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Date: Mon, 3 Sep 2012 23:12:18 -0400 (EDT)
From: jfoug@....net
To: john-dev@...ts.openwall.com
Subject: RE: memory usage (was: [john-users] JTR against 135
millions MD5 hashes)
Thank you for the long and insightful write up.
On Mon, Sep 3, 2012 at 8:49 PM, Solar Designer wrote:
> Jim -
>
> Thank you for sharing your thoughts on this.
>
> On Mon, Sep 03, 2012 at 11:16:44AM -0400, jfoug@....net wrote:
>> The memory reduction due to the source() function, and also due to
>> removing some wasteful allocations within the prepare() and valid()
>> within dynamic. I believe the wasteful allocations in dynamic have
>> been removed, all the way back to the J7-RC. The memory
>> requirements were greatly reduced early on, but have slowly started
>> to be lost some (the bitmaps). However, total mem usage is still
>> much less than it was, when we were keeping the full source buffer
>> for each hash.
>
> I did not realize that there were wasteful allocations in prepare()
> and
> valid(). Weren't they temporary?
They were done using alloc_memory_tiny. Now, I simply use static
buffers. When raw hashes were being used, and a few other cases, there
could be one or more of these type allocations, which are very similar
to memory leaks, when done on temp transient data.
> Another potential source of memory usage reduction are the alignments.
> For raw MD5 hashes, a 4-byte alignment should suffice (they're 4x4
> bytes), yet we were using 8-byte alignment on 64-bit builds.
Very good point. I had not even thought of things like these new
alignment requirements. I really think for most formats, 4 is the
correct alignment, due to dereference of an arch_word_32 in some of the
lookup functions.
I do not have time to fully address all of the issues brought up in your
reply, but will try to look more in depth at this tomorrow afternoon.
Jim.
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