MPSAFE CAM?

[Bruce Evans]

On Mon, 30 May 2005, Robert Watson wrote:

> On Mon, 30 May 2005, M. Warner Losh wrote:
>
>> In message: <20050529121224.L52379 at fledge.watson.org>
>>            Robert Watson <rwatson at FreeBSD.org> writes:
>> : driver, we've gained many of the benefits of making MPSAFE.  Especially 
>> if
>> : we can knock Giant off a lot of the remaining non-CAM device drivers that
>> : it's still stuck over.
>> 
>> usb, psm and atkbd are important ones for interactive performance...
>
> And the tty subsystem becomes an immediate dependency for several of these.

Interactive performance is irrelevant here.  You can can lose efficiency
by competing for Giant a few hundred thousand times in the time that
it takes an interactive user to notice.  The problem is that non-interactive
performance is impaired by using Giant locking anywhere.  Using it for
interactive devices is only a problem if the interaction often requires
holding Giant for a long time.  It doesn't for most input devices
including keyboards, but it might for slow bulk output devices like
syscons consoles.

Bruce