[mpich-discuss] lock/unlock regions and process synchronization

[Robert Kubrick]

I checked the code in the examples, I understand MPI_Accumulate/ 
MPI_Get is the key to achieve read/write atomicity but I actually  
need to perform a set of operations on the window, including random  
assignments. Is there a way to simulate a semaphore through MPI-2  
generalized requests?

Rob.

On Jul 10, 2008, at 4:53 PM, Rajeev Thakur wrote:

> You could use some out-of-band synchronization like that to achieve
> atomicity.
>
> Rajeev
>
>> -----Original Message-----
>> From: owner-mpich-discuss at mcs.anl.gov
>> [mailto:owner-mpich-discuss at mcs.anl.gov] On Behalf Of Robert Kubrick
>> Sent: Thursday, July 10, 2008 3:46 PM
>> To: mpich-discuss at mcs.anl.gov
>> Subject: Re: [mpich-discuss] lock/unlock regions and process
>> synchronization
>>
>> Thanks Rajeev, I already have the book in the mail. On a related
>> note, is there any potential issue using SYSV semaphores with one-
>> sided communications? While the active task is completing the set of
>> instructions, all the other slave processes could wait on a  
>> semaphore.
>>
>> On Jul 10, 2008, at 4:11 PM, Rajeev Thakur wrote:
>>
>>> What you are trying to do is an atomic read-modify-write,
>> which is not
>>> trivial to do with the MPI-2 one-sided operations. There
>> are two ways
>>> described in the book, Using MPI-2. One is easier to understand
>>> than the
>>> other one, but is less scalable. The source code for both is
>>> available in
>>> the MPICH2 distribution. See fetchandadd.c and fetchandadd_tree.c in
>>> test/mpi/rma.
>>>
>>> Rajeev
>>>
>>>> -----Original Message-----
>>>> From: owner-mpich-discuss at mcs.anl.gov
>>>> [mailto:owner-mpich-discuss at mcs.anl.gov] On Behalf Of
>> Robert Kubrick
>>>> Sent: Thursday, July 10, 2008 3:04 PM
>>>> To: mpich-discuss at mcs.anl.gov
>>>> Subject: [mpich-discuss] lock/unlock regions and process
>>>> synchronization
>>>>
>>>> I need to access a one-sided window region in a master
>> from multiple
>>>> slave processes. Each slave needs to read the window contents, then
>>>> update the same window area:
>>>>
>>>> MPI_Win_lock(...);
>>>> MPI_Get(&idx, ...);
>>>> MPI_Win_unlock(...);
>>>>
>>>> // Got idx, check value
>>>> if( idx > 10 ) {
>>>>    idx += 5;
>>>>    MPI_Win_lock(...);
>>>>    MPI_Put(&idx);
>>>>    MPI_Win_unlock(...);
>>>> }
>>>>
>>>> How can I make sure another process does not access 'idx'
>>>> between the
>>>> two lock/unlock regions in the example?
>>>>
>>>>
>>>
>>
>>
>