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Date: Tue, 15 Jan 2008 10:21:08 +0100
From: Kris Kennaway <kris@FreeBSD.org>
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Subject: Re: Swapping caused by very large (regular) file size
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Thomas Hurst wrote:
> * John Baldwin (jhb@freebsd.org) wrote:
> 
>> We have noticed an issue at work but only on faster controllers (e.g.
>> certain mfi(4) drive configurations) when doing I/O to a single file
>> like the dd command mentioned causes the buffer cache to fill up.  The
>> problem being that we can't lock the vm object to recycle pages when
>> we hit the limit that is supposed to prevent this because all the
>> pages in the cache are for the file (vm object) we are working on.
>> Stephan (ups@) says this is fixed in 7.
> 
> Excellent.  I've been seeing this behavior for a long time, mostly on
> backup runs (RAID-1 amr SATA -> 1 disk Marvell ata).  It's pretty odd
> seeing a system with 8G of memory, 60% of which is just cache, swap out
> half a dozen things for no apparant reason.  And to think, people on
> FreeNode ##freebsd just insisted I had a misconfigured system ;)
> 
> My other IO related issue with 6 is IO to independent amr arrays
> blocking each other; e.g. daily run's find over amrd0 will invariably
> cause reads from amrd1 and amrd2 to freeze for seconds at a time (pr
> 114438).  I'll be very interested to see if 7 fixes that.

Sounds like it might be due to Giant contention, which I think is indeed 
fixed in 7.  Can you try it?

Kris