On Sat, Oct 03, 2009 at 03:21:15PM +0200, Jens Axboe wrote: > On Sat, Oct 03 2009, Vivek Goyal wrote: > > On Sat, Oct 03, 2009 at 07:29:15AM -0400, Vivek Goyal wrote: > > > On Sat, Oct 03, 2009 at 07:56:18AM +0200, Mike Galbraith wrote: > > > > On Sat, 2009-10-03 at 07:49 +0200, Mike Galbraith wrote: > > > > > On Fri, 2009-10-02 at 20:19 +0200, Jens Axboe wrote: > > > > > > > > > > > If you could do a cleaned up version of your overload patch based on > > > > > > this: > > > > > > > > > > > > http://git.kernel.dk/?p=linux-2.6-block.git;a=commit;h=1d2235152dc745c6d94bedb550fea84cffdbf768 > > > > > > > > > > > > then lets take it from there. > > > > > > > > > > > Note to self: build the darn thing after last minute changes. > > > > > > > > Block: Delay overloading of CFQ queues to improve read latency. > > > > > > > > Introduce a delay maximum dispatch timestamp, and stamp it when: > > > > 1. we encounter a known seeky or possibly new sync IO queue. > > > > 2. the current queue may go idle and we're draining async IO. > > > > 3. we have sync IO in flight and are servicing an async queue. > > > > 4 we are not the sole user of disk. > > > > Disallow exceeding quantum if any of these events have occurred recently. > > > > > > > > > > So it looks like primarily the issue seems to be that we done lot of > > > dispatch from async queue and if some sync queue comes in now, it will > > > experience latencies. > > > > > > For a ongoing seeky sync queue issue will be solved up to some extent > > > because previously we did not choose to idle for that queue now we will > > > idle, hence async queue will not get a chance to overload the dispatch > > > queue. > > > > > > For the sync queues where we choose not to enable idle, we still will see > > > the latencies. Instead of time stamping on all the above events, can we > > > just keep track of last sync request completed in the system and don't > > > allow async queue to flood/overload the dispatch queue with-in certain > > > time limit of that last sync request completion. This just gives a buffer > > > period to that sync queue to come back and submit more requests and > > > still not suffer large latencies? > > > > > > Thanks > > > Vivek > > > > > > > Hi Mike, > > > > Following is a quick hack patch for the above idea. It is just compile and > > boot tested. Can you please see if it helps in your scenario. > > > > Thanks > > Vivek > > > > > > o Do not allow more than max_dispatch requests from an async queue, if some > > sync request has finished recently. This is in the hope that sync activity > > is still going on in the system and we might receive a sync request soon. > > Most likely from a sync queue which finished a request and we did not enable > > idling on it. > > This is pretty much identical to the scheme I described, except for the > ramping of queue depth. I've applied it, it's nice and simple and I > believe this will get rid of the worst of the problem. > > Things probably end up being a bit simplistic, but we can always tweak > around later. I have kept the overload delay period as "cfq_slice_sync" same as Mike had done. We shall have to experiment what is a good waiting perioed. Is 100ms too long if we are waiting for a request from same process which recently finished IO and we did not enable idle on it. I guess we can tweak the delay period as we move along. Thanks Vivek -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
