On Wed, Nov 30, 2011 at 11:30:01AM +0100, Arne Jansen wrote: > On 29.11.2011 22:47, Chris Mason wrote: > > On Tue, Nov 29, 2011 at 09:40:56PM +0100, Arne Jansen wrote: > >> Write bios are submitted from the submit_worker. The worker pumps down > >> bios into the block layer until it signals a congestion. At least this > >> is the theory. In pratice submit_bio just blocks before any signalling > >> happens. As the bios are queued per device, this can lead to a situation > >> where only one device is served until all bios are submitted, and only > >> then the next device is served. This is obviously suboptimal. > >> This patch just throws out the congestion detection and reschedules the > >> worker every 8 requests. This way, all devices can be kept busy. > >> This is only a temporary fix until the block layer provides a non-blocking > >> submit_bio. Then the whole submit_worker mechanism can be killed. > > > > The problem with the every 8 requests logic is that we've still got a > > pretty good chance of getting stuck behind get_request_wait. The way > > the elevator batching works is that it should give us a batch of > > requests, and once that batch is done we wait. > > > > If we jump around every 8 requests, we've turned this: > > > > [ dev A bio 1-8, dev A bio 8-16, dev A bio 16-32, dev B bio 1-8, dev B ... ] > > currently, it's more like > [ dev A bio 1 - 5000, dev B bio 1-5000 ] > > > > > into: > > > > [ dev A bio 1-8, dev B bio 1-8, dev A bio 8-16, dev B bio 8-16 ] > > so this is a great improvement :) ;) > > > > > They look like the same IO, but if we wait for a request when we do > > (dev B bio 1-8) then our dev A bio 1-8 bio is likely to dispatch without > > all the other dev A bios we had queued. > > > > As you said in IRC, we'd be better off with one thread per device or (my > > preference) with a real non-blocking submit_bio. What kind of results > > did you get with your test from bumping the nr_requests? > > what nr_requests do you mean? btrfs_async_submit_limit? /sys/block/xxx/queue/nr_requests -chris -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
