swap_pager complaints but not using swap
Kris Kennaway
kris at FreeBSD.org
Sun Jan 25 01:30:24 PST 2009
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
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