Date: Fri, 14 Aug 2015 16:31:33 +0300 From: Solar Designer <solar@...nwall.com> To: john-dev@...ts.openwall.com Subject: Re: PHC: Argon2 on GPU On Thu, Aug 13, 2015 at 12:28:57AM +0200, magnum wrote: > On 2015-08-12 23:51, Solar Designer wrote: > >magnum, do you have an explanation why the best benchmark result during > >auto-tuning is usually substantially different from the final benchmark > >in most of Agnieszka's formats? I'm fine with eventually dismissing it > >as "hard to achieve" and "cosmetic anyway", but I'd like to understand > >the cause first. Thanks! > > Generally a mismatch could be caused by using different [cost] test > vectors in auto-tune than the ones benchmarked, or auto-tune using just > one repeated plaintext in a format where length matters for speed (eg. > RAR), or something along those lines. > > Another reason would be incorrect setup of autotune for split kernels. > For example, if auto-tune thinks we're going to call a split kernel 500 > times but the real run does it 1000 times, we'll see inflated figures > from autotune. > > A third reason (seen in early WPA-PSK) is when crypt_all() does > significant post-processing on CPU where auto-tune doesn't. At least the first reason you listed may likely result in suboptimal auto-tuning. Perhaps it wouldn't with simple iterated schemes like PBKDF2, but with memory-hard schemes like Argon2 the cost settings do affect optimal LWS and GWS substantially. So we shouldn't dismiss this without understanding of what exactly is going on in a given case. Alexander
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