On Wed, Dec 12, 2012 at 11:26:17AM +0100, Jan Kara wrote: > On Wed 12-12-12 15:18:21, Dave Chinner wrote: > > On Wed, Dec 12, 2012 at 03:31:37AM +0100, Jan Kara wrote: > > > On Tue 11-12-12 16:44:15, Jeff Moyer wrote: > > > > Jan Kara <jack@xxxxxxx> writes: > > > > > > > > > Hi, > > > > > > > > > > I was looking into IO starvation problems where streaming sync writes (in > > > > > my case from kjournald but DIO would look the same) starve reads. This is > > > > > because reads happen in small chunks and until a request completes we don't > > > > > start reading further (reader reads lots of small files) while writers have > > > > > plenty of big requests to submit. Both processes end up fighting for IO > > > > > requests and writer writes nr_batching 512 KB requests while reader reads > > > > > just one 4 KB request or so. Here the effect is magnified by the fact that > > > > > the drive has relatively big queue depth so it usually takes longer than > > > > > BLK_BATCH_TIME to complete the read request. The net result is it takes > > > > > close to two minutes to read files that can be read under a second without > > > > > writer load. Without the big drive's queue depth, results are not ideal but > > > > > they are bearable - it takes about 20 seconds to do the reading. And for > > > > > comparison, when writer and reader are not competing for IO requests (as it > > > > > happens when writes are submitted as async), it takes about 2 seconds to > > > > > complete reading. > > > > > > > > > > Simple reproducer is: > > > > > > > > > > echo 3 >/proc/sys/vm/drop_caches > > > > > dd if=/dev/zero of=/tmp/f bs=1M count=10000 & > > > > > sleep 30 > > > > > time cat /etc/* 2>&1 >/dev/null > > > > > killall dd > > > > > rm /tmp/f > > > > > > > > This is a buffered writer. How does it end up that you are doing all > > > > synchronous write I/O? Also, you forgot to mention what file system you > > > > were using, and which I/O scheduler. > > > So IO scheduler is CFQ, filesystem is ext3 - which is the culprit why IO > > > ends up being synchronous - in ext3 in data=ordered mode kjournald often ends > > > up submitting all the data to disk and it can do it as WRITE_SYNC if someone is > > > waiting for transaction commit. In theory this can happen with AIO DIO > > > writes or someone running fsync on a big file as well. Although when I > > > tried this now, I wasn't able to create as big problem as kjournald does > > > (a kernel thread submitting huge linked list of buffer heads in a tight loop > > > is hard to beat ;). Hum, so maybe just adding some workaround in kjournald > > > so that it's not as aggressive will solve the real world cases as well... > > > > Maybe kjournald shouldn't be using WRITE_SYNC for those buffers? I > > mean, if there is that many of them then it's really a batch > > submission an dthe latency of a single buffer IO is really > > irrelevant to the rate at which the buffers are flushed to disk.... > Yeah, the idea why kjournald uses WRITE_SYNC is that we know someone is > waiting for transaction commit and that's pretty much definition of what > WRITE_SYNC means. Well, XFS only uses WRITE_SYNC for WB_SYNC_ALL writeback, which means only when a user is waiting on the wdata writeback will it use WRITE_SYNC. I'm really not sure what category journal flushes fall into, because XFS doesn't do data writeback from journal flushes.... > Hum, maybe if DIO wasn't using WRITE_SYNC (one could make similar > argument there as with kjournald). But then the definition of what > WRITE_SYNC should mean starts to be pretty foggy. DIO used WRITE_ODIRECT, not WRITE_SYNC. The difference is that WRITE_SYNC sets REQ_NOIDLE, so DIO is actually different to WRITE_SYNC behaviour for CFQ... Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx -- 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
