[MPICH] Loopback Communication

[Elvedin Trnjanin]

Version:           1.0.2
Device:            ch3:sock
Configure Options:

I didn't install it and those are the only relevant bits about the 
configuration I could find.

Anthony Chan wrote:
> Did you configure mpich2 --with-device=ch3:nemesis which uses shared
> memory for intranode communication ?
>
> A.Chan
>
> On Wed, 27 Feb 2008, Elvedin Trnjanin wrote:
>
>   
>> I have a "ping pong" latency and bandwidth measurement program that I'm
>> using for approximating performance of an Infiniband cluster. I've
>> noticed that for self to self asynchronous communication with
>> mpirun_ssh, to transfer a 1MB message, it takes around 3.2 ms while with
>> node A to node B communication, the latency is around 2.9 ms per 1MB
>> message. On the bandwidth side, self to self communication is 60MB/s
>> slower. Although 60MB/s difference might not seem like a significant
>> problem considering the total bandwidth for the larger message
>> Infiniband transfers, on a small Gigabit Ethernet cluster I use, our
>> node to node asynchronous communication bandwidth is slightly less than
>> 60MB/s. That difference is pretty significant to me.
>>
>> My question is if anyone can explain why self to self communication is
>> slower and also why the interconnect is involved instead of doing a
>> memory copy? I apologize ahead of time if this issue has been discussed
>> in the MPI2 standards but I'm reading the (unofficial) report and
>> haven't found it yet.
>>
>> Regards,
>> Elvedin Trnjanin
>>
>>
>>     
>
>