From owner-freebsd-hackers  Thu Jul  6 13: 0:23 2000
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Date: Thu, 6 Jul 2000 13:00:11 -0700 (PDT)
From: Matthew Jacob <mjacob@feral.com>
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To: Zhihui Zhang <zzhang@cs.binghamton.edu>
Cc: freebsd-hackers@FreeBSD.ORG
Subject: Re: Max DMA size
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> 
> Can anyone tell me what factors determine the max DMA size (DMA counter on
> each controller or PCI bus related)? What is the typical max DMA size for
> a SCSI disk connected to a PCI bus? It seems to be much larger than
> MAXPHYS (128K). If so, does it mean we are not using full potential of
> DMA? So what's the problem if we enlarge MAXPHYS?

You can enlarge MAXPHYS, but watch out that you don't also then make MAXBSIZE
so big that the creation of buffers eats up all of your memory.

IMO MAXPHYS is way too small, but there are a number of current VM
implementation reasons why making it larger by default is not right.





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