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Date: Tue, 6 Oct 2015 11:04:24 +0200
From: Massimo Del Zotto <massimodz8@...il.com>
To: Solar Designer <solar@...nwall.com>
Cc: discussions@...sword-hashing.net
Subject: Re: [PHC] yescrypt on GPU

Hello Alexander, hello all.

For the purpose of the discussion, let's call 'X' (from pwxform) a 'chunk'
of {PWXSimple|S_SIMD} consecutive ulongs.
I assume someone isn't proficient to OpenCL nomenclature so:
- Work Item (WI): element of a SIMD unit. Very often referred as 'thread'
(even though it really isn't).

The rationale is very simple: I have 64 WIs and I have to load (2+2)*4
ulongs ({PWXRounds|S_ROUNDS}=6 consecutive times).
Therefore, loading in two steps as I do gives me 8 ulongs from s0 across
the different chunks, then s1.
As the chunks are independently processed, the fact they are scattered
isn't very relevant in this context. I have those 64 work items and 64
bytes to load so there's no much choice in how much memory a WI can load:
they just map right.

As a side note, I was surprised to see unaligned loads as I understand they
have (had?) considerable performance implications on some CPUs... when
they're legal operations in the first place.

I might try loading ushorts and I have tried loading uints but the OpenCL
compiler emitted something... not very pretty. Loading uints was
considerably slower to me - what should I load instead? Those operations
get dispatched anyway.

I considered doing vload or even async load (meh)... but why would I do
that? Those tests are better performed by people with hardware to test such
as the cryptocurrency mining superstars. Sometimes my users send me perf
data but I'm not all that eager to test under arbitrary non-controllable
conditions.

There is indeed quite some repetition/waste. That's why AMD put a scalar
unit in their compute cluster. It was very typical for graphics, we used to
trash ALU power with no regret. There was a previous formulation where I
had all WIs loop with the same data but apparently the AMD compiler
couldn't figure it out. SALUBusy is around 2%, VALU Busy is up to 80% (I
speculate mostly filled with garbage). I expected something like 10%. We
gotta love those guys, they cannot even SALU the Salsa operations years
after GCN launch!

On coalescing: that's a good question. AMD claims GCN to have no coalescing
but they also claim the best access pattern is to have "stride 1 [bytes]"
across WIs so they probably mean they don't have 'coalescing' in a "VLIW
sense" as they likely required much more complicated machinery to figure
out. The memory usage I got out of the profiler looks odd to me even though
it seems to agree memory usage is sort of ok-ish. In case someone wants to
take a look, I might push everything to CSV.
I think it turned out a to be an interesting case.


Massimo

2015-10-05 21:18 GMT+02:00 Solar Designer <solar@...nwall.com>:

> On Thu, May 07, 2015 at 09:02:41PM +0300, Solar Designer wrote:
> > On Sat, May 02, 2015 at 05:34:27AM +0300, Solar Designer wrote:
> > > The yescrypt cryptocoin stuff is starting to pay off.  djm34 has just
> > > implemented support for BSTY mining on GPU, in both OpenCL and CUDA
> > > (with tiny bits of inline PTX assembly, even - for things such as the
> > > pwxform MULs):
> > >
> > > https://bitcointalk.org/index.php?topic=775289.msg11252741#msg11252741
> > >
> > > The code is still very dirty.  I expect it won't build or work for most
> > > people as-is, yet.  However, it looks reasonably well optimized, and
> > > specialized to the yescrypt settings that BSTY uses (e.g., loop counts
> > > are precomputed and hard-coded, etc.)
> >
> > Further in that thread, djm34 mentions getting 1.5 kh/s on GTX 980.  The
> > best speed another person reported so far is 980 h/s on GTX 750 Ti.
> >
> > > For comparison, a (much cheaper) quad-core CPU does ~3400 h/s.
>
> There's a new BSTY miner for GPUs:
>
> https://github.com/MaxDZ8/M8M/releases/tag/929
>
> https://github.com/MaxDZ8/M8M/commit/a500d4af0c96ee64344dd37797a176bf4cc2d370
>
> The author reports 372 h/s on "Radeon 7750 1 GiB GDDR5 core/mem 850/1200."
>
> The code looks moderately weird to me - there's an optimization(?) in
> M8M/kernels/yescrypt_difficultSmixes.cl: Block_pwxform() that I don't
> understand the rationale for: the 16-byte S-box lookups from global
> memory are split across 16 work-items, loading into local memory and
> followed with a local memory fence, before the values are finally used
> as pairs of ulong's.  I would sort of understand if these were uint's
> (fits the SIMT model and the local memory port width on GCN, so might
> help maximize global memory bandwidth usage, assuming these get
> coalesced for up to 16 bytes anyway), but why individual bytes?  And
> does the code that follows do 16x duplicate work then (this may be fine
> since we have that bottleneck anyway)?
>
> Massimo, did I possibly confuse you into doing that with the indices
> being byte-granular right after applying the mask?  The byte-granular
> offsets don't imply byte-granular loads - you can compute the address at
> byte granularity, yet perform wider loads, like my C code does.
>
> Alexander
>

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