RE: [Xen-devel] [PATCH] TSC scaling for live migration between platforms with different TSC frequecies

["Zhang, Xiantao" <xiantao.zhang@xxxxxxxxx>]

John Levon wrote:
> On Thu, Jun 18, 2009 at 01:57:21PM -0700, Dan Magenheimer wrote:
> 
>>> # dtrace -n 'fbt::tsc_gethrtime:entry /cpu == 0/ { @ =
>>> sum(1); }' -c "sleep 10"
>>> dtrace: description 'fbt::tsc_gethrtime:entry ' matched 1 probe
>>> dtrace: pid 29708 has exited 
>>> 
>>>             27798
>>> 
>>> This is on a basically idle 8-way system. (The other CPUs are
>>> less busy.)
>> 
>> Just checking... this is in 10 seconds and each processor is
>> "ticking" (and possibly a system-wide timer tick as well),
>> so this is ~350 rdtsc/sec/processor, correct?
> 
> No. That's CPU0 only ('cpu == 0'). Solaris only has one system-wide
> timer tick. This is mstate accounting: every kernel/user boundary,
> every interrupt, etc. incurs at least one TSC read. (And of course
> the machine is idle.)

So the rdtsc rate in the system is 2779.8/s per your testing ? If so, the 
performance impact can be ignored. We had done the performance testing with 
sysbench oltp, and in the testing the rdtsc rate exceeds 120000 rdtsc/sec, but 
even in such extreme case perfomrance loss is still less 10%.  In addition, we 
also measured the emulation cost, and the result showes rdtsc can be done in 
1500-1800 cycles in emulation case.
Xiantao 


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