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Subject: Re: [OMPI users] Dynamic processes connection and segfault on MPI_Comm_accept
From: Ralph Castain (rhc_at_[hidden])
Date: 2010-07-07 09:58:00

On Jul 6, 2010, at 8:48 AM, Grzegorz Maj wrote:

> Hi Ralph,
> sorry for the late response, but I couldn't find free time to play
> with this. Finally I've applied the patch you prepared. I've launched
> my processes in the way you've described and I think it's working as
> you expected. None of my processes runs the orted daemon and they can
> perform MPI operations. Unfortunately I'm still hitting the 65
> processes issue :(
> Maybe I'm doing something wrong.
> I attach my source code. If anybody could have a look on this, I would
> be grateful.
> When I run that code with clients_count <= 65 everything works fine:
> all the processes create a common grid, exchange some information and
> disconnect.
> When I set clients_count > 65 the 66th process crashes on
> MPI_Comm_connect (segmentation fault).

I didn't have time to check the code, but my guess is that you are still hitting some kind of file descriptor or other limit. Check to see what your limits are - usually "ulimit" will tell you.

> Another thing I would like to know is if it's normal that any of my
> processes when calling MPI_Comm_connect or MPI_Comm_accept when the
> other side is not ready, is eating up a full CPU available.

Yes - the waiting process is polling in a tight loop waiting for the connection to be made.

> Any help would be appreciated,
> Grzegorz Maj
> 2010/4/24 Ralph Castain <rhc_at_[hidden]>:
>> Actually, OMPI is distributed with a daemon that does pretty much what you
>> want. Checkout "man ompi-server". I originally wrote that code to support
>> cross-application MPI publish/subscribe operations, but we can utilize it
>> here too. Have to blame me for not making it more publicly known.
>> The attached patch upgrades ompi-server and modifies the singleton startup
>> to provide your desired support. This solution works in the following
>> manner:
>> 1. launch "ompi-server -report-uri <filename>". This starts a persistent
>> daemon called "ompi-server" that acts as a rendezvous point for
>> independently started applications. The problem with starting different
>> applications and wanting them to MPI connect/accept lies in the need to have
>> the applications find each other. If they can't discover contact info for
>> the other app, then they can't wire up their interconnects. The
>> "ompi-server" tool provides that rendezvous point. I don't like that
>> comm_accept segfaulted - should have just error'd out.
>> 2. set OMPI_MCA_orte_server=file:<filename>" in the environment where you
>> will start your processes. This will allow your singleton processes to find
>> the ompi-server. I automatically also set the envar to connect the MPI
>> publish/subscribe system for you.
>> 3. run your processes. As they think they are singletons, they will detect
>> the presence of the above envar and automatically connect themselves to the
>> "ompi-server" daemon. This provides each process with the ability to perform
>> any MPI-2 operation.
>> I tested this on my machines and it worked, so hopefully it will meet your
>> needs. You only need to run one "ompi-server" period, so long as you locate
>> it where all of the processes can find the contact file and can open a TCP
>> socket to the daemon. There is a way to knit multiple ompi-servers into a
>> broader network (e.g., to connect processes that cannot directly access a
>> server due to network segmentation), but it's a tad tricky - let me know if
>> you require it and I'll try to help.
>> If you have trouble wiring them all into a single communicator, you might
>> ask separately about that and see if one of our MPI experts can provide
>> advice (I'm just the RTE grunt).
>> HTH - let me know how this works for you and I'll incorporate it into future
>> OMPI releases.
>> Ralph
>> On Apr 24, 2010, at 1:49 AM, Krzysztof Zarzycki wrote:
>> Hi Ralph,
>> I'm Krzysztof and I'm working with Grzegorz Maj on this our small
>> project/experiment.
>> We definitely would like to give your patch a try. But could you please
>> explain your solution a little more?
>> You still would like to start one mpirun per mpi grid, and then have
>> processes started by us to join the MPI comm?
>> It is a good solution of course.
>> But it would be especially preferable to have one daemon running
>> persistently on our "entry" machine that can handle several mpi grid starts.
>> Can your patch help us this way too?
>> Thanks for your help!
>> Krzysztof
>> On 24 April 2010 03:51, Ralph Castain <rhc_at_[hidden]> wrote:
>>> In thinking about this, my proposed solution won't entirely fix the
>>> problem - you'll still wind up with all those daemons. I believe I can
>>> resolve that one as well, but it would require a patch.
>>> Would you like me to send you something you could try? Might take a couple
>>> of iterations to get it right...
>>> On Apr 23, 2010, at 12:12 PM, Ralph Castain wrote:
>>>> Hmmm....I -think- this will work, but I cannot guarantee it:
>>>> 1. launch one process (can just be a spinner) using mpirun that includes
>>>> the following option:
>>>> mpirun -report-uri file
>>>> where file is some filename that mpirun can create and insert its
>>>> contact info into it. This can be a relative or absolute path. This process
>>>> must remain alive throughout your application - doesn't matter what it does.
>>>> It's purpose is solely to keep mpirun alive.
>>>> 2. set OMPI_MCA_dpm_orte_server=FILE:file in your environment, where
>>>> "file" is the filename given above. This will tell your processes how to
>>>> find mpirun, which is acting as a meeting place to handle the connect/accept
>>>> operations
>>>> Now run your processes, and have them connect/accept to each other.
>>>> The reason I cannot guarantee this will work is that these processes
>>>> will all have the same rank && name since they all start as singletons.
>>>> Hence, connect/accept is likely to fail.
>>>> But it -might- work, so you might want to give it a try.
>>>> On Apr 23, 2010, at 8:10 AM, Grzegorz Maj wrote:
>>>>> To be more precise: by 'server process' I mean some process that I
>>>>> could run once on my system and it could help in creating those
>>>>> groups.
>>>>> My typical scenario is:
>>>>> 1. run N separate processes, each without mpirun
>>>>> 2. connect them into MPI group
>>>>> 3. do some job
>>>>> 4. exit all N processes
>>>>> 5. goto 1
>>>>> 2010/4/23 Grzegorz Maj <maju3_at_[hidden]>:
>>>>>> Thank you Ralph for your explanation.
>>>>>> And, apart from that descriptors' issue, is there any other way to
>>>>>> solve my problem, i.e. to run separately a number of processes,
>>>>>> without mpirun and then to collect them into an MPI intracomm group?
>>>>>> If I for example would need to run some 'server process' (even using
>>>>>> mpirun) for this task, that's OK. Any ideas?
>>>>>> Thanks,
>>>>>> Grzegorz Maj
>>>>>> 2010/4/18 Ralph Castain <rhc_at_[hidden]>:
>>>>>>> Okay, but here is the problem. If you don't use mpirun, and are not
>>>>>>> operating in an environment we support for "direct" launch (i.e., starting
>>>>>>> processes outside of mpirun), then every one of those processes thinks it is
>>>>>>> a singleton - yes?
>>>>>>> What you may not realize is that each singleton immediately
>>>>>>> fork/exec's an orted daemon that is configured to behave just like mpirun.
>>>>>>> This is required in order to support MPI-2 operations such as
>>>>>>> MPI_Comm_spawn, MPI_Comm_connect/accept, etc.
>>>>>>> So if you launch 64 processes that think they are singletons, then
>>>>>>> you have 64 copies of orted running as well. This eats up a lot of file
>>>>>>> descriptors, which is probably why you are hitting this 65 process limit -
>>>>>>> your system is probably running out of file descriptors. You might check you
>>>>>>> system limits and see if you can get them revised upward.
>>>>>>> On Apr 17, 2010, at 4:24 PM, Grzegorz Maj wrote:
>>>>>>>> Yes, I know. The problem is that I need to use some special way for
>>>>>>>> running my processes provided by the environment in which I'm
>>>>>>>> working
>>>>>>>> and unfortunately I can't use mpirun.
>>>>>>>> 2010/4/18 Ralph Castain <rhc_at_[hidden]>:
>>>>>>>>> Guess I don't understand why you can't use mpirun - all it does is
>>>>>>>>> start things, provide a means to forward io, etc. It mainly sits there
>>>>>>>>> quietly without using any cpu unless required to support the job.
>>>>>>>>> Sounds like it would solve your problem. Otherwise, I know of no
>>>>>>>>> way to get all these processes into comm_world.
>>>>>>>>> On Apr 17, 2010, at 2:27 PM, Grzegorz Maj wrote:
>>>>>>>>>> Hi,
>>>>>>>>>> I'd like to dynamically create a group of processes communicating
>>>>>>>>>> via
>>>>>>>>>> MPI. Those processes need to be run without mpirun and create
>>>>>>>>>> intracommunicator after the startup. Any ideas how to do this
>>>>>>>>>> efficiently?
>>>>>>>>>> I came up with a solution in which the processes are connecting
>>>>>>>>>> one by
>>>>>>>>>> one using MPI_Comm_connect, but unfortunately all the processes
>>>>>>>>>> that
>>>>>>>>>> are already in the group need to call MPI_Comm_accept. This means
>>>>>>>>>> that
>>>>>>>>>> when the n-th process wants to connect I need to collect all the
>>>>>>>>>> n-1
>>>>>>>>>> processes on the MPI_Comm_accept call. After I run about 40
>>>>>>>>>> processes
>>>>>>>>>> every subsequent call takes more and more time, which I'd like to
>>>>>>>>>> avoid.
>>>>>>>>>> Another problem in this solution is that when I try to connect
>>>>>>>>>> 66-th
>>>>>>>>>> process the root of the existing group segfaults on
>>>>>>>>>> MPI_Comm_accept.
>>>>>>>>>> Maybe it's my bug, but it's weird as everything works fine for at
>>>>>>>>>> most
>>>>>>>>>> 65 processes. Is there any limitation I don't know about?
>>>>>>>>>> My last question is about MPI_COMM_WORLD. When I run my processes
>>>>>>>>>> without mpirun their MPI_COMM_WORLD is the same as MPI_COMM_SELF.
>>>>>>>>>> Is
>>>>>>>>>> there any way to change MPI_COMM_WORLD and set it to the
>>>>>>>>>> intracommunicator that I've created?
>>>>>>>>>> Thanks,
>>>>>>>>>> Grzegorz Maj
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