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Date: Fri, 11 Jul 2008 17:17:51 +0200 From: Martin Sustrik <sustrik@...tmq.com> To: Alan Cox <alan@...rguk.ukuu.org.uk> CC: Andrew Morton <akpm@...ux-foundation.org>, Martin Lucina <mato@...elna.sk>, linux-kernel@...r.kernel.org Subject: Re: Higher than expected disk write(2) latency Hi all, > Which disk scheduler are you using - some of the disk schedulers > intentionally delay writes to try and get better block merging. We've run the test with different I/O schedulers. We've found out that poor performance we've seen before was due to the short AIO queue size. Now, when queue size is big enough to hold all the write requests in the test, the results are much better. Have a look here for the code of the test and the results in form of graphs: http://www.zeromq.org/results:aio There are still at least two problems I see: 1. Enqueueing of writes seems to block every now and then although the size of AIO queue is large enough to hold all the requests in the test. 2. We've observed that messages are batched for writes by approximately 30 per batch. The latency impact of messages 2-30 is almost zero, however, the impact of the first message is much higher than expected. I would expect latency of one disk revolving (8.3 ms) + optionally latency of head movement (say 10 us). What we are seing are latencies of 30-60 ms. Martin -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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