Date: Thu, 26 Mar 2015 12:00:01 +0300
From: Solar Designer <solar@...nwall.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] Another PHC candidates "mechanical" tests (ROUND2)

On Wed, Mar 25, 2015 at 03:40:02PM -0700, Bill Cox wrote:
> I know that many people think, "but I need those other cores available for
> other processes".  I'm afraid that in reality, these SSE optimized memory
> hashing algorithms are so fast, other tasks get their cache data completely
> flushed.  This is one reason I feel it will be hard in practice to mount a
> cache-timing attack against Lyra2 or Yescrypt.  By the time the attackers
> thread is allowed to run, there's nothing left in L3 cache at all.  Giving
> those other cores to other tasks will just cause them to run 10X slower
> than usual due to constant cache misses, slowing down hashing, and causing
> everything to take much longer.  This is a real effect I've measured with
> sse-optimized Scrypt.

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.

Maybe we can create a synthetic benchmark showing that (deliberately hit
the same cache tags in tight loops?), but for most real-world software
the slowdown caused by another program thrashing the caches (especially
if it's just L3+ on a given CPU, with per-core L2's) is much lower.  I'd
expect it's less than 2x.  Possibly way less, e.g. a mere 10% would be
realistic for some real-world software if you try to isolate this effect.

Here's a test you can run.  On a machine with 4 cores, 8 threads, do
e.g. a Linux kernel or a gcc build first with "time make -j8".  Then do
two such builds with two concurrent "time make -j4".  Repeat this once
more if you like, treating the previous run as warm-up of the two trees.
Finally, replace one of those -j4 builds with a SIMD-optimized
memory-hard hashing scheme using 4 threads (or better yet, with a
benchmark running 4 unrelated threads, such as yescrypt's "userom", to
ensure there won't be synchronization delays).  See how much the
remaining -j4 benchmark slows down, if at all, compared to the two
concurrent -j4's run.  I'd expect the difference to be in the 0.7x
(speedup) to 2x (slowdown) range, and most likely 1.1x or so.  (I can
explain why speedup is also possible.)  That's just my gut feeling.

Here's a thought experiment: on the above machine, if you run two
concurrent 4-thread instances of a hashing scheme, how much slower would
each one of them run than if it were run on an otherwise idle machine
(assume no CPU clock rate scaling)?

To put it differently, won't e.g. TwoCats at 8 threads run at least as
fast as it does at 4 threads?  If it remains same speed (as in: same
total running time for same memory usage), then the slowdown of each of
its "4-thread halves" (any groups of 4 threads that we can choose for
this experiment) is exactly 2x.  If it becomes slightly faster, then the
slowdown is less than 2x.  And just why would the impact on typical
real-world software be greater?

... Oh, I recall you actually reporting some slight slowdown with more
threads for TwoCats.  Maybe TwoCats is that extreme.  Maybe it actually
causes a slightly higher than 2x slowdown for the other 4-thread TwoCats
in this experiment.  Why would it, though?  For 4 vs. 2 threads, this
could be via (non-)turbo.  It could also be via L1 cache thrashing when
going 2 threads/core (IIRC, you use 16 KB/thread?)  I doubt L3 cache
thrashing is why.

This is not the case for yescrypt, in my testing.  yescrypt at 8 threads
is generally slightly faster than at 4, on currently typical machines.

Alexander

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