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Date: Sun, 25 Jan 2009 09:30:22 +0000
From: Kris Kennaway <kris@FreeBSD.org>
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To: Dieter <freebsd@sopwith.solgatos.com>
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Cc: freebsd-questions@FreeBSD.org
Subject: Re: swap_pager complaints but not using swap
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Dieter wrote:
>>> AMD64  FreeBSD 7.0  2 GiB main memory
>>>
>>> My console says:
>>>
>>> login: swap_pager: indefinite wait buffer: bufobj: 0, blkno: 22, size: 4096
>>> swap_pager: indefinite wait buffer: bufobj: 0, blkno: 22, size: 4096
>>> swap_pager: indefinite wait buffer: bufobj: 0, blkno: 22, size: 4096
>>> swap_pager: indefinite wait buffer: bufobj: 0, blkno: 22, size: 4096
>>>
>>> pstat -sk
>>> Device          1K-blocks     Used    Avail Capacity
>>> /dev/ad6s10       4590208       96  4590112     0%
>>>
>>> Wow, using a whole 96K of swap.  I don't see any disk related
>>> complaints in dmesg.
>>>
>>> Is this something to worry about?
>> Yes, the system was *trying* to do swap I/O and timing out while doing so.
>>
>> Kris
> 
> Whoops, I forgot to change the subject line after adding the k option
> to pstat.  Without the k it said 0 used.  And this morning it occurs to
> me that even if swap used was zero, it could have been trying to *start*
> using swap.
> 
> Anyway... given this timeout explaination, I'm guessing that page/swap
> has to compete with user processes for disk i/o, and thus probably
> suffers from the same lack of fair i/o scheduling that user processes
> suffer from.  E.g. one process doing disk i/o can lock out another
> process for at least several minutes, probably indefinitely.  :-(

There is a timeout of (from memory) 60 seconds.  I've not seen this 
timeout exceeded on properly functioning disk hardware (even heavily 
loaded), only on broken hardware/controllers, or on I/O devices that are 
intrinsically slow for some reason (USB stick, or swapping to a file).

Unless you're doing something truly unspeakable to that disk's load, I'd 
look at the hardware.

Kris