On Mon, 26 Jul 2010, Vivek Goyal wrote: > On Mon, Jul 26, 2010 at 05:53:40PM -0400, Mikulas Patocka wrote: > > Hi Jens > > > [ Jens's mail id has changed. Ccing him on different mail id ] > > > I found out that when I remove BIO_RW_SYNCIO from dm-kcopyd.c, performance > > when writing to the snapshot origin is increased twice. In this workload, > > snapshot driver reads a lot of 8k chunks from one place on the disk and > > writes them to the other place on the same disk. Without BIO_RW_SYNCIO, it > > is twice faster with CFQ scheduler. > > > > What does BIO_RW_SYNCIO exactly do? Does it attempt to decrease latency > > and decrease throughput? (so that disk head seeks more between the source > > and destination and it causes performance degradation) Is there some > > general guidance, when use BIO_RW_SYNCIO and when not? > > BIO_RW_SYNC marks a request as SYNC. reads are by default sync. Writes > can be marked as SYNC so that they get priority of ASYNC WRITES. > > Marking writes as SYNC has advantage that it gets its own queue, it can > preempt ASYNC WRITES and CFQ also idles on the cfq queue waiting for > more requests. > > I suspect if it is idling on SYNC WRITES which is a problem here. > > - Is it the same thread which is submitting both read and write requests? Yes. > - Are you interleaving reads and writes. Like reading some data, then > writting it and reading again... I issue write immediatelly when read finishes. > I guess after writting some data, CFQ is idling on WRITE_SYNC and next > READ does not get dispatched immediately to the disk. > > Is it possible to provide some 30 second blktrace of the underlying device > where CFQ is running. You can also try setting slice_idle to 0 and see if > it helps. > > If setting slice_idle=0 helps you, can you please also give a try to > following patch. > > https://patchwork.kernel.org/patch/113061/ > > Thanks > Vivek I took the traces and placed them at http://people.redhat.com/mpatocka/data/blktrace/ It shows that WRITE requests are merged without SYNCIO flags and are not merged if SYNCIO is used. slice_idle=0 or the patch don't help. BTW. blkparse ignores the input file, when I use "blkparse -i sda.blktrace.0.sync", blkparse still displays content of "sda.blktrace.0" and not "sda.blktrace.0.sync". I have to use "cat sda.blktrace.0.sync|blkparse -i -" to read th trace. Is it a blkparse bug? I use blkparse from Debian Lenny. Mikulas -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
