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RE: Proposed systemtap access to perfmon hardware
- From: "Stone, Joshua I" <joshua dot i dot stone at intel dot com>
- To: <maynardj at us dot ibm dot com>, "William Cohen" <wcohen at redhat dot com>
- Cc: "SystemTAP" <systemtap at sources dot redhat dot com>
- Date: Wed, 22 Mar 2006 15:46:41 -0800
- Subject: RE: Proposed systemtap access to perfmon hardware
Maynard Johnson wrote:
> William Cohen wrote:
>> The individual start and stop operations would be allowed.
> This is not so good. Besides the fact that it may be difficult (or
> impossible) to do, I don't see it being all that useful. But then,
> I'm a tool developer, not a performance analyst, so I could be
> missing the point.
Enabling start & stop lets you narrow the context that you want to
measure. Perfmon can only give you thread level virtualization of the
counters. With start & stop I can, for example, start the counters when
I enter sys_open and stop when I return. Now if I want I can get a
microbenchmark of IPC for the sys_open call (and its callees).
But this also opens up possibilities for more obscure "contexts" -
perhaps I want to start counting when a network packet is received and
stop when it is delivered to the thread. Any probepoint you can do
today can become a start/stop point for the counters.
Josh