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Subject: Re: [OMPI devel] [RFC] Low pressure OPAL progress
From: Ralph Castain (rhc_at_[hidden])
Date: 2009-06-09 07:54:27


My concern with any form of sleep is with the impact on the proc -
since opal_progress might not be running in a separate thread, won't
the sleep apply to the process as a whole? In that case, the process
isn't free to continue computing.

I can envision applications that might call down into the MPI library
and have opal_progress not find anything, but there is nothing wrong.
The application could continue computations just fine. I would hate to
see us put the process to sleep just because the MPI library wasn't
busy enough.

Hence my suggestion to just change the tick rate. It would definitely
cause a higher latency for the first message that arrived while in
this state, which is bothersome, but would meet the stated objective
without interfering with the process itself.

LANL has also been looking at this problem of stalled jobs, but from a
different approach. We monitor (using a separate job) progress in
terms of output files changing in size plus other factors as specified
by the user. If we don't see any progress in those terms over some
time, then we kill the job. We chose that path because of the concerns
expressed above - e.g., on our RR machine, intense computations can be
underway on the Cell blades while the Opteron MPI processes wait for
us to reach a communication point. We -want- those processes spinning
away so that, when the comm starts, it can proceed as quickly as
possible.

Just some thoughts...
Ralph

On Jun 9, 2009, at 5:28 AM, Terry Dontje wrote:

> Sylvain Jeaugey wrote:
>> 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.
>>
> One way around this is to make all blocked communications (even SM)
> to use poll to block for incoming messages. Jeff and I have
> discussed this and had many false starts on it. The biggest issue
> is coming up with a way to have blocks on the SM btl converted to
> the system poll call without requiring a socket write for every
> packet.
>
> The usleep solution works but is kind of ugly IMO. I think when I
> looked at doing that the overhead increased signifcantly for certain
> communications. Maybe not for toy benchmarks but for less
> synchronized processes I saw the usleep adding overhead where I
> didn't want it too.
>
> --td
>> 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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