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Date: Tue, 17 Jul 2007 17:20:28 -0700
From: "Kip Macy" <kip.macy@gmail.com>
To: "David Christensen" <davidch@broadcom.com>
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Cc: current@freebsd.org
Subject: Re: Getting/Forcing Greater than 4KB Buffer Allocations
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On 7/17/07, David Christensen <davidch@broadcom.com> wrote:
> I'm investigating a problem with my bce driver which occurs when I ask
> for a jumbo
> mbuf cluster (through m_cljget()).  When I map the memory for DMA I
> normally
> get 3 memory segments (4KB + 4KB + 1KB) on my system, but on another
> user's
> system he's seeing 2 memory segments (8KB + 1KB).  Is there a
> configuration
> option that allows this or some other tuning variable involved?  The
> system is a
> Xeon dual-core processor and has 8GB of RAM, running an AMD64 version of
> the kernel.


busdma coalesces the addresses into one segment if they're physically
adjacent. Take a look at busdma_machdep.c.


 -Kip