On Tue, Jun 05, 2012 at 01:23:02PM -0400, Vivek Goyal wrote: > On Wed, May 30, 2012 at 11:21:29AM +0800, Fengguang Wu wrote: > > [..] > > (2) comes from the use of _WAIT_ flags in > > > > sync_file_range(..., SYNC_FILE_RANGE_WAIT_BEFORE|SYNC_FILE_RANGE_WRITE|SYNC_FILE_RANGE_WAIT_AFTER); > > > > Each sync_file_range() syscall will submit 8MB write IO and wait for > > completion. That means the async write IO queue constantly swing > > between 0 and 8MB fillness at the frequency (100MBps / 8MB = 12.5ms). > > So on every 12.5ms, the async IO queue runs empty, which gives any > > pending read IO (from firefox etc.) a chance to be serviced. Nice > > and sweet breaks! > > I doubt that async IO queue is empty for 12.5ms. We wait for previous > range to finish (index-1) and have already started the IO on next 8MB > of pages. So effectively that should keep 8MB of async IO in > queue (until and unless there are delays from user space side). So reason > for latency improvement might be something else and not because async > IO queue is empty for some time. With sync_file_range() test, we can have 8MB of IO in flight. Without that I think we can have more at times and that might be the reason for latency improvement. I see that CFQ has code to allow deeper NCQ depth if there is only a single writer. So once a reader comes along it might find tons of async IO already in flight. sync_file_range() will limit that in flight IO hence the latency improvement. So if we have multiple dd doing sync_file_range() then probably this latency improvement should go away. I will run some tests to verify if my understanding about deeper queue depths in case of single writer is correct or not. Thanks Vivek -- 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
