On Tue, 2009-02-03 at 13:11 +1100, Nick Piggin wrote: > On Tuesday 03 February 2009 12:54:26 Zhang, Yanmin wrote: > > On Tue, 2009-02-03 at 12:24 +1100, Nick Piggin wrote: > > > On Friday 30 January 2009 12:23:15 Jan Kara wrote: > > > > Hi, > > > > > > > > today I found that commit 31a12666d8f0c22235297e1c1575f82061480029 > > > > (mm: write_cache_pages cyclic fix) slows down operations over Berkeley > > > > DB. Without this "fix", I can add 100k entries in about 5 minutes 30s, > > > > with that change it takes about 20 minutes. > > > > What is IMO happening is that previously we scanned to the end of > > > > file, we left writeback_index at the end of file and went to write next > > > > file. With the fix, we wrap around (seek) and after writing some more > > > > we go to next file (seek again). > > > > We also found this commit causes about 40~50% regression with iozone > > mmap-rand-write. #iozone -B -r 4k -s 64k -s 512m -s 1200m > > > > My machine has 8GB memory. > > Ah, thanks. Yes BDB I believe is basically just doing an mmap-rand-write, > so maybe this is a good test case. > > The interesting thing is why is this causing such a slowdown. If there is > only a single main file active in the workload, then I don't see why this > patch should make such a big difference. In either case, wouldn't pdflush > come back and just start writing out from the start of the file anyway? Perhaps the difference is that without the patch, pdflush will return after running congestion_wait()? This would give bdb and iozone a chance to fill in more pages, and increases the chances we'll do sequential IO. -chris -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
