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

[Jens Axboe]

On 03/31/2016 06:56 PM, Dave Chinner wrote:
> 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.

That's the key, with that I get 300-400K ops/sec instead. I'll run some 
testing with this tomorrow and see what I can find, it did one full run 
now and I didn't see any issues, but I need to run it at various 
settings and see if I can find the issue.

> > 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.

OK

> > 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:
> >
> > https://urldefense.proofpoint.com/v2/url?u=http-3A__git.kernel.dk_cgit_linux-2Dblock_log_-3Fh-3Dwb-2Dbuf-2Dthrottle&d=CwIBAg&c=5VD0RTtNlTh3ycd41b3MUw&r=cK1a7KivzZRh1fKQMjSm2A&m=68CEi93IKLje5aOoxk1y9HMe_HF9pAhzxJGTmTZ7_DY&s=NeYNPvJa3VdF_EEsL8VqAQzJ4UycbXZ5PzHihwZAc_A&e=
> >
> > 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.

Right, that'd definitely hide the problem for you. I'll see if I can get 
it in a reproducible state and take it from there.

On your host, you said it's SCSI backed, but what does the device look like?

--
Jens Axboe