Re: [PATCH 3/3] Btrfs: remove nr_async_submits and async_submit_draining

[Liu Bo <bo.li.liu@oracle.com>]

On Wed, Oct 11, 2017 at 07:20:32PM +0200, David Sterba wrote:
> On Wed, Sep 27, 2017 at 02:31:17PM +0200, David Sterba wrote:
> > On Thu, Sep 07, 2017 at 11:22:22AM -0600, Liu Bo wrote:
> > > Now that we have the combo of flushing twice, which can make sure IO
> > > have started since the second flush will wait for page lock which
> > > won't be unlocked unless setting page writeback and queuing ordered
> > > extents, we don't need %async_submit_draining, %async_delalloc_pages
> > > and %nr_async_submits to tell whether IO have actually started.
> > > 
> > > Moreover, all the flushers in use are followed by functions that wait
> > > for ordered extents to complete, so %nr_async_submits, which tracks
> > > whether bio's async submit has made progress, doesn't really make
> > > sense.
> > > 
> > > However, %async_delalloc_pages is still required by shrink_delalloc()
> > > as that function doesn't flush twice in the normal case (just issues a
> > > writeback with WB_REASON_FS_FREE_SPACE).
> > > 
> > > Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
> > 
> > Patches like this are almost impossible to review just from the code.
> > This has runtime implications that we'd need to observe in real on
> > various workloads.
> > 
> > So, the patches "look good", but I did not try to forsee all the
> > scenarios where threads interact through the counters. I think it should
> > be safe to add them to for-next and give enough time to test them.
> > 
> > The performance has varied wildly in the last cycle(s) so it's hard to
> > get a baseline, other than another major release. I do expect changes in
> > performance characteristics but still hope it'll be the same on average.
> 
> Testing appears normal, we get more weirdnes just by enabling the
> writeback throttling, but without that I haven't observed anything
> unusual. Patches go to the misc-next part, meaning I don't expect
> changes other than fixups, as separate patches. Thanks.

Thank you Dave, I'm interesting in that weirdness part, will look into
it.

Thanks,

-liubo