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[Qemu-devel] Re: [PATCH] two level table for IO port lookup

From: Jan Kiszka
Subject: [Qemu-devel] Re: [PATCH] two level table for IO port lookup
Date: Fri, 10 Apr 2009 19:24:13 +0200
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Anthony Liguori wrote:
> Brian Wheeler wrote:
>> The alpha architecture uses 24 bits for the io port address so this
>> patch adds a two level table and puts the IO port data into a
>> struct...because sizeof(void *) * 7 * 16777216 is nearly a 1G on my
>> workstation.
>> I've set the alpha target to use a 12/12 split and everything else to
>> use 8/8.    
> The table lookups really kill performance.  It's probably a better idea
> just to switch to a linear list of IO ports.  There's usually going to
> be a small number of registered IO regions (certainly, less than 100).

Sorry, can't follow: you want to search a linear list of at least a few
ten entries on average instead of doing a simple two-stage table lookup?
  If you had said: "Use a tree." Maybe. But even then you can quickly
end up consulting >2 entries before finding the right one.

Nevertheless, numbers on the performance impact and memory consumption
of this patch would surely help to decide which way to go.


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