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Date: Mon, 19 Dec 2011 16:11:25 +0400
From: Solar Designer <solar@...nwall.com>
To: john-dev@...ts.openwall.com
Subject: Re: DES BS + OMP improvements, MPI direction

On Sun, Dec 18, 2011 at 01:29:02AM -0700, RB wrote:
> On Sat, Dec 17, 2011 at 15:46, Solar Designer <solar@...nwall.com> wrote:
> > Are you suggesting that we'd apply MPI at the same low level where we
> > currently apply OpenMP?  No, that doesn't sound like a good idea to me,
> > except maybe for the slowest hash/cipher types only (BF, MSCash2, RAR,
> > crypt when run against SHA-crypt), where it might be acceptable.
> 
> Yes, it is what I'm suggesting.  Obviously anything that completes in
> less than the RTT of the MPI environment would need to be handled
> differently (if at all), but the problem the MPI implementation has
> historically had is that it splits work at too high a level.  Last I
> checked it was splitting work based on individual work steps - each
> thread took a given length x count and worked it by itself on that.
> For very fast hashes (like LM) I'm sure that's more acceptable, but
> for slower ones and with larger MPI implementations (hundreds to
> thousands of threads), the network very quickly gets to the point that
> every thread is working on an effectively impossible-to-solve step,
> which really isn't the end-user's intent of parallelization.

I think you don't literally mean "every" above (as that would imply that
all easier "steps" were already completed, so everything is as desired),
but I get what you're referring to.

Yes, there's this drawback of the way the current MPI implementation
splits the work for incremental mode.  We need to address it.  However,
I think we should do it without incurring the drawbacks of OpenMP-like
low-level parallelization.

Alexander

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