Re: [Xen-devel] Question about high CPU load during iperf ethernet testing

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Mon, 2014-09-22 at 16:01 +0300, Iurii Konovalenko wrote:

> 
> I decided to debug a bit, so I used "({register uint64_t _r; asm
> volatile("mrrc " "p15, 0, %0, %H0, c14" ";" : "=r" (_r)); _r; })"
> command to read timer counter before and after operations I want to
> test.

I think that is CNTPCT aka the physical timer. This is trapped under
Xen. If you want an untrapped source of time you should use CNTVCT which
is p15,0,c14.

I expect the results are unreliable due to this.

Also watch out that the granularity of those two timers is usually far
below the CPU clock frequency, so one tick of them can potentially
represent quite a few clock cycles/instructions IIRC.

Ian.




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