I am working on monitoring cache coherence events using perf. The more I use
the tool, the more I appreciate its design :)
So, I am using perf on Intel i7 Nehalem machine which supports this event
related to L1 data cache stores :
#-----------------------------
IDX : 31457325
PMU name : nhm (Intel Nehalem)
Name : L1D_CACHE_ST
Equiv : None
Flags : None
Desc : L1 data cache stores
Code : 0x41
Umask-00 : 0x04 : PMU : [E_STATE] : None : L1 data cache stores in E state
Umask-01 : 0x01 : PMU : [I_STATE] : None : L1 data cache store in the I state
Umask-02 : 0x08 : PMU : [M_STATE] : None : L1 data cache stores in M state
Umask-03 : 0x02 : PMU : [S_STATE] : None : L1 data cache stores in S state
Umask-04 : 0x0f : PMU : [MESI] : [default] : L1 data cache store in all states
To obtain these events, I used the command :
"perf -R -e r141:u,r241:u,r441:u,r841:u -c 3 <some binary>"
(a) Now, just to be sure, are my raw event identifiers correct ?
(b) I then used "perf script" on the perf.data file, but here comes the strange
part :
I got this log tuple for 'pbzip2_good' binary using r841:u [ L1 data cache
stores in modified state at user level ] :
pbzip2_good 5857 [004] 15198.792535: raw 0x841: 402c1c
_Z19consumer_decompressPv
(/home/joy/Desktop/Perf/pbzip2/pbzip2-0.9.4/pbzip2_good)
Basically, it points to an instruction 402c1c in consumer_decompress
function which is basically a call to pthread_mutex_lock :
/home/joy/Desktop/Perf/pbzip2/pbzip2-0.9.4/pbzip2.cpp:555
402c18: 48 8b 7b 30 mov 0x30(%rbx),%rdi
402c1c: e8 87 ed ff ff callq 4019a8 <pthread_mutex_lock@plt>
Corresponding code :
pthread_mutex_lock(fifo->mut);
So, I was expecting "402c18" to be the interesting instruction, but I always
see the "402c1c" instruction in the perf log. I see several more
pthread_function calls in the log.
How can a "call" instruction, which from my understanding, has nothing to do
with cache coherence events in data cache, always come up in the output ? While
the "mov" instruction, which is more related to modified stores in data cache,
never comes up in the log.
All suggestions are welcome :)
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