> Von: Ethan Wilson [ethan.wilson@xxxxxxxxxxxxx] > Gesendet: Donnerstag, 4. September 2014 23:12 > An: linux-raid@xxxxxxxxxxxxxxx > Betreff: Re: AW: RAID456 direct I/O write performance > > On 04/09/2014 18:30, Markus Stockhausen wrote: > > A perf record of the 1 writer test gives: > > > > 38.40% swapper [kernel.kallsyms] [k] default_idle > > 13.14% md0_raid5 [kernel.kallsyms] [k] _raw_spin_unlock_irqrestore > > 13.05% swapper [kernel.kallsyms] [k] tick_nohz_idle_enter > > 10.01% iot [raid456] [k] raid5_unplug > > 9.06% swapper [kernel.kallsyms] [k] tick_nohz_idle_exit > > 3.39% md0_raid5 [kernel.kallsyms] [k] __kernel_fpu_begin > > 1.67% md0_raid5 [xor] [k] xor_sse_2_pf64 > > 0.87% iot [kernel.kallsyms] [k] finish_task_switch > > > > I'm confused and clueless. Especially I cannot see where the > > 10% overhead in the source of raid5_unplug might come > > from? Any idea from someone with better insight? > > I am no kernel developer but I have read that the CPU time for serving > interrupts is often accounted to the random process which has the bad > luck to be running at the time the interrupt comes and steals the CPU. I > read this for top, htop etc, which have probably a different accounting > mechanism than perf, but maybe something similar happens here, because > _raw_spin_unlock_irqrestore at 13% looks too absurd to me. > In fact, probably as soon as the interrupts are re-enabled by > _raw_spin_unlock_irqrestore, the CPU often goes serving one interrupt > that was queued, and this is before the function > _raw_spin_unlock_irqrestore exits, so the time is really accounted there > and that's why it's so high. Thanks Ethan for the explanation. In between I found the reason for the slowness. RAID5 handles direct IO write requests sequentially with each one restarting the raid5d() worker thread (more lke sync IO). The raid1d() worker thread can batch those requests. Maybe Neil can tell if that behaviour is desired.
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