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Date: Sun, 10 Jul 2016 00:51:41 +0300
From: Solar Designer <solar@...nwall.com>
To: john-users@...ts.openwall.com
Subject: Re: Loading a large password hash file

On Sun, Jul 10, 2016 at 12:46:14AM +0300, Solar Designer wrote:
> perl -e 'use Digest::SHA1 qw(sha1_hex); for ($i = 0; $i < 200000000; $i++) { print sha1_hex($i), "\n"; }'
> 
> which is 8200000000 bytes.  On a machine with enough RAM, JtR loaded it
> in 6 minutes, and the running "john" process uses 13 GB.
[...]
> There's also the --save-memory option, which may actually speed things
> up when you don't have enough RAM.  But that's sub-optimal, and high
> memory saving levels may hurt cracking speed a lot.  They also hurt
> loading time when there would have been enough RAM to load the hashes
> without memory saving.  I've just tried --save-memory=2 on the 200M
> SHA-1's file, and it looks like it'll load in about 1 hour (instead of
> 6 minutes), consuming something like 11 GB.  So probably not worth it in
> this case.

My --save-memory=2 test completed loading in about 40 minutes, and the
"john" process uses a little over 9 GB.  So in this case it's a 7x
increase in loading time to save 30% of memory.  Cracking is about 5x
slower.  Usually not worth it, but if you had 12 GB RAM and didn't want
to split the input file in two, it could help.

Alexander

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