To: roland <devzero@web.de>
Cc: linux-kernel@vger.kernel.org
Subject: Re: Software based ECC ?
In-Reply-To: Your message of "Fri, 10 Aug 2007 23:16:45 +0200."
             <000a01c7db93$c75b66c0$eeeea8c0@aldipc>
From: Valdis.Kletnieks@vt.edu
References: <000a01c7db93$c75b66c0$eeeea8c0@aldipc>
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Date: Sat, 11 Aug 2007 02:11:15 -0400
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On Fri, 10 Aug 2007 23:16:45 +0200, roland said:

> http://pdos.csail.mit.edu/papers/softecc:ddopson-meng/softecc_ddopson-meng.pdf
> 
> "SoftECC : A System for Software Memory Integrity Checking"
> 
> Is it possible to implement something like this within the Linux virtual
> memory subsystem ?

Anything that can be simulated with a Turing machine is *possible*.

The question is how many rocket boosters the pig needs for takeoff.

Hint: The thesis talks about why he didn't implement it for Linux.

> If it can be done, wouldn`t this be a great feature ?

Read section 5.2 of that thesis, particularly this quote from 5.2.2:

"For random word writes, this implies that SoftECC will need an order of
magnitude more compute time than the user-mode code"

Basically, on every single memory page that gets dirtied, we have to then
re-checksum the page (blowing away cache lines in the process).  If you want
to get a feel for it, find the kernel code that recognizes that a page is
dirtied, and just add a few lines there:

	int foo = 0, i;
	for (i=0;i++;<1024) { // adjust for non-4K pages
		foo ^= *(page+i);
	}

and see how much your system crawls.

Personally, I'd recommend just shelling out the bucks for hardware ECC if
the reliability matters.


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