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Date: Thu, 13 Aug 2015 00:28:57 +0200
From: magnum <>
Subject: Re: PHC: Argon2 on GPU

On 2015-08-12 23:51, Solar Designer wrote:
> On Wed, Aug 12, 2015 at 11:45:35PM +0200, magnum wrote:
>> On 2015-08-12 18:32, Agnieszka Bielec wrote:
>>> gws:      1024        3447 c/s        3447 rounds/s 297.022ms per
>>> crypt_all()+
>>> Local worksize (LWS) 64, global worksize (GWS) 1024
>>> using different password for benchmarking
>>> DONE
>>> Speed for cost 1 (t) of 1, cost 2 (m) of 1500, cost 3 (l) of 1
>>> Many salts:     2925 c/s real, 307200 c/s virtual
>>> Only one salt:  2898 c/s real, 307200 c/s virtual
>> The benchmark figures (last two lines) are the correct ones. If you set
>> up auto-tune correctly, that speed should be similar to the benchmark.
>> For some formats/situations this is hard to achieve and it's just
>> cosmetic anyway.
> 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.


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