phc-discussions - tunable low-level parallelism

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Date: Tue, 14 Apr 2015 23:40:29 +0300
From: Solar Designer <solar@...nwall.com>
To: discussions@...sword-hashing.net
Subject: tunable low-level parallelism

On the panel list, I was asked "what is a reasonable default, and how
often do we have to update it?" referring to tunable instruction-level
parallelism, such as yescrypt pwxform's settings.  Here's my answer:

The default may be provided by something like a yescrypt_default_flags()
function, which may differ between implementations (but will try not to
use too many different settings) - e.g., yescrypt-opt (scalar) may
currently use a single 64-bit lane (to achieve bcrypt-like anti-GPU even
on 32-bit systems), and yescrypt-simd use the current default of 512-bit
as 4x 128-bit SIMD lanes.  AVX2 should run this reasonably, merging
pairs of 128-bit S-box lookups.  (BTW, I previously experimented with
such merging of 64-bit S-box lookups on SSE*/AVX, with good results, but
opted to make 128-bit lookups the default.)  MIC and AVX-512 will have
to merge four 128-bit S-box lookups, which is significantly suboptimal
yet isn't that bad, along with other fully 512-bit processing (for the
MUL, ADD, XOR sequences).  A 512-bit optimized setting (using 512-bit
S-box lookups as well) may be introduced (obviously, it'd hurt anti-GPU
on pre-Haswell CPUs significantly, which is why I don't do it yet).
As to needing more than 512-bit parallelism on currently anticipated
CPUs, I am not sure - with 4-way SMT, perhaps 512-bit will remain
sufficient.  Another factor is L1 data cache size, which is currently
expected to remain at 32 KB, so only accommodating 4 threads * 8 KB
S-boxes anyway.  This will take some testing.

To summarize: two parallelism settings right now (64-bit, and 512-bit
with 128-bit S-box lookups), and a third one in the foreseeable future
(either full 512-bit, or something like 1024-bit with 512-bit S-boxes,
whichever turns out to be optimal on actual AVX-512 CPUs).

After that, update every 10 years or so.

Note that compatibility (both backward and forward) may easily be
maintained, with fallbacks to generic code for settings that have no
optimized code written for them yet or for which such code has already
been dropped.  Generic code for BlockMix_pwxform is quite simple.

The actual settings used will need to be encoded along with hashes, or
along with encrypted filesystems, etc., just like m_cost and t_cost.

Alexander