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Re: Performance of IOMemoryDescriptor::prepare on contiguous, wired memory

Michael Smith (email@hidden) wrote on Thu, 19 Dec 2002 01:18:27 -0800:

>That's not surprising. You know it's wired and contiguous, but the VM
>doesn't (and can't) know or assume that.
>
>> Is my expectation of prepare() invalid?
>
>What expectation do you have?

I guess I expected that the VM system could know its already wired. Now
I know. :)

>In your case, the VM has no way of knowing that the virtual address
>range that
>you are passing it is mapped to memory that is already wired. Wiring
>memory
>is deathly slow in MacOS X, and you pay the entire cost even though the
>pages
>are already pinned down because you're not doing it against the
>original region.

Bummer.

>To avoid this, you'd want to pass a cookie across so that you could use
>the
>originally allocated and wired region from the first driver in the
>second, or

This is what I have done and its working well.

So thanks for your reply Michael, it was most helpful.


--
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Sean McBride, B. Eng email@hidden
Mac Software Designer +1-514-822-6000 x7399
Matrox Electronic Systems Ltd. Montrial, Quibec, Canada
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 >Re: Performance of IOMemoryDescriptor::prepare on contiguous, wired memory (From: Michael Smith <email@hidden>)



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