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Date: Wed, 16 Sep 2015 01:20:14 +0300
From: Solar Designer <solar@...nwall.com>
To: john-dev@...ts.openwall.com
Subject: Re: Judy array

On Tue, Sep 15, 2015 at 11:29:10PM +0200, magnum wrote:
> On 2015-09-15 21:28, Solar Designer wrote:
> >On Tue, Sep 15, 2015 at 09:53:41PM +0300, Solar Designer wrote:
> >>We should be able to avoid the ldr_remove_marked() delay by having it
> >>skip processing when nothing had been marked for removal.  (I am testing
> >>with an initially empty john.pot.)
> >
> >Implemented in the attached patch.  It is possible to do better, such as
> >with per-salt flags.
> >
> >Not much change overall, but anyway:
> >
> >real    1m1.413s
> >user    4m48.603s
> >sys     0m21.961s
> 
> So should I commit it?

Yes, please.

Meanwhile, I also tried reducing PASSWORD_HASH_SHR from 2 to 1:

real    0m58.804s
user    4m26.433s
sys     0m18.934s

and to 0:

real    0m57.022s
user    4m15.630s
sys     0m16.348s

With 0, memory usage by each process at start of cracking is higher by
768 MB (as compared to 2, growing from a little over 2 GB to about 3 GB),
but their total RAM usage by end of the 1-minute run is lower than it
was with 2 (around 7.5 GB instead of around 8.5 GB).  I actually expected
this effect: with a larger hash table, when a password gets cracked,
there's less need for writes to pages holding "struct db_password" and
other things referenced from there.  And overall even at 768 MB the hash
table remains smaller than those other allocations combined.

I am not sure if we should bump up the default.  Perhaps on 64-bit only?
Or maybe we need to introduce a larger bitmap size first or instead (and
then keep SHR=2 or use even larger than 2 relative to that), or maybe
Fred will contribute something even better soon.  Adjusting the default
is easy, so we may as well just do it for 64-bit for now, then revise.

Alexander

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