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Re: [Xen-devel] XEN PV Linux performance

On Tue, 3 Jul 2012, George Dunlap wrote:
> 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.

Actually I only tested 8GB guests and I found that 64 bit PV guests
outperformed 32 bit PV guests
(http://www.slideshare.net/xen_com_mgr/6-stefano-spvhvm, slide 15).
The interesting result that I found is that in the pbzip2 test (slide
17) 32 bit PV guests issue 10 times as many TLB flushes as 64 bit PV

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