[OmniOS-discuss] strange io-pattern

[Tobias Oetiker]

Hi Henk,

Today Henk Langeveld wrote:

> There's a known problem with iostat -xn  on multi-processor systems that I
> posted on the illumos-list
> back in September/October, where we occasionally see an astronomical spike in
> the io wait and service times.
>
> This appears to be caused by the hires kernel timer used by the kstat_io
> routines, which produces increasing
> values of timestamps *per* *physical* *cpu*.  When io events are handled by
> different cpus, the delta_t can
> become negative, as the result of a 64bit int underflow.
>
> These occurrences are rare, but frequent enough to mess up those wait times.
> Also, the wait times only show
> up with the combined '-x' and '-n'  options.
>
> Can you eliminate the possibility of such an incident?
>
> I intended to post a bug report on this, but I've moved on since then, and
> don't have access to
> any multi-cpu hardware right now.  I *think* I've seen it once in a multi-cpu
> virtualbox instance, but have not
> been able to reproduce that.  (This would suggest that virtualbox actually
> emulates the physical cpu registers.)

have a look at the graphs attached to my original post, its not a
spiking problem I think ...

cheers
tobi

>
> Cheers,
> Henk
>
>
> On 13/12/2013 14:13, Tobias Oetiker wrote:
> > I created a little plugin for collectd to interface with iostat. I
> > guess having one for vfsstat and arcstat along the same lines would
> > give a better picture as to what users actually experience but this
> > one gives some impression as to what happens deep down.
> >
> > #!/usr/bin/perl
> > my $filter = $ARGV[0] || '.+';
> >
> > my $pid = open my $iostat, "-|",
> > "/usr/bin/iostat","-Tu","-xnr",int($ENV{COLLECTD_INTERVAL}) or die
> > "launching iostat: $!";
> >
>
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>

-- 
Tobi Oetiker, OETIKER+PARTNER AG, Aarweg 15 CH-4600 Olten, Switzerland
http://it.oetiker.ch tobi at oetiker.ch ++41 62 775 9902 / sb: -9900