On Wed, Jun 23, 2010 at 02:25:40PM +0800, Wu Fengguang wrote: > On Wed, Jun 23, 2010 at 02:03:19PM +0800, Dave Chinner wrote: > > On Wed, Jun 23, 2010 at 11:22:13AM +0800, Wu Fengguang wrote: > > > On Wed, Jun 23, 2010 at 11:06:04AM +0800, Dave Chinner wrote: > > > > On Wed, Jun 23, 2010 at 09:34:26AM +0800, Wu Fengguang wrote: > > > > > On Wed, Jun 23, 2010 at 06:45:51AM +0800, Dave Chinner wrote: > > > > > > By default we set QUEUE_FLAG_SAME_COMP, which means we hand > > > > > > completions back to the submitter CPU during blk_complete_request(). > > > > > > Completion processing is then handled by a softirq on the CPU > > > > > > selected for completion processing. > > > > > > > > > > Good to know about that, thanks! > > > > > > > > > > > This was done, IIRC, because it provided some OLTP benchmark 1-2% > > > > > > better results. It can, however, be turned off via > > > > > > /sys/block/<foo>/queue/rq_affinity, and there's no guarantee that > > > > > > the completion processing doesn't get handled off to some other CPU > > > > > > (e.g. via a workqueue) so we cannot rely on this completion > > > > > > behaviour to avoid cacheline bouncing. > > > > > > > > > > If rq_affinity does not work reliably somewhere in the IO completion > > > > > path, why not trying to fix it? > > > > > > > > Because completion on the submitter CPU is not ideal for high > > > > bandwidth buffered IO. > > > > > > Yes there may be heavy post-processing for read data, however for writes > > > it is mainly the pre-processing that costs CPU? > > > > Could be either - delayed allocation requires significant pre-processing > > for allocation. Avoiding this by using preallocation just > > moves the processing load to IO completion which needs to issue > > transactions to mark the region written. > > Good point, thanks. > > > > So perfect rq_affinity > > > should always benefit write IO? > > > > No, because the flusher thread gets to be CPU bound just writing > > pages, allocating blocks and submitting IO. It might take 5-10GB/s > > to get there (say a million dirty pages a second being processed by > > a single CPU), but that's the sort of storage subsystem XFS is > > capable of driving. IO completion time for such a workload is > > significant, too, so putting that on the same CPU as the flusher > > thread will slow things down by far more than gain from avoiding > > cacheline bouncing. > > So super fast storage is going to demand multiple flushers per bdi. > And once we run multiple flushers for one bdi, it will again be > beneficial to schedule IO completion to the flusher CPU :) Yes - that is where we want to get to with XFS. But we don't have multiple bdi-flusher thread support yet for any filesystem, so I think it will be a while before the we can ignore this issue... 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
