Re: [PATCHSET v3][RFC] Make background writeback not suck

From: Dave Chinner
Date: Thu Mar 31 2016 - 20:56:16 EST

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:
> 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.


Dave Chinner