"High" cpu usage when using ZFS cache device

[Jeremy Chadwick]

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 |