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Date: Tue, 27 Feb 2018 21:14:57 +0100
From: Solar Designer <solar@...nwall.com>
To: john-users@...ts.openwall.com
Subject: Re: Getting full performance out of multiple GPU's

Hi Adam,

On Mon, Feb 26, 2018 at 01:41:34PM -0600, Adam Lininger wrote:
> I've got an 8-gpu cracking rig that I've inherited from someone who had it
> setup with hashcat. Apparently it worked well there.
> 
> As I prefer john, I've been trying to get it set up. I have it up and
> running, but the Utilization output from --list=opencl-devices hovers
> around 0-1%. Is there a way to improve this?
> 
> I'm running John the Ripper 1.8.0.12-jumbo-1-bleeding-6144b8f (freshly
> re-pulled and re-built today).

To use a GPU at all, you need to be invoking one of JtR "formats" with
"-opencl" in its name.  You do this with an option like e.g.
"--format=sha512crypt-opencl", but corresponding to whatever hash type
you're actually cracking.

And if it's one of so-called "fast hashes" - like raw MD5, and thus
"--format=raw-md5-opencl" - then you also need to use "--mask" (and
specify a mask suitable for your attack) in order to have candidate
passwords at least partially generated on GPU.  You may use this mask
mode along with another cracking mode, in which case you'd refer to that
other mode's generated partial candidate password as "?w" in the mask.

Then, as magnum has just pointed out in another thread, to use more than
one GPU you need to list all GPU device numbers and also specify at
least an equivalent number of "forks", like this: "--dev=0,1,2,3,4,5,6,7
--fork=8".  (We got to enhance JtR to print a warning when the specified
number of OpenCL devices is greater than the number of forks.)

Please post your current JtR command line and explain its resulting
behavior if you require further help, or just to share your experience
with the community.

Alexander

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