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Date: Thu, 26 Mar 2015 06:33:54 -0700
From: Bill Cox <waywardgeek@...il.com>
To: "discussions@...sword-hashing.net" <discussions@...sword-hashing.net>
Subject: Re: [PHC] Another PHC candidates "mechanical" tests (ROUND2)

On Thu, Mar 26, 2015 at 2:00 AM, Solar Designer <solar@...nwall.com> wrote:

> The 10x is a huge exaggeration.  I won't believe you when you say you
> measured this on a currently typical machine unless and until you show
> specific numbers confirming it.  There must have been an error or
> something special about your measurements.
>

10X is exaggerated, but not by much.  Results from my laptop this morning
say that my "worker' threads are slowed down by up to 4.97X when TwoCats is
running.  I was off by a factor of 2X :-)

I wrote up a simple "testwork" program that runs "workers" in parallel with
TwoCats hashing.  The worker threads do non-SIMD read/write to L3 cache in
a loop, and increment a counter once they've done it all.  They do some
multiplies and adds in each loop iteration.

The worst case is when I have only 1 worker, using 4MiB of memory (my L3
cache size on my laptop), while TwoCats uses 2 threads to hash 4MiB.  The
workers slow down by less each when I add more workers.  With 2 workers,
the slow-down is less than 2X for the workers.  Here's my run output:

waywardgeek@...wardgeek-glaptop:~/projects/twocats/twocats$ time ./testwork
1 12 2 0
Total work: 169

real 0m1.001s
user 0m1.000s
sys 0m0.000s
waywardgeek@...wardgeek-glaptop:~/projects/twocats/twocats$ time ./testwork
1 12 2 1
Total work: 34

real 0m1.001s
user 0m2.554s
sys 0m0.192s
waywardgeek@...wardgeek-glaptop:~/projects/twocats/twocats$ time ./testwork
2 12 2 1
Total work: 118

real 0m1.003s
user 0m2.780s
sys 0m0.104s
waywardgeek@...wardgeek-glaptop:~/projects/twocats/twocats$ time ./testwork
2 12 2 0
Total work: 210

real 0m1.001s
user 0m1.991s
sys 0m0.004s
waywardgeek@...wardgeek-glaptop:~/projects/twocats/twocats$

Here's the inner loop of the worker threads:

for(uint32_t i = 0; i < len; i++) {
            mem[i] ^= (mem[(i*i*i*i) % len] + i) * (i | 1);
}

The actual "work" done does not have a huge impact on the outcome.  The
important thing is that the worker needs all of its L3 data.  I see use
cases in real life that suffer from this problem when running SSE-optimized
Scrypt.  This effect causes the cost of running Scrypt on servers to be
fairly optimistic compared to what we estimated it costs to use 1 core for
a given runtime.

This impact is not an issue when Scrypt is running by itself, or when
multiple copies are running in parallel on an authentication server.

Bill

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