[mpich-discuss] Re: MPI_Brecv vs multiple MPI_Irecv

[Darius Buntinas]

I'm not sure what you mean by a queue on the receiving side.  If 
receives have been posted, incoming messages that match the posted 
receives will be received directly into the specified location. 
Depending on the particular MPI implementation you're using, progress on 
the receives (i.e., taking them off the network, doing the matching, 
copying/receiving into the user buffer, etc.) may only happen while 
you're calling an MPI function call.  So if you're in a long compute 
loop, the MPI library might not be performing the receives.  But adding 
a user buffer wouldn't help that situation either.

Messages that are received which don't have matching posted receives 
will be "queued" waiting for the matching receives, and either buffered 
internally at the receiver (for small messages) or will "stall" at the 
sender (for large messages).  But I believe you're only concerned with 
the case where receives have been posted.

Does this help?

-d


On 08/27/2008 11:03 AM, Robert Kubrick wrote:
> A buffered receive would allow the implementation to receive and store 
> messages when the application is busy doing something else, like reading 
> messages on a different comm. I now understand why a Brecv is not in the 
> standard and it makes perfect sense, but the result is that on the 
> sending you can control the size of a sending "queue", on the receiving 
> side you can not.
> 
> On Aug 27, 2008, at 11:23 AM, Darius Buntinas wrote:
> 
>>
>> Well, what would it mean to do a buffered receive?
>>
>> This?
>>   buf = malloc(BUF_SZ);
>>   MPI_Irecv(buf,...);
>>   MPI_Wait(...);
>>   memcpy(recv_ptr, buf, BUF_SZ);
>>
>> What would be the benefit?
>>
>> -d
>>
>> On 08/27/2008 10:13 AM, Robert Kubrick wrote:
>>> I just found out that the standard actually doesn't have an MPI_Brecv 
>>> call.
>>> Any reason why the recv can not buffer messages in a user-provided 
>>> memory space, as per MPI_Battach/MPI_Bsend?
>>> On Aug 26, 2008, at 4:17 PM, Robert Kubrick wrote:
>>>> From a performance point of view, which one is better:
>>>>
>>>> MPI_Battach(10*sizeof(MSG))
>>>> MPI_Brecv()
>>>>
>>>> or
>>>>
>>>> MPI_recv_init()
>>>> MPI_recv_init()
>>>> MPI_recv_init()
>>>> ... /* 10 recv handlers */
>>>> MPI_Start(all recv)
>>>> MPI_Waitany()
>>>>
>>>>
>>>> I understand MPI_Brecv will require an extra message copy, from the 
>>>> attached buffer to the MPI_Brecv() buffer. I'd like to know if there 
>>>> other differences between the two methods.
>>>>
>>>> Thanks,
>>>> Rob
>>
>