Date: Wed, 23 Apr 2014 02:29:02 +0200 From: magnum <john.magnum@...hmail.com> To: john-users@...ts.openwall.com Subject: Re: recommended way to crack a descrypt hash in a multi-gpu setup (HD 7990) On 2014-04-23 01:30, yungai wrote: > in recent thread I read about the --fork feature in conjunction with > GPUs but simply trying --fork=2 didn't work for me (running a current > bleeding-jumbo git version). > > So I wanted to ask you what the recommended way to crack a hash with > multiple GPUs currently is? You need to supply a list of devices. Use --list=opencl-devices to see the device numbers: $ ../run/john --list=opencl-devices (...) Platform #2 name: NVIDIA CUDA Platform version: OpenCL 1.1 CUDA 4.2.1 Device #0 (5) name: GeForce GTX TITAN (...) The numbers you need are the ones between parens, in the above example it's 5. You probably have 0 and 1. So you'd do this: ./john -fork=2 -dev=0,1 -format=descrypt-opencl ... or just use the shortcut for "all GPUs": ./john -fork=2 -dev=GPU -format=descrypt-opencl ... > What kind of performance can one expect when cracking descrypt on a HD > 7990? I remember seeing benchmarks on john-dev(?) with ~102000 Kc/s but > I can't seem to find that benchmark anymore Performance has varied a lot over time with different driver versions. For a long period, AMD driver only got slower but I think the situation is better now. > I get on my HD 7990 (with device 0): > ~47000 Kc/s That would be the "many salts" benchmark figure. Attacking just one salt will be more like 36000Kc/s. > so that would be near the value I remembered if you multiply that by two > for the second GPU. Yes. You might be able to push it a little with some LWS/GWS tuning, sacrificing responsiveness. See doc/README-OPENCL magnum
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