Hi Ralph,
I'm entirely convinced that MPI doesn't have to save power in a normal
scenario. The idea is just that if an MPI process is blocked (i.e. has not
performed progress for -say- 5 minutes (default in my implementation), we
stop busy polling and have the process drop from 100% CPU usage to 0%.
I do not call sleep() but usleep(). The result if quite the same, but is
less hurting performance in case of (unexpected) restart.
However, the goal of my RFC was also to know if there was a more clean way
to achieve my goal, and from what I read, I guess I should look at the
"tick" rate instead of trying to do my own delaying.
Don't worry, I was quite expecting the configure-in requirement. However,
I don't think my patch is good for inclusion, it is only an example to
describe what I want to achieve.
Thanks a lot for your comments,
Sylvain
On Mon, 8 Jun 2009, Ralph Castain wrote:
I'm not entirely convinced this actually achieves your goals, but I can see
some potential benefits. I'm also not sure that power consumption is that big
of an issue that MPI needs to begin chasing "power saver" modes of operation,
but that can be a separate debate some day.
I'm assuming you don't mean that you actually call "sleep()" as this would be
very bad - I'm assuming you just change the opal_progress "tick" rate
instead. True? If not, and you really call "sleep", then I would have to
oppose adding this to the code base pending discussion with others who can
corroborate that this won't cause problems.
Either way, I could live with this so long as it was done as a "configure-in"
capability. Just having the params default to a value that causes the system
to behave similarly to today isn't enough - we still wind up adding logic
into a very critical timing loop for no reason. A simple configure option of
--enable-mpi-progress-monitoring would be sufficient to protect the code.
HTH
Ralph
On Jun 8, 2009, at 9:50 AM, Sylvain Jeaugey wrote:
What : when nothing has been received for a very long time - e.g. 5
minutes, stop busy polling in opal_progress and switch to a usleep-based
one.
Why : when we have long waits, and especially when an application is
deadlock'ed, detecting it is not easy and a lot of power is wasted until
the end of the time slice (if there is one).
Where : an example of how it could be implemented is available at
http://bitbucket.org/jeaugeys/low-pressure-opal-progress/
Principle
=========
opal_progress() ensures the progression of MPI communication. The current
algorithm is a loop calling progress on all registered components. If the
program is blocked, the loop will busy-poll indefinetely.
Going to sleep after a certain amount of time with nothing received is
interesting for two things :
- Administrator can easily detect whether a job is deadlocked : all the
processes are in sleep(). Currently, all processors are using 100% cpu and
it is very hard to know if progression is still happening or not.
- When there is nothing to receive, power usage is highly reduced.
However, it could hurt performance in some cases, typically if we go to
sleep just before the message arrives. This will highly depend on the
parameters you give to the sleep mechanism.
At first, we can start with the following assumption : if the sleep takes T
usec, then sleeping after 10000xT should slow down Receives by a factor
less than 0.01 %.
However, other processes may suffer from you being late, and be delayed by
T usec (which may represent more than 0.01% for them).
So, the goal of this mechanism is mainly to detect far-too-long-waits and
should quite never be used in normal MPI jobs. It could also trigger a
warning message when starting to sleep, or at least a trace in the
notifier.
Details of Implementation
=========================
Three parameters fully control the behaviour of this mechanism :
* opal_progress_sleep_count : number of unsuccessful opal_progress() calls
before we start the timer (to prevent latency impact). It defaults to -1,
which completely deactivates the sleep (and is therefore equivalent to the
former code). A value of 1000 can be thought of as a starting point to
enable this mechanism.
* opal_progress_sleep_trigger : time to wait before going to
low-pressure-powersave mode. Default : 600 (in seconds) = 10 minutes.
* opal_progress_sleep_duration : time we sleep at each further unsuccessful
call to opal_progress(). Default : 1000 (in us) = 1 ms.
The duration is big enough to make the process show 0% CPU in top, but low
enough to preserve a good trigger/duration ratio.
The trigger is voluntary high to keep a good trigger/duration ratio.
Indeed, to prevent delays from causing chain reactions, trigger should be
higher than duration * numprocs.
Possible Improvements & Pitfalls
================================
* Trigger could be set automatically at max(trigger, duration * numprocs *
2).
* poll_start and poll_count could be fields of the opal_condition_t struct.
* The sleep section may be exported in a #define and reported in all the
progress pathes (I'm not sure my patch is good for progress threads for
example)
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