Date: Thu, 18 Oct 2012 10:17:05 +0400 From: Solar Designer <solar@...nwall.com> To: john-dev@...ts.openwall.com Subject: Re: bitslice DES on GPU On Tue, Oct 16, 2012 at 10:17:37PM +0530, Sayantan Datta wrote: > I was comparing the statistics of DES_bs_kernel vs the pbkdf2_kernel. The > prime reason for the bottleneck seems to be insufficient number of inflight > wavefronts causing poor ALU utilization. For comparison the ALU utilization > of pbkdf2 is 3 times that of des. Also there are some other factors such as > LDS bank conflicts etc. We're looking for the cause of a much more than a factor of 3 performance hit. Besides, if you reduce pbkdf2_kernel's number of in-flight wavefronts by a factor of 3, the slowdown will probably be less than a factor of 3 (maybe a lot less). So I think there's something else as well, maybe something more important. > Also is there any specific reason for doing 32 hashes per kernel? You mean per work-item? Yes: that's the bit width of SIMD vector elements in GPU hardware. With bitslicing, we have to match this width, or we'd be wasting it. > Can it be > lowered to something like 8 so that the size of the data block could be > reduced to 16 integers. With bitslicing, our 64 data bits - for one hash - are spread across 64 different array elements (but are in the same bit layer). A more compact representation would mean that we implement something other than bitslicing. > If we could do so, would it reduce the size of K ? It would imply something other than bitslicing for K as well. And the S-boxes would be represented differently, too. But I think you should stop thinking in this direction. I don't expect there's another useful representation inbetween bitslicing and straightforward table lookups. > Also is it possible to reduce the number of regs used by the sboxes ? Only a little bit - by tuning those #define's. Reducing the number of regs needed for temporaries was definitely among the selection criteria for the S-box expressions when Roman and I worked on this last year. (There were thousands of other versions that would require more regs.) Alexander
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