Re: [Xen-devel] Runaway real/sys time in newer paravirt domUs?

[Jed Smith <jed@xxxxxxxxxx>]

On Jul 9, 2010, at 4:46 AM, Jan Beulich wrote:

> Is this perhaps also dependent on the CPU make/model?

I have a mostly-homogenous environment, but I'll see what I can do.  I have one
box in mind, actually...

Since I spoke with Jeremy yesterday, I've become suspicious that the issue I am
reporting here is likely the same that I've seen for some time.  A quick
background: In our domU environment, Munin incorrectly reports idle time.  All
Linode domUs have 4 VCPUs, so user/sys travel freely to 400%.  Idle, however,
has always reported 800% under paravirtualized kernels.  This led me to
investigate a bit, and this is what I deduced:

 - In /proc/uptime, idle time outruns system uptime significantly.

 - This ratio is seemingly affected by the number of VCPUs the domU is
   configured for.  With only one VCPU, the ratio is roughly 2.0, leading me to
   think that idle ticks are counted twice per-VCPU.

 - The ratio between idle/system is inconclusive after a lengthy uptime.

 - In a 2.6.29 domU, two things happen:

  - a) The original bug does not manifest after 50 attempts.

  - b) In /proc/uptime, idle time is always precisely 0.00.  It never counts.

 - Between 2.6.29 and 2.6.30, /proc/uptime behaves much differently, and the bug
   then exposes itself.  Something changed there.

I have seen the /proc/uptime behavior on i386.  In fact, a personal domU:

   10:51 jsmith@undertow% cat /proc/uptime
   1984022.43 2954870.51

   10:51 jsmith@undertow% awk '{printf("%f\n", $2 / $1)}' /proc/uptime
   1.489342

   10:51 jsmith@undertow% uname -a
   Linux undertow.jedsmith.org 2.6.32.12-linode25 #1 SMP Wed Apr 28 19:25:11 
UTC 2010 i686 GNU/Linux

I am not sure if the fact that I can only make the original bug appear on x86_64
is a red herring.  Maybe the timer {ov,und}erflow display is different depending
on word size, and this is all the same issue?

I will see if I can try this on a vastly different CPU later today.

Regards,

Jed Smith
Systems Administrator
Linode, LLC
+1 (609) 593-7103 x1209
jed@xxxxxxxxxx


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