From owner-freebsd-current  Sun Oct 27 23:15:32 2002
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Date: Mon, 28 Oct 2002 18:26:34 +1100 (EST)
From: Bruce Evans <bde@zeta.org.au>
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To: Seigo Tanimura <tanimura@axe-inc.co.jp>
Cc: Julian Elischer <julian@elischer.org>,
	Jeff Roberson <jroberson@chesapeake.net>, <current@FreeBSD.ORG>,
	<tanimura@FreeBSD.ORG>
Subject: Re: Dynamic growth of the buffer and buffer page reclaim
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On Mon, 28 Oct 2002, Seigo Tanimura wrote:

> On Thu, 24 Oct 2002 15:05:30 +1000 (EST),
>   Bruce Evans <bde@zeta.org.au> said:
>
> bde> Almost exactly what we have.  It turns out to be not very good, at least
> bde> in its current implementation, since remapping is too expensive.  Things
> bde> work OK to the extent that remapping is not required, but so would a
> bde> much simpler implementation that uses less vm and more copying of data
> bde> (copying seems to be faster than remapping).
>
> Which process is expensive in remapping?  Allocation of a KVA space?
> Page wiring?  Or pmap operation?

The allocation seemed to be most expensive when I looked at this about 2
years ago.  The cause of the remapping seemed to be that different amounts
of buffer kva were allocated for different buffer sizes.  Copying between
filesystems with different block sizes therefore caused lots of remapping.
I think this cause of remapping has been fixed.  VM has been improved too.
I'm not sure how much in this area.

Bruce


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