Re: [PATCHSET v5] Make background writeback great again for the first time

[Chris Mason <clm@fb.com>]

On Tue, May 03, 2016 at 03:06:09PM +0200, Jan Kara wrote:
> On Tue 03-05-16 08:40:11, Chris Mason wrote:
> > On Tue, May 03, 2016 at 02:17:19PM +0200, Jan Kara wrote:
> > > On Thu 28-04-16 12:46:41, Jens Axboe wrote:
> > > > >>-	rwb->wb_max = 1 + ((depth - 1) >> min(31U, rwb->scale_step));
> > > > >>-	rwb->wb_normal = (rwb->wb_max + 1) / 2;
> > > > >>-	rwb->wb_background = (rwb->wb_max + 3) / 4;
> > > > >>+	if (rwb->queue_depth == 1) {
> > > > >>+		rwb->wb_max = rwb->wb_normal = 2;
> > > > >>+		rwb->wb_background = 1;
> > > > >
> > > > >This breaks the detection of too big scale_step in scale_up() where we key
> > > > >of wb_max == 1 value. However even with that fixed no luck :(:
> > > > 
> > > > Yeah, I need to look at that. For QD=1, I think the only sensible values for
> > > > max/normal/bg is 2/2/1 and 1/1/1 if we step down.
> > > > 
> > > > >dd if=/dev/zero of=/mnt/file bs=1M count=10000 conv=fsync
> > > > >Runtime: 105.126 107.125 105.641
> > > > >
> > > > >So about the same as before. I'll try to debug this later today...
> > > > 
> > > > Thanks, I'm very interested in what you find!
> > > 
> > > OK, so the reason was relatively standard in the end. I was using ext3 (or
> > > more exactly ext4 without delayed allocation) for the test. The throttling
> > > of background writes gave more priority to writes from the journalling
> > > thread which happen with WRITE_SYNC and thus are not throttled. Thus the
> > > journalling thread ended up having to do more data writeback to be able to
> > > commit a transaction (due to requirements of data=ordered mode) and it is
> > > less efficient at that than the normal flusher thread.
> > > 
> > > So this is an example where throttling background writeback effectively
> > > just pushes more work into another context which does it less efficiently
> > > and indirectly makes everyone wait for it. ext3 has been always sensitive to
> > > issues like this. ext4 is using delayed allocation and thus only data
> > > writes into holes end up being part of a transaction -> simple dd test case
> > > doesn't hit that path. And indeed when I repeat the same test with ext4,
> > > the numbers with and without your patch are exactly the same.
> > > 
> > > The question remains how common a pattern where throttling of background
> > > writeback delays also something else is. I'll schedule a couple of
> > > benchmarks to measure impact of your patches for a wider range of workloads
> > > (but sadly pretty limited set of hw). If ext3 is the only one seeing
> > > issues, I would be willing to accept that ext3 takes the hit since it is
> > > doing something rather stupid (but inherent in its journal design) and we
> > > have a way to deal with this either by enabling delayed allocation or by
> > > turning off the writeback throttling...
> > 
> > At least in the case of io that we know is going to be data=ordered, we
> > can bump the prio of those pages?
> 
> But how would flusher thread, which is submitting IO, know that? We would
> have to somehow mark inodes that are part of the running transaction and
> flusher thread could give more priority to such writeback - e.g. by using
> WRITE_SYNC or at least plain writes. Hmm, if we use an inode flag for that,
> it could be doable.

This would be specific to the data=ordered code in the FS.  If there's
some way to test for an inode or a page's status in the data=ordered
list, the FS writepages call could flag the IO as higher prio?

-chris