Date: Fri, 3 Aug 2012 00:18:34 +0800 From: myrice <qqlddg@...il.com> To: john-dev@...ts.openwall.com Subject: Re: Result of hard core password generation on 7970 On Thu, Jul 26, 2012 at 1:05 AM, Bit Weasil <bitweasil@...il.com> wrote: > I do not know if my previous email made it through. I got an error from the > mailing list server regarding a disk space error. > > Apologies if it goes through twice. > > The GPU is not a CPU - you cannot treat it like one! You cannot safely > treat it as "a bunch of CPUs running in parallel" - this leads to memory > contention. It must be coded as a very wide vector engine. > > The "hard coded" MD5 kernel is written like CPU code, not like GPU code. > > I see no use of local memory. This is very bad. Global memory is high > bandwidth, but very high latency. > > You're doing a linear search through the main global memory to check > passwords, as far as I can tell. From each thread. On many AMD GPUs, this > does not broadcast the read (I believe nVidia will, at least on newer GPUs). > Thanks for your advice. I think NVidia could do broadcast but not sure about AMD GPUs. > Further, there's not a lookup bitmap in sight. They're used for very good > reasons, and are absolutely critical for good performance on the GPU. I'm > using a 3 layer bitmap system (local, global-but-cached, > global-and-not-cached) to make sure I only do a binary search through the > sorted hashes if there's a very high probability of finding the hash. A > walk through global memory space from one thread is incredibly expensive. > Yes, this is only first version of GPU password comparison. I am implemting bitmap on GPU now and try to use local memory. Thanks myrice
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