Recently, I've been getting myself more familiar with the FreeBSD kernel internals, and some new features brought along with FreeBSD 7 (and 8-CURRENT, which I typically run). I've been picking at the kernel sources to convert places where timeout(9) is used into callout(9) implementations of the same logic. This should remove extra use of the Giant-lock and improve concurrency in the long-run. Most recently, I have converted the if_ndis device to use the new callout(9) API. If you're running -CURRENT and use the NDISulator, I would ask that you give it a try and email a report back to me of how well it works (or doesn't) for you.
I put together a page for the work here: callout-conversion
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