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Date: Sat, 5 Sep 2015 15:48:23 +0300
From: Solar Designer <solar@...nwall.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] Low Argon2 performance in L3 cache

Bill,

We're mostly in agreement, except:

On Sat, Sep 05, 2015 at 05:07:39AM -0700, Bill Cox wrote:
> On Fri, Sep 4, 2015 at 5:11 PM, Solar Designer <solar@...nwall.com> wrote:
> > Can you tune Argon2d and TwoCats for same defensive throughput per CPU
> > chip (with multiple independent concurrent instances), rather than for
> > same defensive latency, for a comparison like this?
> 
> This would be ideal, in that having only password hashing running on a CPU
> solves problems related to how these algorithms do not work and play well
> with other services on the same CPU.

That's not what I meant.  While I suggest benchmarking per CPU chip,
this doesn't mean that deployments will necessarily have to dedicate CPU
chips to the task.  Sharing of CPUs with other authentication service
functionality is fine, and is in fact preferable over introducing an
extra network connection between authentication and hashing services.
Sharing with unrelated services isn't great from a security standpoint,
but is OK from resource usage standpoint.

You seem to imply that sharing with other unrelated processing will
result in a substantial efficiency drop for the hashing, but I doubt it
will, not even when you tune for targeting L3 cache rather than RAM
bandwidth.  Few other tasks are nearly as bandwidth demanding and have
similar locality of reference.  So even if you see substantial
efficiency drop when exceeding 3x 4 MiB concurrent hashes on your 12 MiB
L3 cache CPU, this doesn't mean there would be as significant an
efficiency drop for the hashing when running other unrelated tasks
concurrently.  And for yescrypt's defaults, which assume that the memory
is primarily RAM and not L3 cache, the impact of other tasks' cache
thrashing would be even less.  And the hashing's impact on other tasks
is small too.  We've been through this discussion in here already.

Another aspect is that during a request rate spike such that password
hashing becomes the primary bottleneck most of the CPU time would in
fact be spent on the password hashing, by definition of it being the
primary bottleneck.  And it's target request rate capacity in a scenario
like this that will determine the password hashing scheme choice and
cost settings.  1 vs. 2 milliseconds latency when using 1 CPU core is
mostly irrelevant, regardless of whether the (multi-core) CPU is shared
with other tasks or not.

Alexander

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