[ltt-dev] LTTng and multi-core

[Francois]

Hi guys,

This was posted on the Eclipse mailing list but I think it belongs to
you...

Sent to you by Francois via Google Reader: LTTng and multi-core via
Eclipse Community Forums RDF feed by Russell Klein on 10/13/10
I am trying to find a race condition in my system using LTTng. I am
running an application with 42 threads. There is a lot of locking and
and memory contention between the threads. It is running on an 8 core
system (Freescale P4080). I am running on a Linux 2.6.30 base. I have
patched it with LTTng v 0.159 (the latest which works with 2.6.30 - I
believe) with version 2.3 trace.

What I am seeing is that 41 of the 42 threads are reported to be
running on one of the CPUs. The CPUID field in the events for the PID
for these threads is always the same (note that the threads - pthreads,
specifically - seem to be implemented as processes as each thread has a
pid associated with it).

However, when I look at the control flow viewer - it appears that at
any point in time that up to 8 threads are in the "running" state
(shown as bright green) at any one time. This consistent with what I
believe should be happening - but conflicts with the CPUID field being
reported with the events.

One other thing, at the same time that there are 8 "running" user space
threads/processes - there are up to 8 (typically 6) system level
processes which are reported (in the control flow view) as "running"
(bright blue). While I have 8 PPC cores, they are not hyperthreaded to
provide the illusion of 2 or more logical cores per physical core.

Can anyone shed some light on this?

How does LTTng determine the CPUID field? Can this be relied upon?

How does LTTng determine that a process is "running'? Is it possible to
have more processes in the "running" state than cores available?

How can I correlate a running process with a specific CPU?

Thanks, Russ Klein
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