sched: document nice levels

Document the design thinking behind nice levels.
Signed-off-by: NIngo Molnar <mingo@elte.hu>

Documentation/sched-nice-design.txt

+This document explains the thinking about the revamped and streamlined
+nice-levels implementation in the new Linux scheduler.
+
+Nice levels were always pretty weak under Linux and people continuously
+pestered us to make nice +19 tasks use up much less CPU time.
+
+Unfortunately that was not that easy to implement under the old
+scheduler, (otherwise we'd have done it long ago) because nice level
+support was historically coupled to timeslice length, and timeslice
+units were driven by the HZ tick, so the smallest timeslice was 1/HZ.
+
+In the O(1) scheduler (in 2003) we changed negative nice levels to be
+much stronger than they were before in 2.4 (and people were happy about
+that change), and we also intentionally calibrated the linear timeslice
+rule so that nice +19 level would be _exactly_ 1 jiffy. To better
+understand it, the timeslice graph went like this (cheesy ASCII art
+alert!):
+
+
+                   A
+             \     | [timeslice length]
+              \    |
+               \   |
+                \  |
+                 \ |
+                  \|___100msecs
+                   |^ . _
+                   |      ^ . _
+                   |            ^ . _
+ -*----------------------------------*-----> [nice level]
+ -20               |                +19
+                   |
+                   |
+
+So that if someone wanted to really renice tasks, +19 would give a much
+bigger hit than the normal linear rule would do. (The solution of
+changing the ABI to extend priorities was discarded early on.)
+
+This approach worked to some degree for some time, but later on with
+HZ=1000 it caused 1 jiffy to be 1 msec, which meant 0.1% CPU usage which
+we felt to be a bit excessive. Excessive _not_ because it's too small of
+a CPU utilization, but because it causes too frequent (once per
+millisec) rescheduling. (and would thus trash the cache, etc. Remember,
+this was long ago when hardware was weaker and caches were smaller, and
+people were running number crunching apps at nice +19.)
+
+So for HZ=1000 we changed nice +19 to 5msecs, because that felt like the
+right minimal granularity - and this translates to 5% CPU utilization.
+But the fundamental HZ-sensitive property for nice+19 still remained,
+and we never got a single complaint about nice +19 being too _weak_ in
+terms of CPU utilization, we only got complaints about it (still) being
+too _strong_ :-)
+
+To sum it up: we always wanted to make nice levels more consistent, but
+within the constraints of HZ and jiffies and their nasty design level
+coupling to timeslices and granularity it was not really viable.
+
+The second (less frequent but still periodically occuring) complaint
+about Linux's nice level support was its assymetry around the origo
+(which you can see demonstrated in the picture above), or more
+accurately: the fact that nice level behavior depended on the _absolute_
+nice level as well, while the nice API itself is fundamentally
+"relative":
+
+   int nice(int inc);
+
+   asmlinkage long sys_nice(int increment)
+
+(the first one is the glibc API, the second one is the syscall API.)
+Note that the 'inc' is relative to the current nice level. Tools like
+bash's "nice" command mirror this relative API.
+
+With the old scheduler, if you for example started a niced task with +1
+and another task with +2, the CPU split between the two tasks would
+depend on the nice level of the parent shell - if it was at nice -10 the
+CPU split was different than if it was at +5 or +10.
+
+A third complaint against Linux's nice level support was that negative
+nice levels were not 'punchy enough', so lots of people had to resort to
+run audio (and other multimedia) apps under RT priorities such as
+SCHED_FIFO. But this caused other problems: SCHED_FIFO is not starvation
+proof, and a buggy SCHED_FIFO app can also lock up the system for good.
+
+The new scheduler in v2.6.23 addresses all three types of complaints:
+
+To address the first complaint (of nice levels being not "punchy"
+enough), the scheduler was decoupled from 'time slice' and HZ concepts
+(and granularity was made a separate concept from nice levels) and thus
+it was possible to implement better and more consistent nice +19
+support: with the new scheduler nice +19 tasks get a HZ-independent
+1.5%, instead of the variable 3%-5%-9% range they got in the old
+scheduler.
+
+To address the second complaint (of nice levels not being consistent),
+the new scheduler makes nice(1) have the same CPU utilization effect on
+tasks, regardless of their absolute nice levels. So on the new
+scheduler, running a nice +10 and a nice 11 task has the same CPU
+utilization "split" between them as running a nice -5 and a nice -4
+task. (one will get 55% of the CPU, the other 45%.) That is why nice
+levels were changed to be "multiplicative" (or exponential) - that way
+it does not matter which nice level you start out from, the 'relative
+result' will always be the same.
+
+The third complaint (of negative nice levels not being "punchy" enough
+and forcing audio apps to run under the more dangerous SCHED_FIFO
+scheduling policy) is addressed by the new scheduler almost
+automatically: stronger negative nice levels are an automatic
+side-effect of the recalibrated dynamic range of nice levels.