Re: Getting started with perf; a couple of questions

[Andrew Davidoff <davidoff@qedmf.net>]

Hi,

Thanks for the quick response.

> add a -v and see if more info comes out.
>
> perf script can be used to dump events.

I tried again with -v on record, but I am still not getting any event
samples in perf.data. Looking at an ASCII dump with report -D, I see
43 PERF_RECORD_MMAP entries, each of which appears to be related to a
kernel module, but nothing for any of the events I have selected to
sample (each is simply listed like "page-faults stats:").

The run ends like this every time:

[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 0.004 MB perf.data (~194 samples) ]

"Woken up 1 times" seemed suspicious, if record should be sampling at
1000Hz. I tried using -F 1000 and adding cycles to my events list both
in an effort to get 1000Hz samples, but as far as I could tell, there
was no change in the recorded data.

Also note that when using -v with record, I get the message "failed to
write feature 2" at the very end of the run output. I do not get that
message without -v. I don't know what it's trying to tell me and
haven't looked at the source code yet.

The command I am running is a threaded python script (spawns workers).
As I understand the documentation, I shouldn't need any additional
flags for perf to collect data on all the threads, since it reads like
counter inheritance is on by default. In other words, I believe perf
record should be working in what is documented as "per-process" mode
by default. I tried using -i with record in case I wasn't
understanding correctly, but that didn't change anything.

I will play with this more tomorrow but any further advice is
appreciated. I feel like I'm probably just missing a switch or
something that's not entirely obvious from the scant documentation I
have found.

Thanks again.
Andy