Messages in this thread | | | Subject | Re: question about IO-sched | Date | Sun, 15 Jul 2012 15:08:12 +0800 | From | gaoqiang <> |
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many thanks. but why the sys_read operation hangs on sync_page ? there are still many free memory.I mean ,the actually free memory,excluding the various kinds of caches or buffers.
在 Fri, 13 Jul 2012 22:15:31 +0800,Corrado Zoccolo <czoccolo@gmail.com> 写道:
> Hi, > the catch is that writes are "fire and forget", so they keep accumulating > in the I/O sched, and there is always plenty of them to schedule (unless > you explicitly make sync writes). > > The reader, instead, waits for the result of each read operation before > scheduling a new read, so there is at most one outstanding read, and some > time nothing. > > The deadline scheduler is work conserving, meaning that it never leaves > the > disk idle when there is work queued, and most of the time after an > operation completes, there is only write work queued, so you see much > more > writes being sent to the device. > > Only schedulers that delay writes waiting for reads (as Anticipatory in > old > kernels, and now CFQ) can achieve higher read to write ratios. > > Cheers > Corrado > > On Thu, Jul 12, 2012 at 11:01 AM, gaoqiang <gaoqiangscut@gmail.com> > wrote: > >> Hi,all >> >> I have long known that deadline is read-prefered. but a simple >> test gives the opposite result. >> >> with two processes running at the same time,one for read and one >> for write.actually,they did nothing bug IO operation. >> while(true) >> { >> read(); >> } >> the other: >> while(true) >> { >> write(); >> } >> >> with deadline IO-sched and ext4 filesystem.as a result, read >> ratio was about below 3M/s.and write about 100M/s. I have tested both >> kernel-2.6.18 and kernel-2.6.32,getting the same result. >> >> I add some debug information in the kernel and recompile,found >> that,it has little to do with IO-sched layer because read request >> dropped >> into deadline was 5% of write request .from /proc/<pid>/stack,the read >> process hands on sync_page most of the time. >> what is the matter ? anyone help me ? >> -- >> To unsubscribe from this list: send the line "unsubscribe linux-kernel" >> in >> the body of a message to majordomo@vger.kernel.org >> More majordomo info at >> http://vger.kernel.org/**majordomo-info.html<http://vger.kernel.org/majordomo-info.html> >> Please read the FAQ at http://www.tux.org/lkml/ >> > > >
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