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Subject: Re: [OMPI users] Message compression in OpenMPI
From: George Bosilca (bosilca_at_[hidden])
Date: 2008-04-24 11:06:45


Actually, even in this particular condition (over internet)1
compression make sense only for very specific data. The problem is
that usually the compression algorithm is very expensive if you want
to really get a interesting factor of size reduction. And there is the
tradeoff, what you save in terms of data transfer you lose in terms of
compression time. In other terms, the compression became interesting
in only 2 scenarios: you have a very congested network (really very
very congested) or you have a network with a limited bandwidth.

The algorithm use in the paper you cited is fast, but unfortunately
very specific for MPI_DOUBLE and only works if the data exhibit the
properties I cited in my previous email. The generic compression
algorithms are at least one order of magnitude slower. And then again,
one needs a very slow network in order to get any benefits from doing
the compression ... And of course slow networks is not exactly the
most common place where you will find MPI applications.

But as Jeff stated in his email, contributions are always welcomed :)

   george.

On Apr 24, 2008, at 8:26 AM, Tomas Ukkonen wrote:

> George Bosilca wrote:
>>
>> The paper you cited, while presenting a particular implementation
>> doesn't bring present any new ideas. The compression of the data
>> was studied for long time, and [unfortunately] it always came back
>> to the same result. In the general case, not worth the effort !
>>
>> Now of course, if one limit itself to very regular applications
>> (such as the one presented in the paper), where the matrices
>> involved in the computation are well conditioned (such as in the
>> paper), and if you only use MPI_DOUBLE (\cite{same_paper}), and
>> finally if you only expect to run over slow Ethernet (1Gbs)
>> (\cite{same_paper_again})... then yes one might get some benefit.
>>
> Yes, you are probably right that its not worth the effort in general
> and
> especially not in HPC environments where you have very fast network.
>
> But I can think of (rather important) special cases where it is
> important
>
> - non HPC environments with slow network: beowulf clusters and/or
> internet + normal PCs where you use existing workstations and
> network
> for computations.
> - communication/io-bound computations where you transfer
> large redundant datasets between nodes
>
> So it would be nice to be able to turn on the compression (for spefic
> communicators and/or data transfers) when you need it.
>
> --
> Tomas
>> george.
>>
>> On Apr 22, 2008, at 9:03 AM, Tomas Ukkonen wrote:
>>
>>> Hello
>>>
>>> I read from somewhere that OpenMPI supports
>>> some kind of data compression but I couldn't find
>>> any information about it.
>>>
>>> Is this true and how it can be used?
>>>
>>> Does anyone have any experiences about using it?
>>>
>>> Is it possible to use compression in just some
>>> subset of communications (communicator
>>> specific compression settings)?
>>>
>>> In our MPI application we are transferring large
>>> amounts of sparse/redundant data that compresses
>>> very well. Also my initial tests showed significant
>>> improvements in performance.
>>>
>>> There are also articles that suggest that compression
>>> should be used [1].
>>>
>>> [1] J. Ke, M. Burtcher and E. Speight.
>>> Runtime Compression of MPI Messages to Improve the
>>> Performance and Scalability of Parallel Applications.
>>>
>>>
>>> Thanks in advance,
>>> Tomas
>>>
>>> _______________________________________________
>>> users mailing list
>>> users_at_[hidden]
>>> http://www.open-mpi.org/mailman/listinfo.cgi/users
>>
>>
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