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Date: Sun, 4 Sep 2016 21:35:16 +0200
From: Sebastian Heyn <>
Subject: Re: Question regarding CPU load on GPU tasks

Thank you for clarification. I think that means for now: a new psu and 
graphcics card. Excellent!

Am 02.09.2016 um 19:20 schrieb magnum:
> 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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