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Date: Tue, 9 Mar 2010 17:37:09 -0700
From: RB <aoz.syn@...il.com>
To: john-users@...ts.openwall.com
Subject: Re: Is JTR MPIrun can be optimized for more cores ?
On Tue, Mar 9, 2010 at 12:50, websiteaccess@...il.com
<websiteaccess@...il.com> wrote:
> Why I get only 1558 more hashes cracked with 8 cores than 2 cores ???
Because of the way the current MPI implementation splits the work. If
you look at the log for an --incremental run, you'll notice JtR is
splitting the password space up into 'passes' - length@...th. Each of
those passes is incrementally larger (harder) than the last. The
current MPI implementation splits work by these passes: the first
process takes the first pass, the 2nd the 2nd pass, and so on. When
they complete a pass, they leapfrog to the next pass in their order -
on a 4-processor run, the 2nd process will do passes 2, 6, and 10; the
3rd will do 3, 7, and 11, and so on.
What this really means is that it attacks the harder passes sooner,
before the easy passes are done. For every additional processor you
add, you will see diminishing returns in hashes per minute, but more
of the harder passwords will be cracked sooner.
> Is it possible to optimise MPIrun to work better with 4 cores and much
> much better with 8 cores ?
Yes, but understand your expectations are skewed because you didn't
know how the work was being split. Keep in mind the original
implementation was a single person's university project, and was
probably designed more for completion than perfection. I don't have
the time to even maintain it (thanks, Magnum!), much less improve it.
> Otherwise, why use 8 cores if I get only few hashes cracked more ?
Because you're doing 8x the work - the gain is not linear (thanks to
MPI overhead), but it is sufficiently so and measured in c/s, not
cracked hashes per minute. A given pass will still take the same
amount of time on a given core, but several passes are being done at
once.
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