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Date:      Tue, 31 May 2005 11:39:47 +1000 (EST)
From:      Bruce Evans <bde@zeta.org.au>
To:        Robert Watson <rwatson@FreeBSD.org>
Cc:        daniel_k_eriksson@telia.com, freebsd-arch@FreeBSD.org
Subject:   Re: MPSAFE CAM?
Message-ID:  <20050531113008.R91505@delplex.bde.org>
In-Reply-To: <20050530182650.R90885@fledge.watson.org>
References:  <!~!UENERkVCMDkAAQACAAAAAAAAAAAAAAAAABgAAAAAAAAA0VcX9IoJqUaXPS8MjT1PdsKAAAAQAAAAvHxW/c0DGk+3ez1DZggmFgEAAAAA@telia.com> <4298E316.9020303@samsco.org> <20050529121224.L52379@fledge.watson.org> <20050530.102803.74664036.imp@bsdimp.com> <20050530182650.R90885@fledge.watson.org>

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On Mon, 30 May 2005, Robert Watson wrote:

> On Mon, 30 May 2005, M. Warner Losh wrote:
>
>> In message: <20050529121224.L52379@fledge.watson.org>
>>            Robert Watson <rwatson@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



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