply after a few 10s of processes.
>> > 
>> > There is a small cost to this extra complexity.
>> > At 20 processes running a particular test on 72 cores, the lock
>> > acquisitions per second drops from 1.8 million to 1.4 million with
>> > this patch.  For 100 processes, this patch still provides 1.4 million
>> > while without this patch there are about 700,000.
>> > 
>> > NeilBrown
>> > 
>> > ---
>> > 
>> > NeilBrown (4):
>> >       fs/locks: rename some lists and pointers.
>> >       fs/locks: allow a lock request to block other requests.
>> >       fs/locks: change all *_conflict() functions to return bool.
>> >       fs/locks: create a tree of dependent requests.
>> > 
>> > 
>> >  fs/cifs/file.c                  |    2 -
>> >  fs/locks.c                      |  142 +++++++++++++++++++++++++--------------
>> >  include/linux/fs.h              |    5 +
>> >  include/trace/events/filelock.h |   16 ++--
>> >  4 files changed, 103 insertions(+), 62 deletions(-)
>> > 
>> 
>> Nice work! I looked over this and I think it looks good.
>> 
>> I made an attempt to fix this issue several years ago, but my method
>> sucked as it ended up penalizing the unlocking task too much. This is
>> much cleaner and should scale well overall, I think.
>
> I think I also took a crack at this at one point while I was at UM/CITI
> and never got anything I was happy with.  Looks like good work!
>
> I remember one main obstacle that I felt like I never had a good
> benchmark....
>
> How did you choose this workload and hardware?  Was it in fact udev
> (booting a large machine?), or was there some other motivation?

I'm hoping Martin will chime in here - her identified the problem and
did most of the testing...

NeilBrown


>
> Not that I'm likely to do it any time soon, but could you share
> sufficient details for someone else to reproduce your results?
>
> --b.

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