On Tue, Jan 16 2007, Jeff Garzik wrote: > Mark Hahn wrote: > >>>>I though that NCQ was intended to increase performance ?? > > > >intended to increase _sales_ performance ;) > > Yep. > > > >remember that you've always had command queueing (kernel elevator): the > >main difference with NCQ (or SCSI tagged queueing) is when > >the disk can out-schedule the kernel. afaikt, this means sqeezing > >in a rotationally intermediate request along the way. > > > >that intermediate request must be fairly small and should be a read > >(for head-settling reasons). > > > >I wonder how often this happens in the real world, given the relatively > >small queues the disk has to work with. > > ISTR either Jens or Andrew ran some numbers, and found that there was > little utility beyond 4 or 8 tags or so. It entirely depends on the access pattern. For truly random reads, performance does seem to continue to scale up with increasing drive queue depths. It may only be a benchmark figure though, as truly random read workloads probably aren't that common :-) For anything else, going beyond 4 tags doesn't improve much. > >>My hdparm test is a sequential read-ahead test, so it will > >>naturally perform worse on a Raptor when NCQ is on. > > > >that's a surprisingly naive heuristic, especially since NCQ is concerned > >with just a max of ~4MB of reads, only a smallish > >fraction of the available cache. > > NCQ mainly helps with multiple threads doing reads. Writes are largely > asynchronous to the user already (except for fsync-style writes). You > want to be able to stuff the disk's internal elevator with as many read > requests as possible, because reads are very often synchronous -- most > apps (1) read a block, (2) do something, (3) goto step #1. The kernel's > elevator isn't much use in these cases. Au contraire, this is one of the cases where intelligent IO scheduling in the kernel makes a ton of difference. It's the primary reason that AS and CFQ are able to maintain > 90% of disk bandwidth for more than one process, idling the drive for the duration of step 2 in the sequence above (step 2 is typically really small, time wise). If the next block read is close to the first one, that is. If you do that, you will greatly outperform the same workload pushed to the drive scheduling. I've done considerable benchmarks on this. Only if the processes are doing random IO should the IO scheduler punt and push everything to the drive queue. -- Jens Axboe - To unsubscribe from this list: send the line "unsubscribe linux-ide" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
