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@xxxxxxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe linux-block" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
