phc-discussions - Re: [PHC] avoiding cache thrashing

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Date: Wed, 19 Feb 2014 18:39:48 +0400
From: Solar Designer <solar@...nwall.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] avoiding cache thrashing

On Wed, Feb 19, 2014 at 06:25:24PM +0400, Solar Designer wrote:
> A drawback of this workaround is that it effectively halves our usable
> sizes of both caches.  What can we do about that?  Another workaround:
> once in a while, alter our choice of address bit that we split the two
> types of accesses on.
[...]
> amortize the cost of this AND in software, which is desirable since this
> cost is fully avoided in ASIC.

Correction: if we do alter the choice of our selector bit, then the mask
is not entirely constant (some of its bits are, some are not), so its
cost is no longer fully avoided in ASIC.  This is an additional slight
benefit to altering the choice (and to using as wide a range of selector
bits as practical).  (Although no longer being able to use an immediate
value for the mask might have a cost in software, too - depending on
architecture, available registers, specific CPU type.)

I care about these minor costs because random L1 accesses need to be
very frequent in order to match bcrypt's GPU unfriendliness.

Alexander