Subject: Re: "fast" syscalls?
To: None <>
From: Chapman Flack <>
List: tech-kern
Date: 03/17/2006 14:33:28
Garrett D'Amore wrote:
>>You may want to look at some of the audio drivers which allow you to
>>mmap a buffer into userland.  From memory they handle the
>>userland/kernel interaction by updating the head/tail pointers in a
>>circular queue, userland updates one pointer and the kernel updates
>>the other... when they pointers are equal the queue is empty.

And if you adapt the classic 1 reader 1 writer circular queue convention
where the queue is 'full' when w+1 == r (mod size), that is, when
exactly one slot is unused, then you're in business as long as the
pointer updates in your architecture are indivisible (which might
require carefully making sure the pointers do not straddle page
boundaries but otherwise ought to be a safe assumption - at least that
seemed to be the consensus when I asked). The only time you'll ever need
to use a u/k interaction to synchronize is when the reader or writer
actually starves or blocks, and if you can adjust buffer sizes and
processing rates so that happens seldom, you could keep the cost of that
interaction down by not needing to do it often.