Messages in this thread | | | From | Ed Tomlinson <> | Subject | Re: Ten percent test | Date | Mon, 9 Apr 2007 07:26:00 -0400 |
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On Monday 09 April 2007 01:38, Mike Galbraith wrote: > 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.)
Interesting. I run UP amd64, 1000HZ, 1.25G, preempt off (on causes kernel stalls with no messages - but that is another story). I do not notice a single make. When several are running the desktop slows down a bit. I do not have X niced. Wonder why we see such different results?
I am not saying that SD is perfect - I fully expect that more bugs will turn up in its code (some will affect mainline too). I do however like the idea of a scheduler that does not need alchemy to achieve good results. Nor do I necessarily expect it to be 100% transparent. If one changes something as basic as the scheduler some tweaking should be expected. IMO this is fine as long as we get consistant results.
> > 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 I made no mention of low latency. I did mention predictable latency. If you are 100% utilized, and have a nice -20 task cpu hog, I would expect it to run and that it _should_ affect other tasks - thats why it runs with -20...
This is why I suggest that user space may be a better place to boost interactive tasks. A daemon that posted a message telling me that the nice -20 cpu hog is causing 300ms delays for X would, IMHO, be a good thing. That same daemon could then propose a fix telling me the expected latencies and let me decide if I want to change priorities. It could also be set to automaticily adjust nice levels...
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