Re: Ten percent test

Mike Galbraith
2007-04-09T05:38:27Z

On Sun, 2007-04-08 at 09:08 -0400, Ed Tomlinson wrote:
> Hi,
>
> I am one of those who have been happily testing Con's patches.
>
> They work better than mainline here.

(I tried a UP kernel yesterday, and even a single kernel build would
make noticeable hitches if I move a window around. YMMV etc.)

> If one really needs some sort of interactivity booster (I do not with SD), why
> not move it into user space? With SD it would be simple enough to export
> some info on estimated latency. With this user space could make a good
> attempt to keep latency within bounds for a set of tasks just by renicing....

I don't think you can have very much effect on latency using nice with
SD once the CPU is fully utilized. See below.

/*
* This contains a bitmap for each dynamic priority level with empty slots
* for the valid priorities each different nice level can have. It allows
* us to stagger the slots where differing priorities run in a way that
* keeps latency differences between different nice levels at a minimum.
* ie, where 0 means a slot for that priority, priority running from left to
* right:
* nice -20 0000000000000000000000000000000000000000
* nice -10 1001000100100010001001000100010010001000
* nice 0 0101010101010101010101010101010101010101
* nice 5 1101011010110101101011010110101101011011
* nice 10 0110111011011101110110111011101101110111
* nice 15 0111110111111011111101111101111110111111
* nice 19 1111111111111111111011111111111111111111
*/

Nice allocates bandwidth, but as long as the CPU is busy, tasks always
proceed downward in priority until they hit the expired array. That's
the design. If X gets busy and expires, and a nice 20 CPU hog wakes up
after it's previous rotation has ended, but before the current rotation
is ended (ie there is 1 task running at wakeup time), X will take a
guaranteed minimum 160ms latency hit (quite noticeable) independent of
nice level. The only way to avoid it is to use a realtime class.

A nice -20 task has maximum bandwidth allocated, but that also makes it
a bigger target for preemption from tasks at all nice levels as it
proceeds downward toward expiration. AFAIKT, low latency scheduling
just isn't possible once the CPU becomes 100% utilized, but it is
bounded to runqueue length. In mainline OTOH, a nice -20 task will
always preempt a nice 0 task, giving it instant gratification, and
latency of lower priority tasks is bounded by the EXPIRED_STARVING(rq)
safety net.

-Mike