phc-discussions - Re: [PHC] A review per day

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Date: Sat, 30 Aug 2014 09:05:23 -0400
From: Bill Cox <waywardgeek@...il.com>
To: "discussions@...sword-hashing.net" <discussions@...sword-hashing.net>
Subject: Re: [PHC] A review per day - TwoCats

On Sat, Aug 30, 2014 at 7:35 AM, Bill Cox <waywardgeek@...il.com> wrote:

> On Sat, Aug 30, 2014 at 12:50 AM, Solar Designer <solar@...nwall.com>
> wrote:
>
>> > How fast does bcrypt do random reads?
>>
>> According to that posting I referenced above, it's:
>>
>> 2176000*3072 = 6.7 billion/s per FX-8120 chip
>> 2176000*493 = 1.1 billion/s with 1 instance (with other cores idle)
>>
>> That's for defensive-use bcrypt code, which is what's relevant here.
>>
>> So your "2.8B 16-byte random reads per second on Ivy Bridge" is very
>> good if it's for 1 thread, but not good enough if it's for entire chip.
>> And you also need to compare the available parallelism vs. bcrypt's.
>>
>
> 2.8B reads/s was for 4 threads on my quad-core Ivy Bridge processor.  I
> tweaked the inner loop just a bit, and benchmarked it against the simplest
> loop I could write for doing unpredictable reads.  Here's my one thread
> numbers vs what is possible:
>

I must have made a mistake on the 2.8B reads/second.  The max possible with
no parallelism on 4 threads should be less than about 2.2B/s with no read
parallelism.  I also tested 8-way read parallelism.  It was 35% faster at
doing unpredictable reads than 4-way.

Except for the multiply, my inner loop is also very simple, and can easily
be done in an ASIC in one clock, probably at 3.4GHz in Intel's process,
just like bcrypt.  I need at least one multiply for good compute time
hardening.  Unfortunately, even 1 multiply increases the 1B pseudo-random
L1-cache read test from 2.3s to 3.6s!  That's not bad compute time
hardening, though, given that it does 2B serial 3-cycle multiplies, which
require 1.9s on an ASIC (in Intel's process, using Intel's multiplier).

By increasing my lane size and sub-block size to 256 bits, twice as much
work gets done serially in the CPU per multiply, and the runtime drops back
down to 2.4s, but with half the compute-time and unpredictable-read
hardening.

Bill

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