Re: [Xen-devel] Poor HVM performance with 8 vcpus

[Gianluca Guida <gianluca.guida@xxxxxxxxxxxxx>]

Hi,

On Wed, Oct 7, 2009 at 8:55 AM, Juergen Gross
<juergen.gross@xxxxxxxxxxxxxx> wrote:
> we've got massive performance problems running a 8 vcpu HVM-guest (BS2000)
> under XEN (xen 3.3.1).
>
> With a specific benchmark producing a rather high load on memory management
> operations (lots of process creation/deletion and memory allocation) the 8
> vcpu performance was worse than the 4 vcpu performance. On other platforms
> (/390, MIPS, SPARC) this benchmark scaled rather well with the number of cpus.
>
> The result of the usage of the software performance counters of XEN seemed
> to point to the shadow lock being the reason. I modified the Hypervisor to
> gather some lock statistics (patch will be sent soon) and found that the
> shadow lock is really the bottleneck. On average 4 vcpus are waiting to get
> the lock!
>
> Is this a known issue?

Acutally, I think so. The OOS optimization is widely known not to be
too scalable at 8vcpus in the current state, since its weak point is
the CR3 switching time increasing linearly with the number of cpus. If
you have lot of processes switches together with lot of PTE writings
(as it seems to be the case for your benchmark) then that's probably
the cause.

Could you try disabling the OOS optimization from the
SHADOW_OPTIMIZATIONS definition?

Thanks,
Gianluca

> Is there a chance to split the shadow lock into sub-locks or to use a
> reader/writer lock instead?
> I just wanted to ask before trying to understand all of the shadow code :-)
>
>
> Juergen
>
> --
> Juergen Gross                 Principal Developer Operating Systems
> TSP ES&S SWE OS6                       Telephone: +49 (0) 89 636 47950
> Fujitsu Technolgy Solutions               e-mail: juergen.gross@xxxxxxxxxxxxxx
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>
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