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Date: Tue, 31 Dec 2013 03:31:07 +0000
From: Samuel Neves <>
Subject: Re: [PHC] Best RNG for filling memory?

On 31-12-2013 02:46, Solar Designer wrote:
> Will ChaCha8 get below 0.85 cpb per core or 1.70 cpb per thread with 2
> threads/core on SB? This is unclear - would need to test. Alexander 

The speeds reported in [1] are much higher than the ones in eBACS; I
don't know whether this is due to the measuring mechanism or the
implementation. They seem to be more in line with your OpenSSL results,
though: a good AES-CTR implementation should dispatch an AESENC at every
cycle (by running 8 parallel instances in SB), which will bottleneck on
Port5, shared competitively by both threads. That is, the effect of
running a good AES-CTR implementation on two threads should be similar
to that of running Chacha8.

The best Chacha implementations already pretty much max out the
available ILP. So AES-NI is likely, I suppose, to win on pre-Haswell
processors. On the other hand, my experiments put Chacha8 on Haswell at
0.5-0.6cpb in a single thread. AES-NI also gets a little faster on
Haswell, but by a small margin [1, slide 20].


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