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Date: Sun, 20 Nov 2011 02:58:38 +0400
From: Solar Designer <solar@...nwall.com>
To: john-users@...ts.openwall.com
Subject: Re: OpenMP not using all threads

On Sat, Nov 19, 2011 at 04:29:12PM -0500, Stephen Reese wrote:
> I have patched john-1.7.8.tar.gz with john-1.7.8-omp-des-7.diff.gz in

-omp-des-7 is good if you want to attack just one salt or very few
salts.  For many salts, -omp-des-4 provides better performance.  (This
is mentioned on the wiki.)

Alternatively, if you feel adventurous, you may do a CVS checkout for
even newer code (currently known as development version 1.7.8.7), which
combines the best properties of these two patches into one source code
tree (no patches are needed).  The CVS checkout instructions are here:

http://www.openwall.com/Owl/DOWNLOAD.shtml

> order to utilize four threads from a E5520 on a Debian system but it

Actually, you should be running 8 threads on this CPU unless you have
Hyperthreading disabled.  But you don't need to worry about that - gcc's
libgomp will run as many threads as you have logical CPUs by default
(most likely 8).

> instead seem like it is only using two. When testing DES I see around
> 2500K c/s and when patched about 5000K. I was hoping for closer to
> 8000K to 10000K.

Yes, you should get about 9000K (for 8 threads combined).

The increase may be less than 4x because the thread-safe code is slower,
because the CPU clock rate is lower when all cores are in use (E5520 has
Turbo Boost), and for certain other reasons.  Yet you should in fact get
9000K or so.

> I also edited the Makefile as follows:
> 
> # gcc with OpenMP
> OMPFLAGS = -fopenmp -msse2
> 
> Another strangeness is when testing is I am not seeing the -16
> appending to the following:
> 
> Benchmarking: Traditional DES [128/128 BS SSE2]... DONE
> 
> Is this normal or did something go wrong.

It looks like you made a 32-bit build.  That is, you probably used the
linux-x86-sse2 make target instead of linux-x86-64.  When you build
without OpenMP, the -sse2 target uses assembly code supplied with JtR,
however when you go for OpenMP, gcc has to generate thread-safe code
instead.  It does this well for x86-64, but not for 32-bit x86 (there
are too few registers on 32-bit x86).

To get decent performance at DES with OpenMP builds on your machine, you
ought to make 64-bit builds.  And indeed your install of Debian should
be 64-bit, too.

I hope this helps.

Alexander

P.S. The Subject is almost certainly wrong - there's no indication that
the build doesn't use all threads.  Rather, the threads are slow.

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