On Fri, Jun 25, 2010 at 01:03:20PM +0200, Christoph Hellwig wrote: > On Wed, Jun 23, 2010 at 09:44:20PM -0400, Vivek Goyal wrote: > > Let me explain the general idling logic and then see if it makes sense in case > > of WRITE_SYNC. > > > > Once a request has completed, if the cfq queue is empty, we have two choices. > > Either expire the cfq queue and move on to dispatch requests from a > > different queue or we idle on the queue hoping we will get more IO from > > same process/queue. > > queues are basically processes in this context? > > > Idling can help (on SATA disks with high seek cost), if > > our guess was right and soon we got another request from same process. We > > cut down on number of seeks hence increased throghput. > > I don't really understand the logic behind this. If we lots of I/O > that actually is close to each other we should generally submit it in > one batch. That is true for pagecache writeback, that is true for > metadata (at least in XFS..), and it's true for any sane application > doing O_DIRECT / O_SYNC style I/O. > > What workloads produde I/O that is local (not random) writes with small > delays between the I/O requests? Biggest thing is multiple small files operations like on the same directory. Best case I measured back when doing AS io scheduler versus deadline was about 100x improvement on a uncached kernel grep workload when competing with a streaming writeout (the writeout probably ended up going somewhat slower naturally, but it is fairer). -- 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
