"High" cpu usage when using ZFS cache device
Jeremy Chadwick
freebsd at jdc.parodius.com
Tue Nov 16 12:54:34 UTC 2010
On Tue, Nov 16, 2010 at 01:15:32PM +0100, Christer Solskogen wrote:
> On Tue, Nov 16, 2010 at 12:47 PM, Ivan Voras <ivoras at freebsd.org> wrote:
>
> > You can easily test it - use the stick as a simple disk device with UFS and
> > see how much CPU does it take simply to talk to the device.
>
> See, that is why I think it is a ZFS issue. Because I did that.
> I created a UFS filesystem on the same usb stick. Mounted it and did a
> "dd if=/dev/zero of=/mnt/file".
> The systemload goes +0.6 instead if +10.3.
>
> See:
> CPU: 0.0% user, 0.0% nice, 0.6% system, 0.0% interrupt, 99.3% idle
> Mem: 832M Active, 960M Inact, 7017M Wired, 2600K Cache, 1237M Buf, 3063M Free
> Swap: 8192M Total, 8192M Free
>
> PID USERNAME THR PRI NICE SIZE RES STATE C TIME WCPU COMMAND
> 38261 root 1 46 0 5776K 1112K wdrain 7 0:07 4.98% dd
>
> But when using it as cache device for zfs:
>
> CPU: 0.0% user, 0.0% nice, 11.9% system, 0.0% interrupt, 88.1% idle
> Mem: 832M Active, 193M Inact, 5782M Wired, 2592K Cache, 1237M Buf, 5066M Free
> Swap: 8192M Total, 8192M Free
>
> The funny thing is that when I add the device (and some cache is added
> to it) the load is normal. But the load goes up when nothing is
> written to it (or beeing read from it)
Since you're running 8.1-RELEASE, can you please test this issue on
RELENG_8 (8.1-STABLE) and see if it exists there?
You can download a livefs snapshot or equivalent and test via that
(preferably one which has ZFS v15 support; you'll need to make a new
pool rather than upgrade your existing pool, unless you plan on moving
to RELENG_8 permanently). Here's such a snapshot:
ftp://ftp4.freebsd.org/pub/FreeBSD/snapshots/201011/
--
| Jeremy Chadwick jdc at parodius.com |
| Parodius Networking http://www.parodius.com/ |
| UNIX Systems Administrator Mountain View, CA, USA |
| Making life hard for others since 1977. PGP: 4BD6C0CB |
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