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 -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
