Re: Performance of 40-way guest running 2.6.32-220 (RHEL6.2) vs. 3.3.1 OS

[Gleb Natapov <gleb@redhat.com>]

On Thu, Apr 12, 2012 at 02:21:06PM -0400, Rik van Riel wrote:
> On 04/11/2012 01:21 PM, Chegu Vinod wrote:
> >
> >Hello,
> >
> >While running an AIM7 (workfile.high_systime) in a single 40-way (or a single
> >60-way KVM guest) I noticed pretty bad performance when the guest was booted
> >with 3.3.1 kernel when compared to the same guest booted with 2.6.32-220
> >(RHEL6.2) kernel.
> 
> >For the 40-way Guest-RunA (2.6.32-220 kernel) performed nearly 9x better than
> >the Guest-RunB (3.3.1 kernel). In the case of 60-way guest run the older guest
> >kernel was nearly 12x better !
> 
How many CPUs your host has?

> >Turned on function tracing and found that there appears to be more time being
> >spent around the lock code in the 3.3.1 guest when compared to the 2.6.32-220
> >guest.
> 
> Looks like you may be running into the ticket spinlock
> code. During the early RHEL 6 days, Gleb came up with a
> patch to automatically disable ticket spinlocks when
> running inside a KVM guest.
> 
> IIRC that patch got rejected upstream at the time,
> with upstream developers preferring to wait for a
> "better solution".
> 
> If such a better solution is not on its way upstream
> now (two years later), maybe we should just merge
> Gleb's patch upstream for the time being?
I think the pv spinlock that is actively discussed currently should
address the issue, but I am not sure someone tests it against non-ticket
lock in a guest to see which one performs better.

--
			Gleb.