linux-kernel - Re: [PATCH] RTC: Add mmap method to rtc character driver

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Message-ID: <44C6875F.4090300@zytor.com>
Date:	Tue, 25 Jul 2006 14:04:31 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	jg@...top.org
CC:	Neil Horman <nhorman@...driver.com>,
	Dave Airlie <airlied@...il.com>,
	Segher Boessenkool <segher@...nel.crashing.org>,
	linux-kernel@...r.kernel.org, a.zummo@...ertech.it,
	jg@...edesktop.org
Subject: Re: [PATCH] RTC: Add mmap method to rtc character driver

Jim Gettys wrote:
> 
> Some of us want/need both tickless and smart scheduling in the X server.
> 
> To give a bit of data: on my machine, a 1x1 rectangle can go almost 2
> million rectangles/second; a 500x500 rectangle is only 2800/second.  You
> can see the tremendous variation (and this is on accelerated hardware;
> the variation can in fact be much larger than this, if the operation has
> to be done in software fall-backs). 
> 
> This is why the X server needs to know the time so much, so cheaply; we
> have to be able to tell how much time a given client has been using, and
> it can't be computed from anything but the time; otherwise individual
> clients can "starve" other clients, and interactive feel goes to pot.
>                                - Jim
> 

That's why I'm suggesting adding a cheap, possibly low-res, gettimeofday 
virtual system call in case there is no way for the kernel to provide 
userspace with a cheap full-resolution gettimeofday.  Obviously, if a 
high-quality gettimeofday is available, then they can be linked together 
by the kernel.

	-hpa
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