On Thu 21-04-11 12:57:17, Chris Mason wrote: > Excerpts from Jan Kara's message of 2011-04-21 12:55:29 -0400: > > On Thu 21-04-11 11:25:41, Chris Mason wrote: > > > Excerpts from Chris Mason's message of 2011-04-21 07:09:11 -0400: > > > > Excerpts from Vivek Goyal's message of 2011-04-20 18:06:26 -0400: > > > > > > > > > > > > In this case the 128s spent in write was on a single 4K overwrite on a > > > > > > 4K file. > > > > > > > > > > Chris, You seem to be doing 1MB (32768*32) writes on fsync file instead of 4K. > > > > > I changed the size to 4K still not much difference though. > > > > > > > > Whoops, I had that change made locally but didn't get it copied out. > > > > > > > > > > > > > > Once the program has exited because of high write time, i restarted it and > > > > > this time I don't see high write times. > > > > > > > > I see this for some of my runs as well. > > > > > > > > > > > > > > First run > > > > > --------- > > > > > # ./a.out > > > > > setting up random write file > > > > > done setting up random write file > > > > > starting fsync run > > > > > starting random io! > > > > > write time: 0.0006s fsync time: 0.3400s > > > > > write time: 63.3270s fsync time: 0.3760s > > > > > run done 2 fsyncs total, killing random writer > > > > > > > > > > Second run > > > > > ---------- > > > > > # ./a.out > > > > > starting fsync run > > > > > starting random io! > > > > > write time: 0.0006s fsync time: 0.5359s > > > > > write time: 0.0007s fsync time: 0.3559s > > > > > write time: 0.0009s fsync time: 0.3113s > > > > > write time: 0.0008s fsync time: 0.4336s > > > > > write time: 0.0009s fsync time: 0.3780s > > > > > write time: 0.0008s fsync time: 0.3114s > > > > > write time: 0.0009s fsync time: 0.3225s > > > > > write time: 0.0009s fsync time: 0.3891s > > > > > write time: 0.0009s fsync time: 0.4336s > > > > > write time: 0.0009s fsync time: 0.4225s > > > > > write time: 0.0009s fsync time: 0.4114s > > > > > write time: 0.0007s fsync time: 0.4004s > > > > > > > > > > Not sure why would that happen. > > > > > > > > > > I am wondering why pwrite/fsync process was throttled. It did not have any > > > > > pages in page cache and it shouldn't have hit the task dirty limits. Does that > > > > > mean per task dirty limit logic does not work or I am completely missing > > > > > the root cause of the problem. > > > > > > > > I haven't traced it to see. This test box only has 1GB of ram, so the > > > > dirty ratios can be very tight. > > > > > > Oh, I see now. The test program first creates the file with a big > > > streaming write. So the task doing the streaming writes gets nailed > > > with the per-task dirty accounting because it is making a ton of dirty > > > data. > > > > > > Then the task forks the random writer to do all the random IO. > > > > > > Then the original pid goes back to do the fsyncs on the new file. > > > > > > So, in the original run, we get stuffed into balance_dirty_pages because > > > the per-task limits show we've done a lot of dirties. > > > > > > In all later runs, the file already exists, so our fsyncing process > > > hasn't done much dirtying at all. Looks like the VM is doing something > > > sane, we just get nailed with big random IO. > > Ok, so there isn't a problem with fsync() as such if I understand it > > right. We just block tasks in balance_dirty_pages() for a *long* time > > because it takes long time to write out that dirty IO and we make it even > > worse by trying to writeout more on behalf of the throttled task. Am I > > right? The IO-less throttling will solve this regardless of patchset we > > choose so I wouldn't be too worried about the problem now. > > You're right. With one small exception, we probably do want to rotor > out of the random buffered writes in hopes of finding some sequential IO > even with the IO-less dirty throttling. Flusher thread should do this - it writes at most 1024 pages (as much as ->writepages call does) from the big file and then put inode to b_more_io queue and go on with the next dirty inode. So if there is some sequential IO as well, we should get to it... 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
