Re: [PerfSuite-users] Questions about "Wall clock time"

[Rick K. <rk...@il...>]

Jie -

I am glad the apparent discrepancy in elapsed time has been accounted 
for.  Regarding your other question:  I cannot locate the metric 
calculation you refer to in the Intel manuals, so unfortunately cannot 
comment or speculate.

Rick


Jie Jiang wrote:
> Hi Rick,
>
> Thanks for your reply.
>
> I checked my platform and found that the CPU frequency will scale down
> automatically when it is idle.
>
> After using "cpuspeed" command to adjust CPU speed to 2.53GHz, I got the
> expected, right wallclock time. Thanks again.
>
> But I wonder why psrun can get the scale-down frequency. I need to check
> when psrun reads it from /proc/cpuinfo.
>
>
> Another question. When measuring cg.A with event
> MEM_LAOD_RETIRED:LLC_UNSHARED_HIT, I got a counter value of 331743878.
>
> According to Intel manual Vol3b, the percentage of the load latency in
> total run time can be calculated as following:
>
> ((MEM_LOAD_RETIRED.LLC_UNSHARED_HIT * 35) +
> (MEM_LOAD_RETIRED.OTHER_CORE_L2_HIT_HITM * 74)) /
> CPU_CLK_UNHALTED.THREAD) * 100
>
> Here,the result percentage will be about 158.827%. This is intuitively
> wrong since all overhead should be smaller than the total run time.
> What's wrong? 
> P.S. I test it on my platform with the latest pfmon-3.9/perfmon2. It
> gives a similar count value.
>
> Any idea?
>
> Regards,
> Jie
>
>
>
>
>
> On 二, 2009-11-17 at 10:33 -0600, Rick Kufrin wrote:
>   
>> Jie -
>>
>> It seems that the cause of the discrepancy in elapsed time reported is due to the differences in reporting of your machine's clock speed.  I see from the content of the "brand string" element in your XML document that it is a Xeon E5540, 2.53 GHz.  This information comes from the CPUID instruction.  However, the "clockspeed" element reported in the document is 1.6 GHz; that information comes from /proc/cpuinfo.  If you replace the clockspeed of 1600 with 2530, the numbers will be much closer.
>>
>> I am guessing there is some variable speed going on with your platform, and that the discrepancy stems from that, not from overhead generated by PerfSuite.
>>
>> Rick
>>
>>