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Date: Tue, 6 Oct 2015 13:27:00 +0300
From: Solar Designer <solar@...nwall.com>
To: Massimo Del Zotto <massimodz8@...il.com>
Cc: discussions@...sword-hashing.net
Subject: Re: [PHC] yescrypt on GPU

Hi Massimo,

Thank you for your work on this, and for your comments.

On Tue, Oct 06, 2015 at 11:04:24AM +0200, Massimo Del Zotto wrote:
> 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.

Yes, but why waste all 64 work-items on one yescrypt hash computation?
You could as well load in uint's rather than bytes, and have 2 or 4 (or
more) yescrypt's across those 64 WIs, no?  (And you could also spend
fewer WIs per yescrypt instance, for up to the obvious 64 yescrypt's in
64 WIs, in which case you wouldn't need any communication via local
memory, but that's not necessarily more optimal.)

> 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.

Do you see unaligned loads somewhere in yescrypt?  There shouldn't be any.

> 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?

When you tried uints, did you adjust your use of the work-items
accordingly, so that you'd have more instances of yescrypt in the 64?

> Those operations get dispatched anyway.

Yes, but you're not limited to computing one yescrypt hash in there.

In fact, it doesn't make any sense to waste an entire CU on one hash,
yet keep the S-boxes in global memory.  You have 64 KB of local memory
per CU, enough for several yescrypt's.  Have you tried keeping the
S-boxes in local memory?

(For bcrypt, which is in many ways similar to pwxform, but uses 4 KB
S-boxes, it is more optimal to keep them in local memory on GCN.)

I think you happen to achieve decent performance (compared to other
results for yescrypt on GPU) at all because the S-boxes are actually
loaded from the same CU's cache.  Well, the cache is 16 KB (and there's
also L2) - enough for the 8 KB S-boxes - but you certainly can do better
by simply keeping the S-boxes in local memory.

> 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

What makes you think it's "1 [bytes]"?  That would be weird.

Alexander

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