On Thu, Apr 12, 2012 at 04:26:34PM +0200, Jan Kara wrote:
> Hi Fengguang,
>
> On Thu 12-04-12 10:20:40, Wu Fengguang wrote:
> > > > > > Note that plugging for O_SYNC writes is also removed. The user may pass
> > > > > > arbitrary @size arguments, which may be much larger than the preferable
> > > > > > I/O size, or may cross extent/device boundaries. Let the lower layers
> > > > > > handle the plugging. Otherwise the plugging code here will turn the
> > > > > > low level plugging into no-ops.
> > > > >
> > > > > I assume you have some numbers to back this up, right? Care to share
> > > > > those?
> > > >
> > > > Yes please.
> > > >
> > > > We've broken this stuff a few times recently - we should review and
> > > > test carefully.
> > >
> > > Yes sure. Last time I posted the patch, I did some tests and found no
> > > performance changes. Now for 3.3, the tests started days ago have not
> > > finished now (partly because it is stalled for quite long time due to
> > > unknown reason). The now-available numbers for bs=4k dd's look fine.
> > > The pending tests are for bs=1M dd's and some random fio workloads.
> > >
> >
> > The changes are basically small enough to be considered noises.
> > But anyway here are some interpretations:
> >
> > - application visible data write performance (write_bw) is almost the same
> > - it slightly reduces the real IOs that hit disk (io_wkB_s, io_rkB_s)
> > - disk utilization slightly increased
> > - CPU time is slightly reduced
> >
> Well, two of the throughput numbers stand out (in both directions
> actually) although they seem to be more extreme configurations so maybe it
> is a noise. But maybe it would deserve further check:
>
> > $ ./compare-io bay/*/*-{3.3.0,3.3.0-plug+}
> > 3.3.0 3.3.0-plug+
> > ------------------------ ------------------------
> ...
> > 2.60 +7.1% 2.78 bay/thresh=1M/btrfs-10dd-1-3.3.0
> > 3.72 -12.5% 3.25 bay/thresh=1M/btrfs-1dd-1-3.3.0
> ...
Yeah. In fact I noticed btrfs performing badly on the 1MB dirty
threshold cases comparing to other filesystems. thresh=10MB cases are
fine. It looks better to explore this first. My hunch is that fixing
this issue may also change its sensitiveness to the plugging.
3.3.0 3.3.0-plug+
------------------------ ------------------------
2.60 +7.1% 2.78 bay/thresh=1M/btrfs-10dd-1-3.3.0
3.72 -12.5% 3.25 bay/thresh=1M/btrfs-1dd-1-3.3.0
62.41 +0.3% 62.60 bay/thresh=1M/ext3-10dd-1-3.3.0
58.61 +0.4% 58.82 bay/thresh=1M/ext3-1dd-1-3.3.0
70.70 -1.1% 69.92 bay/thresh=1M/ext4-10dd-1-3.3.0
84.81 +0.1% 84.90 bay/thresh=1M/ext4-1dd-1-3.3.0
66.05 -0.1% 65.97 bay/thresh=1M/xfs-10dd-1-3.3.0
74.81 +0.1% 74.85 bay/thresh=1M/xfs-1dd-1-3.3.0
> I looked at other iostat numbers as well, but seeing quite some changes
> in both directions I'd say that those iostat numbers are too noisy to draw
> serious conclusion from them.
Agreed.
Thanks,
Fengguang
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