Thanks for the answer.
It is good to know that I can look through some routines in distributed
source code.
It was also helpful to know that collective calls are just send, receive and
some algorithm by reading source codes.
Allgatherv is simple and clear, but reduce seems to have some algorithm to
improve speed.
Anyway, I appreciate for your kind answer.
Byungchan Kim
>From: Amey Dharurkar <adharurk_at_[hidden]>
>Reply-To: General LAM/MPI mailing list <lam_at_[hidden]>
>To: General LAM/MPI mailing list <lam_at_[hidden]>
>Subject: Re: LAM: Allgatherv & reduce_scatter performance
>Date: Mon, 20 Oct 2003 15:49:47 -0500 (EST)
>
>
> > Hi All.
> >
> > I just wonder that which of allgatherv and reduce_scatter is faster?
> > Basically, they deal with same amout of data but in opposite way.
> > In my experience, reduce_scatter is much slower that allgatherv.
> > I don't know how they work differently in implementation level.
> > Can anybody help me understand the speed difference of allgatherv and
> > reduce_scatter?
>
>In LAM, Allgatherv is implemented using N calls to Gatherv while
>Reduce_scatter is implemented using a Reduce followed by a Scatterv. You
>might want to see the source code (/share/ssi/coll/lam_basic/src) for
>above collective operations.
>
>When you say that reduce_scatter is slower, it depends upon how you
>measure the completion time, the message size and the number of processes.
>I think that for large number of processes and sufficiently large message
>sizes, allgatherv becomes slower compared to the reduce_scatter because of
>the implementation. Further, in allgatherv, rank0 finishes early compared
>to rank1 and so on. In reduce_scatter all the processes finish almost at
>the same time.
>
>Hope this helps.
>
> > Thank you so much
> >
> > Byungchan Kim
> >
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