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

[Jens Axboe]

On 03/31/2016 09:29 PM, Jens Axboe wrote:
> > > 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.

No stalls seen, I get the same performance with it disabled and with it 
enabled, at both default settings, and lower ones (wb_percent=20). 
Looking at iostat, we don't drive a lot of depth, so it makes sense, 
even with the throttling we're doing essentially the same amount of IO.

What does 'nr_requests' say for your virtio_blk device? Looks like 
virtio_blk has a queue_depth setting, but it's not set by default, and 
then it uses the free entries in the ring. But I don't know what that is...

--
Jens Axboe