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From: Ian Lepore <freebsd@damnhippie.dyndns.org>
To: Adrian Chadd <adrian@freebsd.org>
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Date: Wed, 05 Sep 2012 09:28:33 -0600
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Cc: freebsd-arm@freebsd.org, freebsd-mips@freebsd.org
Subject: Re: busdma buffer management enhancements - call for review and test
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On Tue, 2012-09-04 at 20:06 -0700, Adrian Chadd wrote:
> I just want to make sure that you don't assume individual memory
> access costs are the same regardless of how big the allocations are.
> 
> Ie, if you have a bunch of scattered L1-sized allocations, versus them
> all being nearby in the same 512/1024 byte page; the underlying memory
> architecture may be doing memory transactions at a slightly larger
> size than your L1 assumption.
> 
> Damn these hierarchicial memory systems.

I think nothing about my code significantly changes the current
situation in this regard.  Right now bus_dmamem_alloc() gets small
chunks of memory from malloc(9), which means they come from a set of
power-of-two-sized uma zones.  With my changes the memory comes from a
similar set of 2^N sized uma zones, but they're segregated from the
zones used by malloc(9) so that memory in a single cache line never
contains data for both DMA and non-DMA (or two unrelated DMA) users at
once.

-- Ian