Re: Question about LLC-load-misses event

[Chulmin Kim <cmkim@core.kaist.ac.kr>]

2012-10-23 오후 2:39, Namhyung Kim 쓴 글:
> Hi Chulmin,
>
> On Mon, 15 Oct 2012 22:31:34 +0900, Chulmin Kim wrote:
>> 2012-10-15 오후 10:07, Chulmin Kim 쓴 글:
>>> (perf command : perf stat -a -A -e LLC-loads -e LLC-load-misses -e
>>> instructions sleep 3)
>>>
>>> The problem is,, the bandwidth from STREAM benchmark does not match with
>>> the monitored value.
>>>
>>> e.g.
>>> I got 9395MB/s from Stream.
>>>
>>> "perf" shows 134,642,063 LLC-load-misses for 3 seconds.
>>> -> BW = ((# of events)/(3 seconds)) * 64 bytes / (1024*1024) = 2739MB/s
>>> In this equation, the term (64bytes) is for cache line size, and the
>>> term(1024*1024) is for (MB/s).
>>>
>>> Why does this mismatch occur?
>> In case of Oprofile, the value for a certain event represents the number
>> of the overflows which occur when the number of the event exceeds the
>> predefined value.
>> Is it a similar case with that?
> I guess not.  And what's the result of the LLC-loads?  AFAIK it counts
> all cache accesses including hits and misses.

Sorry for the lack of information.

I used STREAM benchmark which generates 100% cache miss.
Of course, the value of LLC-loads shows bit larger number than that of 
LLC-load-misses (but, they are almost same).


>    Did you calculate the
> bandwidth using the result of LLC-loads?

Bandwidth results:
9395MB/s from Stream
2739MB/s from LLC-load (including both hit and miss)
I also want to add the BW from the mem write (about 3000MB/s from 
LLC-store (including both hit and miss) )

I'm wondering why this difference happens?  (9395MB/s vs about 5739MB/s)



> I suspect the h/w *might*
> prefetches a couple of lines when cache-miss occurred, but I'm not
> sure. :)

I also suspected PREFETCH.

After i uploaded this question, i checked prefetch events using "perf".
I got 100% prefetch miss also. (but i don't know the meaning of this 
result thoroughly.)

Do you know what this means?
Are Prefetch events and LLC-load (or store) events exclusive? or 
correlated?

Perf tool is too hard for me.. No remarkable document or site to help 
the user.  (If you know, recommend it please! :) )


Thanks for your attention!




> Thanks,
> Namhyung
>