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Date: Wed, 10 Mar 2010 13:15:48 -0500 From: Christoph Hellwig <hch@...radead.org> To: Hans-Peter Jansen <hpj@...la.net> Cc: linux-kernel@...r.kernel.org Subject: Re: howto combat highly pathologic latencies on a server? On Wed, Mar 10, 2010 at 06:17:42PM +0100, Hans-Peter Jansen wrote: > While this system usually operates fine, it suffers from delays, that are > displayed in latencytop as: "Writing page to disk: 8425,5 ms": > ftp://urpla.net/lat-8.4sec.png, but we see them also in the 1.7-4.8 sec > range: ftp://urpla.net/lat-1.7sec.png, ftp://urpla.net/lat-2.9sec.png, > ftp://urpla.net/lat-4.6sec.png and ftp://urpla.net/lat-4.8sec.png. > > >From other observations, this issue "feels" like it is induced by single > syncronisation points in the block layer, eg. if I create heavy IO load on > one RAID array, say resizing a VMware disk image, it can take up to a > minute to log in by ssh, although the ssh login does not touch this area at > all (different RAID arrays). Note, that the latencytop snapshots above are > made during normal operation, not this kind of load.. I had very similar issues on various systems (mostly using xfs, but some with ext3, too) using kernels before ~ 2.6.30 when using the cfq I/O scheduler. Switching to noop fixed that for me, or upgrading to a recent kernel where cfq behaves better again. -- 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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