Re: [Xen-devel] XEN PV Linux performance

[George Dunlap <George.Dunlap@xxxxxxxxxxxxx>]

On Tue, Jul 3, 2012 at 11:37 AM, Tim Deegan <tim@xxxxxxx> wrote:
> At 22:26 +0800 on 01 Jul (1341181569), Zhou Jacky wrote:
>> Hello,
>>
>> I see what's the gettimeofday behavior depends on the glic config macro &
>> compile.
>> I just demonstrate the issue that 64bit PV DomU performance is very poor
>> using   gettimeofday .
>> In fact, the performance is poor when call read,write, fork,exec and almost
>> all system calls.
>
> Yes, system calls from 64-bit PV guests are expensive.  If your guest
> doesn't have a lot of memory (less than about 4GB) it's probably faster
> to use a 32-bit kernel, which doesn't have this problem.  If it has a
> lot of memory, the cost of extra pagetable manipulations may mean that
> the 32-bit kernel is actually slower, though.

IIRC, I think Stefano did some tests with kernel compile (a very
system-call-heavy workload), and found that a 32-bit PV kernel
outperformed 64-bit PV for less than 768MiB -- basically, the cost of
extra TLB flushes due to HIGHMEM outweighed the extra cost of system
calls.

 -George

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