Date: Fri, 2 Sep 2016 19:20:03 +0200 From: magnum <john.magnum@...hmail.com> To: john-users@...ts.openwall.com Subject: Re: Question regarding CPU load on GPU tasks On 2016-09-02 13:53, Sebastian Heyn wrote: > Now I noticed when I run a wpapsk-opencl vs a wordlist, the cpu load on a single core is 100% solid. I presume we're not talking about running OpenCL on CPU here. Are you using an nvidia GPU? A silly property of their OpenCL runtime is it busy-waits for GPU events. So it looks like it's pegging the CPU but in reality it does no work on it. > When I fork the task to 4, it is 4x100% (its a quad core cpu) - however the amount of passwords per second is the same. I dont know how to interpret that result. Is the cpu 100% flat out due to some loop in the code, or is it really 100% busy? If so the forking should show some improvement, right? For WPAPSK there's no point in forking a run with OpenCL formats (unless you have several GPU's - then you'll fork one process per GPU). If you really have 4 GPU's, it's the same answer as before: Busy-waits. No real bottleneck. > I am asking because I wonder what would happen if I bought a larger gpu (or 2) - would the cpu become a bottleneck? Not for this format anyway. It runs almost entirely on GPU(s). > Second question: Is openmpi also able to run gpu tasks on several machines? Sure. magnum
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