Re: Ten percent test

[Ed Tomlinson <edt@aei.ca>]

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...

Thanks
Ed
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