Date: Tue, 16 Nov 2010 13:55:57 +0100 From: Alexander Leidinger <Alexander@Leidinger.net> To: Christer Solskogen <christer.solskogen@gmail.com> Cc: freebsd-stable@freebsd.org, Ivan Voras <ivoras@freebsd.org> Subject: Re: "High" cpu usage when using ZFS cache device Message-ID: <20101116135557.66601irt6kd1gndw@webmail.leidinger.net> In-Reply-To: <AANLkTi=tJ-Hf%2BrMqG0=tEzNV2jJW-B_7Yu_ftW4tAMqT@mail.gmail.com> References: <AANLkTinzwyhABYxzWknzRFzLCbcDSd3BU2kQ5tX_SSk-@mail.gmail.com> <20101116003029.GC79816@numachi.com> <AANLkTinfTgXzf7t3PtO2VAef7NSkKWc0RnGdpv=6_-Vj@mail.gmail.com> <ibtqvp$bfq$1@dough.gmane.org> <AANLkTi=tJ-Hf%2BrMqG0=tEzNV2jJW-B_7Yu_ftW4tAMqT@mail.gmail.com>
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Quoting Christer Solskogen <christer.solskogen@gmail.com> (from Tue,
16 Nov 2010 13:15:32 +0100):
> On Tue, Nov 16, 2010 at 12:47 PM, Ivan Voras <ivoras@freebsd.org> wrote:
>
>> You can easily test it - use the stick as a simple disk device with UFS and
>> see how much CPU does it take simply to talk to the device.
>
> See, that is why I think it is a ZFS issue. Because I did that.
> I created a UFS filesystem on the same usb stick. Mounted it and did a
> "dd if=/dev/zero of=/mnt/file".
> The systemload goes +0.6 instead if +10.3.
>
> See:
> CPU: 0.0% user, 0.0% nice, 0.6% system, 0.0% interrupt, 99.3% idle
> Mem: 832M Active, 960M Inact, 7017M Wired, 2600K Cache, 1237M Buf, 3063M Free
> Swap: 8192M Total, 8192M Free
>
> PID USERNAME THR PRI NICE SIZE RES STATE C TIME WCPU COMMAND
> 38261 root 1 46 0 5776K 1112K wdrain 7 0:07 4.98% dd
>
> But when using it as cache device for zfs:
>
> CPU: 0.0% user, 0.0% nice, 11.9% system, 0.0% interrupt, 88.1% idle
> Mem: 832M Active, 193M Inact, 5782M Wired, 2592K Cache, 1237M Buf, 5066M Free
> Swap: 8192M Total, 8192M Free
>
> The funny thing is that when I add the device (and some cache is added
> to it) the load is normal. But the load goes up when nothing is
> written to it (or beeing read from it)
How do you measure that nothing is read or written to it?
Please check with
gstat -f '^<DEVICE>$'
if there are really no reads/writes to the device (please replace
<DEVICE> with the name of your USB device, e.g. da0).
If you see writes, I would say
- this is the reason for the load
- your cache is on the way to be filled with
useful data
If gstat shows zero activity, I suggest to run 'top -S' and look at
the process(es) which consume about 10% CPU (do not take care about
the idle process). Based upon this we can maybe suggest further things
to investigate.
Bye,
Alexander.
--
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