linux-kernel - Re: [PATCH 06/13] async_tx: add support for asynchronous GF multiplication

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Date:	Thu, 19 Mar 2009 09:06:41 -0700
From:	"H. Peter Anvin" <h.peter.anvin@...el.com>
To:	Dan Williams <dan.j.williams@...el.com>
CC:	linux-raid@...r.kernel.org, linux-kernel@...r.kernel.org,
	neilb@...e.de, maciej.sosnowski@...el.com,
	Ilya Yanok <yanok@...raft.com>, Yuri Tikhonov <yur@...raft.com>
Subject: Re: [PATCH 06/13] async_tx: add support for asynchronous GF multiplication

Dan Williams wrote:
> + * @scfs: array of source coefficients used in GF-multiplication

Array of source coefficients?  Are you doing a vector-vector
multiplication here?

Given this code:

> 	for (d = 0; d < len; d++) {
> +		wq = wp = ptrs[0][d];
> +		for (z = 1; z < src_cnt; z++) {
> +			wd = ptrs[z][d];
> +			wp ^= wd;
> +			wq ^= raid6_gfmul[scfs[z]][wd];
> +		}

... it kinds of looks like that.

This is really quite expensive!  The whole point of the restore code
that exists is that we never do a two-dimensional lookup, instead
caching a pointer to the multiplication table that we intend to use,
because the RAID-6 code only ever contains scalar-vector multiplications.

I really don't get this, and I think it's broken.

	-hpa

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