Re: Problems with perf

[Tommy <tommy24@gatech.edu>]

Tommy <tommy24 <at> gatech.edu> writes:

> 
> Jen-Cheng(Tommy) Huang <tommy24 <at> gatech.edu> writes:
> > 
> > Hi
> > 
> > I am trying to use perf with event grouping to profile Java apps.
> > My kernel is 3.16.1-1.el6.elrepo.x86_64
> > perf version 3.16.1-1.el6.elrepo.x86_64
> > The Java app I use is hadoop TaskTracker, which spawns multiple child 
> JVMs.
> > There seems to be something wrong with event group, since very few
> > samples were collected if I use more than one event in a group.
> > The following shows an example
> > 
> > I used
> > perf record -e '{instructions}:S' -c 100000 [TaskTracker command]
> > This one works fine and it collects a reasonable amount of samples and
> > several threads are sampled.
> > 
> > However, the following command does not work.
> > perf record -e '{instructions,cycles}:S' -c 100000 [TaskTracker command]
> > Less than 1000 samples were collected and only one thread was sampled
> > 
> > If I don't use :S to read samples, the sample counts are reasonable,
> > but the event grouping is gone. (both events were sampled, no leader
> > event.)
> > perf record -e '{instructions,cycles}' -c 100000 [TaskTracker command]
> > 
> > I tested single-threaded app, such as SPEC CPU.
> > Everything works fine. The event grouping works fine, so as the :S flag.
> > However, the Java app that I tested does not.
> > Please let me know if any suggestions on this issue.
> > 
> > Thanks.
> > 
> To make it clear about the sample count difference, this is the stats 
> collected from a few seconds run of the same app: With only one event in a 
> group, it has ~362339 samples while 2 events in a group, it has only 938 
> samples (only 1 thread was profiled). I think the sample recording is 
> somehow stopped.
> Thanks.

I found that even the woken counts are different
1 event in a group:
[ perf record: Woken up 27 times to write data ]
[ perf record: Captured and wrote 7.165 MB perf.data (~313046 samples) ]

2 events in a group:
[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 0.051 MB perf.data (~2223 samples) ]

It seems that something has been disabled, but no error/warning messages 
were shown.
Thanks.