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Date: Sun, 12 Jan 2014 11:36:38 +0400
From: Solar Designer <solar@...nwall.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] escrypt memory access speed (Re: [PHC] Reworked KDF available on github for feedback: NOELKDF)

On Sat, Jan 11, 2014 at 08:47:07PM -0800, Andy Lutomirski wrote:
> To be clear, I was thinking about the possibility of hammering cache
> and main memory at the same time.

We're playing with that, but it's tricky.

> Imagine some matrix multiplies or
> other cache-heavy operations needed to select the next memory index to
> access.  If you could get 30 or 40 GB/sec and, say, 100GB/sec to L2,
> you'd probably be doing even better.  (No clue whether this is
> possible -- I'm not at all sure whether modern CPUs can sustain that
> much bandwidth anywhere.  Something that depends on low-latency access
> to a block of cache might be a better bet.)

Modern CPUs can sustain that bandwidth and more - I am getting ~400 GB/s
cumulative for sequential reads from L3 cache on the 2x E5-2670 machine
(it got 20+20 MB L3 cache in the two CPUs), with 32 threads each reading
from its own region.  However, getting a KDF to use that is not trivial.

For example, making only one change relative to the 50 GB/s escrypt with
Salsa20/1 at 2 GB (one of the examples I posted recently) - reducing
scrypt's N so that the memory usage is only 32 MB - surprisingly results
in the speed dropping to 10 GB/s.  Having many threads running suddenly
provides little or no speedup over 1 thread (it's around 10 GB/s either
way).  I think this has to do with the way L3 cache is structured: split
between the two CPU sockets and having 2.5 MB sub-regions close to each
CPU core.

Running many independent instances of escrypt with p=1 works fine
(providing slight increase in speed as we reduce memory usage, with no
unexpected performance drops), but does not (yet) reach cumulative
speeds in excess of RAM bandwidth (even though on the sequential read
test the L3 cache is much faster, as I mentioned above).

So it appears that each thread needs to use its own portion of the cache
(or we can have p=1, in which case the issue does not arise).  Then
simultaneous use of cache and RAM may be beneficial.  In escrypt, we
currently only have something like this when there's a "ROM" (such as in
RAM on 2 MB pages in SysV shared memory) and also a smaller "RAM".  One
of the specific size combinations we're using for testing on this
machine is 112 GiB ROM (shared) + 1.75 MiB RAM (per instance).  With 32
concurrent instances (each with p=1, since we're talking mass user
authentication where the parallelism will come from nearly concurrent
authentication attempts), this works reasonably well, but unfortunately
does not (yet) benefit from L3 cache's higher bandwidth potential as
much as I think it could (given that most of the 1.75*32 = 56 MB can fit
in the 40 MB of L3 cache).  There's room for improvement, but it gets
tricky, and even if we come up with some approach that provides good
results I'm afraid we won't like the added complexity.

So we're already making use of two layers in the memory subsystem, via
having "ROM" and "RAM", and we intend to add greater dependency on nice
properties of the L1 cache (ability to access smaller than cache line
pieces of data efficiently and with low latency).  That will be three
layers when a "ROM" is present, and two layers when not.  Adding another
layer (such as to better use L2/L3 caches when there's no "ROM") might
not be worth the complexity.

Alexander

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