Messages in this thread | | | Subject | Re: [PATCH 1/2]block cfq: make queue preempt work for queues from different workload | From | Shaohua Li <> | Date | Tue, 18 Jan 2011 08:55:13 +0800 |
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On Mon, 2011-01-17 at 22:26 +0800, Vivek Goyal wrote: > On Tue, Jan 11, 2011 at 04:51:56PM +0800, Shaohua Li wrote: > > I got this: > > fio-874 [007] 2157.724514: 8,32 m N cfq874 preempt > > fio-874 [007] 2157.724519: 8,32 m N cfq830 slice expired t=1 > > fio-874 [007] 2157.724520: 8,32 m N cfq830 sl_used=1 disp=0 charge=1 iops=0 sect=0 > > fio-874 [007] 2157.724521: 8,32 m N cfq830 set_active wl_prio:0 wl_type:0 > > fio-874 [007] 2157.724522: 8,32 m N cfq830 Not idling. st->count:1 > > cfq830 is an async queue, and preempted by a sync queue cfq874. But since we > > have cfqg->saved_workload_slice mechanism, the preempt is a nop. > > Looks currently our preempt is totally broken if the two queues are not from > > the same workload type. > > Below patch fixes it. This will might make async queue starvation, but it's > > what our old code does before cgroup is added. > > I am not sure how good a idea this is. So effectively now if there is a > preemtion across workload, the workload being preempted can lose its > share completely and be starved. So it is not just about async WRITES. > sync-noidle can starve sync-idle workload too as meta data request will > preempt any regular sync-idle queue and then sync-idle queue workload > can be starved. So this is not exactly going back to old CFQ behavior > but something more than that too. > > On one hand you are going to great lengths to fix the issues where if a > cfqq is preempted, it does not lose its share (patch 2 in the series) and > on the other hand you don't mind all the queues in a workload losing their > share due to preemption. > > I think atleast we should limit this to async workload. Or solve the > problem by giving even smaller slice length to async workload. I haven't too much idea about this, but disabling preempt might harm performance even for sync too, for example, the cfq_rq_close() case. can we do something like this: sync preempts async: expire the whole workload slice sync queue A preempt sync queue B: the workload of queue A will immediately expire after queue A is expired.
Thanks, Shaohua
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