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Subject: Re: [OMPI users] Meaning and the significance of MCA parameter "opal_cr_use_thread"
From: Josh Hursey (jjhursey_at_[hidden])
Date: 2010-03-29 11:32:23


So the MCA parameter that you mention is explained at the link below:
   http://osl.iu.edu/research/ft/ompi-cr/api.php#mca-opal_cr_use_thread

This enables/disables the C/R thread a runtime if Open MPI was
configured with C/R thread support:
   http://osl.iu.edu/research/ft/ompi-cr/api.php#conf-enable-ft-thread

The C/R thread enables asynchronous processing of checkpoint requests
when the application process is not inside the MPI library. The
purpose of this thread is to improve the responsiveness of the
checkpoint operation. Without the thread, if the application is in a
computation loop then the checkpoint will be delayed until the process
enters the MPI library. With the thread enabled, the checkpoint will
start in the C/R thread if the application is not in the MPI library.

The primary advantages of the C/R thread are:
  - Response time to the C/R request since the checkpoint is not
delayed until the process enters the MPI library,
  - Asynchronous processing of the checkpoint while the application is
executing outside the MPI library (improves the checkpoint overhead
experienced by the process).

The primary disadvantage of the C/R thread is the additional
processing task running in parallel with the application. If the C/R
thread is polling too often it could slow down the main process by
forcing frequent context switches between the C/R thread and the main
execution thread. You can adjust the aggressiveness by adjusting the
parameters at the link below:
   http://osl.iu.edu/research/ft/ompi-cr/api.php#mca-opal_cr_thread_sleep_check

-- Josh

On Mar 24, 2010, at 11:24 AM, <ananda.mudar_at_[hidden]> <ananda.mudar_at_[hidden]
> wrote:

> The description for MCA parameter “opal_cr_use_thread” is very short
> at URL: http://osl.iu.edu/research/ft/ompi-cr/api.php
>
> Can someone explain the usefulness of enabling this parameter vs
> disabling it? In other words, what are pros/cons of disabling it?
>
> I found that this gets enabled automatically when openmpi library
> is configured with –ft-enable-threads option.
>
> Thanks
> Ananda
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