On Mon, 30 Aug 2010, Ted Ts'o wrote: > On Sun, Aug 29, 2010 at 11:11:26PM -0400, Bill Fink wrote: > > A 50% ext4 disk write performance regression was introduced > > in 2.6.32 and still exists in 2.6.35, although somewhat improved > > from 2.6.32. Read performance was not affected). > > Thanks for reporting it. I'm going to have to take a closer look at > why this makes a difference. I'm going to guess though that what's > going on is that we're posting writes in such a way that they're no > longer aligned or ending at the end of a RAID5 stripe, causing a > read-modify-write pass. That would easily explain the write > performance regression. I'm not sure I understand. How could calling or not calling ext4_num_dirty_pages() (unpatched versus patched 2.6.35 kernel) affect the write alignment? I was wondering if the locking being done in ext4_num_dirty_pages() could somehow be affecting the performance. I did notice from top that in the patched 2.6.35 kernel, the I/O wait time was generally in the 60-65% range, while in the unpatched 2.6.35 kernel, it was at a higher 75-80% range. However, I don't know if that's just a result of the lower performance, or a possible clue to its cause. > The interesting thing is that we don't actually do anything in > ext4_da_writepages() to assure that we are making our writes are > appropriate aligned and sized. We do pay attention to make sure they > are alligned correctly in the allocator, but _not_ in the writepages > code. So the fact that apparently things were well aligned in 2.6.32 > seems to be luck... (or maybe the writes are perfectly aligned in > 2.6.32; they're just much worse with 2.6.35, and with explicit > attention paid to the RAID stripe size, we could do even better :-) It was 2.6.31 that was good. The regression was in 2.6.32. And again how does the write alignment get modified simply by whether or not ext4_num_dirty_pages() is called? > If you could run blktraces on 2.6.32, 2.6.35 stock, and 2.6.35 with > your patch, that would be really helpful to confirm my hypothesis. Is > that something that wouldn't be too much trouble? I'd be glad to if you explain how one runs blktraces. -Thanks -Bill -- 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
