Re: Low latency for recent kernels

[Dan Maas]

> I'm a little surprised that desktop users do notice significant
> benefits with all the latency/preempt patches.  If you actually
> instrument the kernel's behaviour, the stalls are in fact
> quite small and infrequent.

Havoc Pennington, Soeren Sandmann, and I have been investigating causes of
UI unresponsiveness in Xfree86/Linux. I would agree that in most situations,
on a mostly-idle machine, low-latency/preempt patches should *not* enhance
the overall feel of the desktop. (if they do measurably increase
responsiveness, then that would suggest inefficiencies in X/the WM/the X
client - a definite possibility, of course).

Two situations where I would expect low-latency/preemption to have a
positive effect on responsiveness are 1) when the system is under heavy CPU
and disk load (e.g. kernel compile); due to the interactive tasks being able
to run earlier/more often, and 2) when performing UI operations that depend
on tight synchronization between X/the WM/the X client, particularly opaque
window resizing. (my theory is that low-latency/preemption results in the
CPU switching more rapidly or evenly among these processes, reducing the
perceptible "lag" between the client window and its WM frame)

Regards,
Dan