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Date: Sat, 4 Jan 2014 12:19:10 +0400
From: Solar Designer <>
Subject: Re: [PHC] Initial hashing function. Feedback welcome

On Thu, Jan 02, 2014 at 09:44:20AM -0500, Bill Cox wrote:
> I've done some hand optimization, and gotten the speed to be only about
> 2.2X slower than memmove when copying 2GB over itself.  To get the speed, I
> stopped reading randomly from the "from page", and read it linearly
> instead.  This seemed to help the optimizer quite a bit

I think what helped the optimizer is your addition of lots of
parallelism (way too much for current CPUs, in fact).  The sequential
nature of reads allows PAGE_LENGTH to be beyond L1 data cache size.

I think it'd be better to have random reads, keep PAGE_LENGTH*3*2 within
L1 data cache size, and have just enough parallelism for current and
near-future CPUs (preferably, have it tunable).  Also, make it SIMD
friendly (tricky with random reads - would need to make them the size of
a SIMD vector, so that you don't require gather loads).


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