Message-ID: <49C26D91.10804@intel.com>
Date: Thu, 19 Mar 2009 09:06:41 -0700
From: "H. Peter Anvin" <h.peter.anvin@intel.com>
Organization: Intel Corporation
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To: Dan Williams <dan.j.williams@intel.com>
CC: linux-raid@vger.kernel.org, linux-kernel@vger.kernel.org, neilb@suse.de,
       maciej.sosnowski@intel.com, Ilya Yanok <yanok@emcraft.com>,
       Yuri Tikhonov <yur@emcraft.com>
Subject: Re: [PATCH 06/13] async_tx: add support for asynchronous GF multiplication
References: <20090318191248.20375.40560.stgit@dwillia2-linux.ch.intel.com> <20090318192046.20375.89854.stgit@dwillia2-linux.ch.intel.com>
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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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