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SubjectRe: Low latency and fbcon [0.5 sec is not considered low latency] [Re: new IRQ scalability changes in 2.3.48]

> i know about fbcon's latency problems, have you tried fixing the
> screen-scrolling function?

Thats a mess in more than one way. The code is design with have only one
card in your system in mind. Multihead it crokes.

> It's by far the worst offender. Another
> function: if your card does bitblt and the bitblt command is posted to the
> videocard then typically the code polls the videocard status register to
> wait for the bitblt to finish - that can be long as well.

Accels help alot. Semaphore would be the best solution.

> a semaphore plus a conditional reschedule in the above two places should
> already help. (also the semaphore has to be try_down_semaphore()-ed from
> the kernel-message function, because we obviously must not block in IRQ
> contexts which call printk.)
>
> > (I replaced if (need_resched) schedule() with just schedule() -- I
> > hope that can not break anything...)
>
> you have to add the semaphore (or semaphores) to the generic console code
> to be safe.

Yep. This is one of the thing we are working on.

> The console code assumes that the individual lowlevel console
> functions are atomic (well, most of them). You should also probably boost
> the priority of the process holding this semaphore to the highest
> user-priority, otherwise console output can be blocked by normal
> processes. (If you manage to get this working, i'd be very interested in
> it.)

Me too. Email me the patch I will send it off to the linuxconsole project.


"Look its a text editor, no its a OS, no its Emacs"
James Simmons (o_
fbdev/gfx developer (o_ (o_ //\
http://www.linux-fbdev.org (/)_ (/)_ V_/_
http://linuxgfx.sourceforge.net


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