Re: ext4 data=writeback performs worse than data=ordered now

Wu Fengguang
2011-12-14T23:48:23Z

On Wed, Dec 14, 2011 at 10:52:00PM +0800, Tao Ma wrote:
> Hi Ted/Fengguang,
> On 12/14/2011 10:30 PM, Ted Ts'o wrote:
> > On Wed, Dec 14, 2011 at 09:34:00PM +0800, Wu Fengguang wrote:
> >> Hi,
> >>
> >> Shaohua recently found that ext4 writeback mode could perform worse
> >> than ordered mode in some cases. It may not be a big problem, however
> >> we'd like to share some information on our findings.
> >>
> >> I tested both 3.2 and 3.1 kernels on normal SATA disks and USB key.
> >> The interesting thing is, data=writeback used to run a bit faster
> >> than data=ordered, however situation get inverted presumably by the
> >> IO-less dirty throttling.
> >
> > Interesting. What sort of workloads are you using to do these
> > measurements? How many writer threads; I assume you are doing
> > sequential writes which are extending one or more files, etc?
> >
> > I suspect it's due to the throttling meaning that each thread is
> > getting to send less data to the disk, and so there is more seeking
> > going on with data=writeback, where as with data=ordered, at each
> > journal commit we are forcing all of the dirty pages out to disk, one
> > inode at a time, and this is resulting in a more efficient writeback
> > compared to when the writeback code is getting to make its own choices
> > about how much each inode gets to write out at at time.
> >
> > It would be interesting to see what would happen if in
> > ext4_da_writepages(), we completely ignore how many pages are
> > requested to be written back by the writeback code, and just simply
> > write back all of the dirty pages, and see if that brings the
> > performance back.
> I guess fengguang's test is a buffer write dd test. Here we have found
> some performance regression from 18 because of the delayed allocation.
> In case of delayed allocation, we will create the extent tree during
> writepages which would delay the write because ext4_da_write_begin would
> down_read the i_data_sem to map the block while writepages would
> down_write it so we have seen some severe delay in ext4_da_write_begin
> (around 3s). And instead of increasing the page numbers of every
> writepages, some tests shows that the decrease makes the performance
> increase. I will dive into it soon to see what's going on there.
>
> So Fengguang, would you please keep the page number in
> ext4_da_writepages passed by writeback(instead of the bumping) and check
> the result?

Sure, can you provide a patch for me to test?

Thanks,
Fengguang