Messages in this thread | | | Date | Wed, 24 Aug 2011 10:06:38 +0800 | Subject | Re: [PATCH 1/1] CFQ: fix handling 'deep' cfqq | From | Shaohua Li <> |
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2011/8/23 Maxim Patlasov <maxim.patlasov@gmail.com>: > Hi, > >>> An aproach suggested here avoids performance degradations mentioned above. >>> With this patch applied, the performance on slow hdd is the same as it used >>> to be before 8e1ac6655104bc6e1e79d67e2df88cc8fa9b6e07, and, on fast h/w-raids, >>> it's roughly the same as for noop scheduler or CFQ with slice_idle=0. >> idle is a usual cause of cfq performance issue. did you have test in >> disk without NCQ? > > Yes, on the node with "slow hdd" CFQ detected it as hw_tag=0. I > explained what I tested in cover message (subj: [PATCH 0/1] CFQ: > fixing performance issues). > >> And did you test if this will hurt the performance of Vivek's original problem? > > No. What's Vivek's original problem? > >> snip >>> + if (cfq_cfqq_deep_early(cfqq) && cfqq->n_dispatched >= CFQQ_DEEP_THR) { >>> + if (cfqq->first_dispatch == jiffies) >>> + cfqd->cfq_disk_looks_fast++; >>> + else >>> + cfqd->cfq_disk_looks_slow++; >>> + >> jiffies is too coarse here. A disk with NCQ can dispatch several >> requests within one jiffy. > > If a disk with NCQ dispatches four requests in raw within one jiffy > regularly, the patch I suggested will claim it as "fast enough". It > should be beneficial to disable idling for deep&seeky cfqq in this > case, imho. Anyway, existing code: > >> /* >> * This is a deep seek queue, but the device is much faster than >> * the queue can deliver, don't idle >> **/ >> if (CFQQ_SEEKY(cfqq) && cfq_cfqq_idle_window(cfqq) && >> (cfq_cfqq_slice_new(cfqq) || >> (cfqq->slice_end - jiffies > jiffies - cfqq->slice_start))) { >> cfq_clear_cfqq_deep(cfqq); >> cfq_clear_cfqq_idle_window(cfqq); >> } >> > > would surely disable idling in this case. So, the patch I suggested > doesn't make things worse (as compared with existing implementation). but the cfq_disk_looks_slow isn't updated if the queue doesn't have 4 requests or doesn't dispatch > 4 requests, so you always have CFQD_DISK_LOOKS_FAST() return true if the first slice gets it to true. And if the queue does dispatch > 4 requests in one jiffy, only cfq_disk_looks_fast is updated, CFQD_DISK_LOOKS_FAST returns true too. I don't understand when CFQD_DISK_LOOKS_FAST can be false.
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