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Message-ID: <20230423110803.GA17261@openwall.com>
Date: Sun, 23 Apr 2023 13:08:03 +0200
From: Solar Designer <solar@...nwall.com>
To: john-dev@...ts.openwall.com
Subject: Re: Birthday paradox
On Sun, Apr 23, 2023 at 12:28:27PM +0200, magnum wrote:
> Oh, I see now I need longer tests because of the speed involved. At
> 30Gp/s a nano-second timer indeed *is* low resolution. Still, the
> difference is very small. I now bumped a 2.5 second test to 25 seconds
> for the below. Difference still is small and shows "steps" in reported
> speed.
>
> Starting without a bitmap:
>
> $ for num in 256 ; do head -$num scraped-bare.in > test.$num.in && for k
> in 0 1 2 3 4 5 6 ; do rm -f ../run/john.pot && BLOOM_K=$k
> BITMAP_SIZE=$(($num*64)) ../run/john -form:nt-opencl -mask:?a -len=6
> test.$num.in -lws=512 -gws=34816 -v:1 || break 2 ; done ; rm
> test.$num.in ; done
>
> Starting with no bitmap (BLOOM_K=0 disables):
>
> Using default input encoding: UTF-8
> Loaded 256 password hashes with no different salts (NT-opencl [MD4 OpenCL])
> 256 hashes: Hash table in local memory (2468 B); no filter.
> Offset tbl 380 B, Hash tbl 2088 B, Results 3076 B, Dupe bmp 36 B, TOTAL
> on GPU: 5580 B
> LWS=256 GWS=34816 (136 blocks) x9025
> Press 'q' or Ctrl-C to abort, 'h' for help, almost any other key for status
> 25g 0:00:00:24 DONE (2023-04-23 12:04) 1.004g/s *30628Mp/s* 30628Mc/s
> 7190GC/s Dev#2:66°C util:95% aaUh}|..aaUh}|
> Remaining 231 password hashes with no different salts
This isn't 256 hashes. Maybe your other tests are not, either?
> Session completed.
> 735091890625 crypts, fp from bitmap: 1/4153061529 (0.00%), 177 hash
> table lookups
735091890625/24/10^6 = 30628.8
Looks like the low resolution timer is what we use for the figures on
the status line, and it has 1-second resolution in your tests. On new
sessions, we normally initially use clock ticks in that computation:
use_ticks = (c <= 0xffffffffU && !c_ehi && !status_restored_time);
However, in this case the speeds are so high that we disable this early.
I think we should switch to using floating-point inside that function so
that we do not have to explicitly lower precision to avoid overflows
(but rather this would happen implicitly in much finer-grained steps and
would maintain greater precision of the result). In other words, let's
continue to use wide integers (up to 96-bit like we use now) to count
things (so that any precision loss doesn't accumulate), but use "double"
within the rate calculations.
Alexander
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