Poor HVM performance with 8 vcpus

[Juergen Gross <juergen.gross@ts.fujitsu.com>]

Hi,

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?
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@ts.fujitsu.com
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