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Subject: Re: [OMPI users] overlapping memcpy in ompi_coll_tuned_allgather_intra_bruck
From: Number Cruncher (number.cruncher_at_[hidden])
Date: 2008-02-04 11:56:06


George Bosilca wrote:
>
> Now, the overlapping case is a real exception. Obviously, it happened
> for at least two peoples (as per mailing list search) in about 4 years,
> but without affecting the correctness of the application. Is that a
> reason good enough to effect the overall performance of all parallel
> applications using Open MPI ? You can already guess my stance.
>

Thanks for the reply. I agree with your pragmatic approach in general,
and the lack of widespread problems certainly puts this low priority.
However, there *is* a reason for the memmove/memcpy distinction,
otherwise there'd only be a single API point in libc. And, as you state,
that reason is performance. One day someone will write some optimized
memcpy that *isn't* a simple forward copy.

I'm old enough to remember the Z80 instructions LDDR and LDIR
(http://www.sincuser.f9.co.uk/044/mcode.htm) for assembly-level memory
copying. A memmove would have to choose between the two; memcpy could
legitimately use either and would corrupt overlapping memory 50% of the
time.

> However, I can imagine a way to rewrite the last step of the bruck
> algorithm to avoid this problem, and without affecting the overall
> performance.

Totally agree. The vast majority of OpenMPI stuff uses memcpy fine. It
would just be a local bug fix. Can I volunteer?

Regards,
Simon
>
> Thanks,
> George.
>
> On Jan 30, 2008, at 9:41 AM, Number Cruncher wrote:
>
>> I'm getting many "Source and destination overlap in memcpy" errors when
>> running my application on an odd number of procs.
>>
>> I believe this is because the Allgather collective is using Bruck's
>> algorithm and doing a shift on the buffer as a finalisation step
>> (coll_tuned_allgather.c):
>>
>> tmprecv = (char*) rbuf;
>> tmpsend = (char*) rbuf + (size - rank) * rcount * rext;
>>
>> err = ompi_ddt_copy_content_same_ddt(rdtype, rank * rcount,
>> tmprecv, tmpsend);
>>
>> Unfortunately ompi_ddt_copy_content_same_ddt does a memcpy, instead of
>> the memmove which is needed here. For this buffer-left-shift, any
>> forward-copying memcpy should actually be OK as it won't overwrite
>> itself during the copy, but this violates the precondition of memcpy and
>> may break for some implementations.
>>
>> I think this issue was dismissed too lightly previously:
>> http://www.open-mpi.org/community/lists/users/2007/08/3873.php
>>
>> Thanks,
>> Simon
>>
>>
>> _______________________________________________
>> users mailing list
>> users_at_[hidden]
>> http://www.open-mpi.org/mailman/listinfo.cgi/users
>
>
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