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Date: Fri, 18 Apr 2014 14:34:26 -0400
From: Bill Cox <waywardgeek@...il.com>
To: discussions@...sword-hashing.net
Subject: Any other authores ready for benchmarking? (was [PHC] Lyra2 initial review)

Yesterday, I had some fun benchmarking Lyra2, TwoCats, and Yescrypt.  Are
there any other authors who would like their algorithms benchmarked?  I'd
be happy to start tabulating data on all the entries that are ready for
benchmarking.  After yesterday's hack-fest, I don't feel simply running the
PHS routines without the involvement of the author is likely to generate
useful data, so I'm only going to do benchmarking the authors request for
now.

We should discuss what to measure and how to report it.  Is there a better
place than posting it here?

I think we maybe we need three memory size categories, where PHC entries
can be benchmarked in any or all of them.  Basically, the 3 memory sizes
correspond to:

- L1 cache bound:  how about 8KiB?
- Cache bound: how about 8MiB?
- External DRAM bound: how about 2GiB?

Also, some are not memory bound, for PBKDF2-like entries.  It's best if we
stick to powers of two for memory size, as some entries require that.

The simplest thing to report is how long a algorithm takes to run to hash
for a given amount of memory, so I think we should report that.  However,
it needs to be interpreted based on what the algorithm is doing.  Assuming
the algorithm is linear in runtime for the range of memory in the category
(for example, heavily external DRAM bound), then we can can convert runtime
to now many MiB or GiB would be hashed in a 1 second runtime, by dividing
the memory hashed by the runtime.  This let's us compare the memory*time
cost for a 1 second runtime, and obviously higher is better.

An important modification to the GiB/s hashed is the expected "average"
memory usage for a 1 second run.  Several entries call malloc or a similar
function to allocate uninitialized memory, and in a first loop they fill
that memory.  Linux does not allocate physical memory to malloc-ed address
space until it is accessed, so memory usage increases roughly linearly for
the first memory filling loop.  TwoCats fills memory and is done, so it's
average memory usage is only 1/2 of the total.  Others mostly write to
memory more than once, and so they reach peak memory early in the
algorithm.  For example Scrypt would be expected to have around a 75%
average memory usage relative to the total used, since it fills memory
linearly in the first loop and hashes it once in the second loop.

Finally, there's memory bandwidth, which should be in terms of MiB or GiB
per second read/write to DRAM or cache.  Memory bandwidth is the primary
runtime defense for a lot of the entries, so having high bandwidth is very
important.  We can estimate memory bandwidth by the total memory hashed
divided by runtime, times the number of memory accesses on average.  Cache
adds a complication when estimating external DRAM bandwidth, but so far
we've been able to guestimate which reads are most likely cached, and
discount them for external DRAM bandwidth.  For example, for TwoCats, I
estimate each DRAM location is written on average once, and read on average
once, the same as Scrypt.

Are these the right metrics?

Bill

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