On Mon 08-06-09 14:28:34, Jens Axboe wrote: > On Mon, Jun 08 2009, Jan Kara wrote: > > On Mon 08-06-09 11:23:38, Jens Axboe wrote: > > > On Sat, Jun 06 2009, Frederic Weisbecker wrote: > > > > On Sat, Jun 06, 2009 at 02:23:40AM +0200, Jan Kara wrote: > > > > > On Fri 05-06-09 20:18:15, Chris Mason wrote: > > > > > > On Fri, Jun 05, 2009 at 11:14:38PM +0200, Jan Kara wrote: > > > > > > > On Fri 05-06-09 21:15:28, Jens Axboe wrote: > > > > > > > > On Fri, Jun 05 2009, Frederic Weisbecker wrote: > > > > > > > > > The result with noop is even more impressive. > > > > > > > > > > > > > > > > > > See: http://kernel.org/pub/linux/kernel/people/frederic/dbench-noop.pdf > > > > > > > > > > > > > > > > > > Also a comparison, noop with pdflush against noop with bdi writeback: > > > > > > > > > > > > > > > > > > http://kernel.org/pub/linux/kernel/people/frederic/dbench-noop-cmp.pdf > > > > > > > > > > > > > > > > OK, so things aren't exactly peachy here to begin with. It may not > > > > > > > > actually BE an issue, or at least now a new one, but that doesn't mean > > > > > > > > that we should not attempt to quantify the impact. > > > > > > > What looks interesting is also the overall throughput. With pdflush we > > > > > > > get to 2.5 MB/s + 26 MB/s while with per-bdi we get to 2.7 MB/s + 13 MB/s. > > > > > > > So per-bdi seems to be *more* fair but throughput suffers a lot (which > > > > > > > might be inevitable due to incurred seeks). > > > > > > > Frederic, how much does dbench achieve for you just on one partition > > > > > > > (test both consecutively if possible) with as many threads as have those > > > > > > > two dbench instances together? Thanks. > > > > > > > > > > > > Is the graph showing us dbench tput or disk tput? I'm assuming it is > > > > > > disk tput, so bdi may just be writing less? > > > > > Good, question. I was assuming dbench throughput :). > > > > > > > > > > Honza > > > > > > > > > > > > Yeah it's dbench. May be that's not the right tool to measure the writeback > > > > layer, even though dbench results are necessarily influenced by the writeback > > > > behaviour. > > > > > > > > May be I should use something else? > > > > > > > > Note that if you want I can put some surgicals trace_printk() > > > > in fs/fs-writeback.c > > > > > > FWIW, I ran a similar test here just now. CFQ was used, two partitions > > > on an (otherwise) idle drive. I used 30 clients per dbench and 600s > > > runtime. Results are nearly identical, both throughout the run and > > > total: > > > > > > /dev/sdb1 > > > Throughput 165.738 MB/sec 30 clients 30 procs max_latency=459.002 ms > > > > > > /dev/sdb2 > > > Throughput 165.773 MB/sec 30 clients 30 procs max_latency=607.198 ms > > Hmm, interesting. 165 MB/sec (in fact 330 MB/sec for that drive) sounds > > like quite a lot ;). This usually happens with dbench when the processes > > manage to delete / redirty data before writeback thread gets to them (so > > some IO happens in memory only and throughput is bound by the CPU / memory > > speed). So I think you are on a different part of the performance curve > > than Frederic. Probably you have to run with more threads so that dbench > > threads get throttled because of total amount of dirty data generated... > > Certainly, the actual disk data rate was consistenctly in the > 60-70MB/sec region. The issue is likely that the box has 6GB of RAM, if > I boot with less than 30 clients will do. Yes, that would do as well. > But unless the situation changes radically with memory pressure, it > still shows a fair distribution of IO between the two. Since they have > identical results throughout, it should be safe to assume that the have > equal bandwidth distribution at the disk end. A fast dbench run is one Yes, I agree. Your previous test indirectly shows fair distribution on the disk end (with blktrace you could actually confirm it directly). > that doesn't touch the disk at all, once you start touching disk you > lose :-) Honza -- Jan Kara <jack@xxxxxxx> SUSE Labs, CR -- 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
