From: Dave Chinner <david@fromorbit.com>
To: Jens Axboe <axboe@fb.com>
Cc: linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
linux-block@vger.kernel.org
Subject: Re: [PATCHSET v3][RFC] Make background writeback not suck
Date: Fri, 1 Apr 2016 11:56:08 +1100 [thread overview]
Message-ID: <20160401005608.GU11812@dastard> (raw)
In-Reply-To: <56FD4E70.3090203@fb.com>
On Thu, Mar 31, 2016 at 10:21:04AM -0600, Jens Axboe wrote:
> On 03/31/2016 08:29 AM, Jens Axboe wrote:
> >>What I see in these performance dips is the XFS transaction
> >>subsystem stalling *completely* - instead of running at a steady
> >>state of around 350,000 transactions/s, there are *zero*
> >>transactions running for periods of up to ten seconds. This
> >>co-incides with the CPU usage falling to almost zero as well.
> >>AFAICT, the only thing that is running when the filesystem stalls
> >>like this is memory reclaim.
> >
> >I'll take a look at this, stalls should definitely not be occurring. How
> >much memory does the box have?
>
> I can't seem to reproduce this at all. On an nvme device, I get a
> fairly steady 60K/sec file creation rate, and we're nowhere near
> being IO bound. So the throttling has no effect at all.
That's too slow to show the stalls - your likely concurrency bound
in allocation by the default AG count (4) from mkfs. Use mkfs.xfs -d
agcount=32 so that every thread works in it's own AG.
> On a raid0 on 4 flash devices, I get something that looks more IO
> bound, for some reason. Still no impact of the throttling, however.
> But given that your setup is this:
>
> virtio in guest, XFS direct IO -> no-op -> scsi in host.
>
> we do potentially have two throttling points, which we don't want.
> Is both the guest and the host running the new code, or just the
> guest?
Just the guest. Host is running a 4.2.x kernel, IIRC.
> In any case, can I talk you into trying with two patches on top of
> the current code? It's the two newest patches here:
>
> http://git.kernel.dk/cgit/linux-block/log/?h=wb-buf-throttle
>
> The first treats REQ_META|REQ_PRIO like they should be treated, like
> high priority IO. The second disables throttling for virtual
> devices, so we only throttle on the backend. The latter should
> probably be the other way around, but we need some way of conveying
> that information to the backend.
I'm not changing the host kernels - it's a production machine and so
it runs long uptime testing of stable kernels. (e.g. catch slow
memory leaks, etc). So if you've disabled throttling in the guest, I
can't test the throttling changes.
Cheers,
Dave.
--
Dave Chinner
david@fromorbit.com
next prev parent reply other threads:[~2016-04-01 0:56 UTC|newest]
Thread overview: 33+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-03-30 15:07 [PATCHSET v3][RFC] Make background writeback not suck Jens Axboe
2016-03-30 15:07 ` [PATCH 1/9] writeback: propagate the various reasons for writeback Jens Axboe
2016-03-30 15:07 ` [PATCH 2/9] writeback: add wbc_to_write() Jens Axboe
2016-03-30 15:07 ` [PATCH 3/9] writeback: use WRITE_SYNC for reclaim or sync writeback Jens Axboe
2016-03-30 15:07 ` [PATCH 4/9] writeback: track if we're sleeping on progress in balance_dirty_pages() Jens Axboe
2016-04-13 13:08 ` Jan Kara
2016-04-13 14:20 ` Jens Axboe
2016-03-30 15:07 ` [PATCH 5/9] block: add ability to flag write back caching on a device Jens Axboe
2016-03-30 15:42 ` Christoph Hellwig
2016-03-30 15:46 ` Jens Axboe
2016-03-30 16:23 ` Jens Axboe
2016-03-30 17:29 ` Christoph Hellwig
2016-03-30 15:07 ` [PATCH 6/9] sd: inform block layer of write cache state Jens Axboe
2016-03-30 15:07 ` [PATCH 7/9] NVMe: " Jens Axboe
2016-03-30 15:07 ` [PATCH 8/9] block: add code to track actual device queue depth Jens Axboe
2016-03-30 15:07 ` [PATCH 9/9] writeback: throttle buffered writeback Jens Axboe
2016-03-31 8:24 ` [PATCHSET v3][RFC] Make background writeback not suck Dave Chinner
2016-03-31 14:29 ` Jens Axboe
2016-03-31 16:21 ` Jens Axboe
2016-04-01 0:56 ` Dave Chinner [this message]
2016-04-01 3:29 ` Jens Axboe
2016-04-01 3:33 ` Jens Axboe
2016-04-01 3:39 ` Jens Axboe
2016-04-01 6:16 ` Dave Chinner
2016-04-01 14:33 ` Jens Axboe
2016-04-01 5:04 ` Dave Chinner
2016-04-01 0:46 ` Dave Chinner
2016-04-01 3:25 ` Jens Axboe
2016-04-01 6:27 ` Dave Chinner
2016-04-01 14:34 ` Jens Axboe
2016-03-31 22:09 ` Holger Hoffstätte
2016-04-01 1:01 ` Dave Chinner
2016-04-01 16:58 ` Holger Hoffstätte
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