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Date: Wed, 12 Aug 2009 22:54:31 +0200
Message-ID: <7c86c4470908121354q33a82914x6cf63eb375b6fb59@mail.gmail.com>
Subject: Re: perf_counters issue with PERF_SAMPLE_GROUP
From: stephane eranian <eranian@googlemail.com>
To: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Ingo Molnar <mingo@elte.hu>, LKML <linux-kernel@vger.kernel.org>,
       Andrew Morton <akpm@linux-foundation.org>,
       Thomas Gleixner <tglx@linutronix.de>,
       Robert Richter <robert.richter@amd.com>,
       Paul Mackerras <paulus@samba.org>, Andi Kleen <andi@firstfloor.org>,
       Maynard Johnson <mpjohn@us.ibm.com>, Carl Love <cel@us.ibm.com>,
       Corey J Ashford <cjashfor@us.ibm.com>,
       Philip Mucci <mucci@eecs.utk.edu>, Dan Terpstra <terpstra@eecs.utk.edu>,
       perfmon2-devel <perfmon2-devel@lists.sourceforge.net>
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Hi,

Another thing that occurred to me  while using PERF_SAMPLE_GROUP
is about that unique ID which you get out of read() with PERF_FORMAT_ID.

That number if generated by the kernel and it is a globally incrementing
64-bit counter.

An application may not get subsequent numbers even though it is issuing
perf_open_counter() in sequence. Another applications may cause the
number to change.

I believe this is not very convenient because imagine in your signal handler
you parse the GROUP and you want to relate id to the event. You need
to have a lookup table.

I believe it would be more convenient if the tool could pass a 64-bit number
itself when the event is created. It would get it back as part of
PERF_SAMPLE_GROUP.
The id could then be more related to the tool's data structure, no need for a
lookup table, overall more efficient. If I recall, this is what
epoll() can do and
this is very convenient.