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Date: Sun, 9 Mar 2014 09:52:56 +0400
From: Solar Designer <>
Subject: ROM-on-SSD (Re: [PHC] escrypt 0.3.1)

On Wed, Mar 05, 2014 at 04:44:04AM +0400, Solar Designer wrote:
> - ROM-on-SSD support.  See PERFORMANCE-SSD for a usage example and some
> performance figures.
> For ROM-on-SSD, support for read-ahead may need to be added, although

Read-ahead was a wrong term to use here.  I meant computing the index in
advance, to allow for prefetch.

> we've achieved reasonable results even without it (with many hardware
> threads sharing one SSD).

One thing I didn't test before posting the above was running more
threads than are supported in hardware.  With ROM-on-SSD, we incur
context switches anyway, so there's no reason to expect exactly matching
the hardware thread count to be optimal.

I tested this now, and there's good speedup with higher thread counts.
Specifically, this result from PERFORMANCE-SSD:

$ ./userom 64 16 rom64.dat
r=512 N=2^8 NROM=2^20
Will use 67108864.00 KiB ROM
         16384.00 KiB RAM
ROM access frequency mask: 0xe
Benchmarking 1 thread ...
43 c/s real, 65 c/s virtual (63 hashes in 1.45 seconds)
Benchmarking 8 threads ...
180 c/s real, 37 c/s virtual (189 hashes in 1.05 seconds)

improves to:

$ OMP_NUM_THREADS=24 ./userom 64 16 ../rom64.dat
r=512 N=2^8 NROM=2^20
Will use 67108864.00 KiB ROM
         16384.00 KiB RAM
ROM access frequency mask: 0xe
Benchmarking 1 thread ...
42 c/s real, 64 c/s virtual (63 hashes in 1.49 seconds)
Benchmarking 24 threads ...
215 c/s real, 29 c/s virtual (441 hashes in 2.05 seconds)

(same code revision, same ROM).

This is 19% higher performance with 24 threads than with 8 threads (on a
CPU supporting only 8 hardware threads).  In terms of bandwidth, this
corresponds to 14.0 GB/s from RAM and 225 MB/s from SSD.  Relative to an
SSD-less, RAM-only run (also 16 MiB RAM/hash) on the same machine, this
is 89% of the c/s rate and 86% of RAM bandwidth usage.

I think these results are good enough as-is that advance availability of
the lookup index (and implementation of prefetch) is not worth adding.
The intended use case for ROM-on-SSD is authentication servers, where
the cost settings are limited by what happens at high concurrency -
which is precisely what's optimal for this approach to ROM-on-SSD.
Using e.g. 3 times more RAM for optimal performance is not a problem,
and may actually be an advantage (an attacker with lots of SSDs in a
machine would also need to provide as much RAM per SSD to achieve
similar efficiency).  So we have a good match here.

The SSD read speed may be improved by about a factor of 2 (so to 450 MB/s
for this SSD) by using a much larger block size (than the 64 KiB used in
the tests above), but then there would be less of a dependency on this
being an SSD rather than a HDD (or an array of HDDs).  I ran such tests
as well, and did reach ~450 MB/s from SSD in escrypt, but I dislike
relaxing that dependency on a local SSD (vs. HDD or storage in a distant
network location).  Also, with larger block size the number of random
lookups per hash computed becomes too low.  Anyway, this sort of tuning
is possible, and a decision may be made for each deployment separately.

> Support for simultaneous use of multiple ROMs (with different access
> frequencies) may need to be added, so that when using ROM-on-SSD it is
> possible to also use a ROM-in-RAM.  (Alternatively, the APIs may be
> simplified assuming that such support would never be added.)

Any comments on this?

> Alternatively, ROM-on-SSD may be considered too exotic, and
> simplifications may be made by excluding support for adjusting ROM
> access frequency.

And this?


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