Re: driver concurrency

[Thor Lancelot Simon <tls%panix.com@localhost>]

On Mon, Dec 01, 2014 at 05:53:12PM +0100, Manuel Bouyer wrote:
> On Mon, Dec 01, 2014 at 03:02:39PM +0000, David Holland wrote:
> > How many drivers are there (hardware-level drivers, not things like
> > raidframe) where it really matters for more than one lwp to be able to
> > be running (not stopped) in the driver at once?
> > 
> > I'm thinking probably network cards but not much else.
> 
> disk controllers which supports lots of concurent transactions could
> probably benefit from this too.

They would, and many are simple enough to make this reasonably easy to do,
but in practice, the giant locking of our SCSI code makes it pointless.

I have experience with a popular (in the high performance crypto market,
anyway) crypto accellerator which can be configured to present multiple
command queues to allow CPU affinity for requests.  When run that way the
driver still needs locks (one per queue) but the locks are almost always
uncontended.  It's a big win.

Thor