Performance difference between Xen versions

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

Hi,

comparing performance of different Xen versions with BS2000 as HVM guest
showed some weird data I'd like to understand.

All measurements were done on an Intel Xeon E7220 box. We used a disk-
benchmark and found the cpu utilization was much higher with Xen 4.0 compared
to Xen 3.3. I did some more investigation and narrowed things down to calls of
the hypervisor (implicit or explicit).

Following is a table with timing data for different low-level functions, all
timing values are tsc ticks obtained via rdtsc:

Xen 3.3     Xen 4.0      Function
       88        165      just the measurement overhead
      176        330      rdtsc-instruction + cli/sti
     5896      11044      lapic timer query
     7381      13519      setting lapic timer
     4653       8987      reload of cr3
     3124       5709      invlpg instruction
   792253     792264      wbinvd instruction
      748       1375      int + iret
     5203       9317      hypervisor yield call
12598102   12597882      memory access loop

All operations involving the hypervisor take nearly twice the time on 4.0.
Operations not involving the hypervisor (wbinvd and memory access loop) are
the same on both systems (this rules out the possibility of different rdtsc
behavior).

Is there any easy explanation for this? Both Xen versions are from SLES
(SLES11 or SLES11 SP1).


Juergen

-- 
Juergen Gross                 Principal Developer Operating Systems
TSP ES&S SWE OS6                       Telephone: +49 (0) 89 3222 2967
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