1=====================
  2Scheduler Nice Design
  3=====================
  4
  5This document explains the thinking about the revamped and streamlined
  6nice-levels implementation in the new Linux scheduler.
  7
  8Nice levels were always pretty weak under Linux and people continuously
  9pestered us to make nice +19 tasks use up much less CPU time.
 10
 11Unfortunately that was not that easy to implement under the old
 12scheduler, (otherwise we'd have done it long ago) because nice level
 13support was historically coupled to timeslice length, and timeslice
 14units were driven by the HZ tick, so the smallest timeslice was 1/HZ.
 15
 16In the O(1) scheduler (in 2003) we changed negative nice levels to be
 17much stronger than they were before in 2.4 (and people were happy about
 18that change), and we also intentionally calibrated the linear timeslice
 19rule so that nice +19 level would be _exactly_ 1 jiffy. To better
 20understand it, the timeslice graph went like this (cheesy ASCII art
 21alert!)::
 22
 23
 24                   A
 25             \     | [timeslice length]
 26              \    |
 27               \   |
 28                \  |
 29                 \ |
 30                  \|___100msecs
 31                   |^ . _
 32                   |      ^ . _
 33                   |            ^ . _
 34 -*----------------------------------*-----> [nice level]
 35 -20               |                +19
 36                   |
 37                   |
 38
 39So that if someone wanted to really renice tasks, +19 would give a much
 40bigger hit than the normal linear rule would do. (The solution of
 41changing the ABI to extend priorities was discarded early on.)
 42
 43This approach worked to some degree for some time, but later on with
 44HZ=1000 it caused 1 jiffy to be 1 msec, which meant 0.1% CPU usage which
 45we felt to be a bit excessive. Excessive _not_ because it's too small of
 46a CPU utilization, but because it causes too frequent (once per
 47millisec) rescheduling. (and would thus trash the cache, etc. Remember,
 48this was long ago when hardware was weaker and caches were smaller, and
 49people were running number crunching apps at nice +19.)
 50
 51So for HZ=1000 we changed nice +19 to 5msecs, because that felt like the
 52right minimal granularity - and this translates to 5% CPU utilization.
 53But the fundamental HZ-sensitive property for nice+19 still remained,
 54and we never got a single complaint about nice +19 being too _weak_ in
 55terms of CPU utilization, we only got complaints about it (still) being
 56too _strong_ :-)
 57
 58To sum it up: we always wanted to make nice levels more consistent, but
 59within the constraints of HZ and jiffies and their nasty design level
 60coupling to timeslices and granularity it was not really viable.
 61
 62The second (less frequent but still periodically occurring) complaint
 63about Linux's nice level support was its asymmetry around the origin
 64(which you can see demonstrated in the picture above), or more
 65accurately: the fact that nice level behavior depended on the _absolute_
 66nice level as well, while the nice API itself is fundamentally
 67"relative":
 68
 69   int nice(int inc);
 70
 71   asmlinkage long sys_nice(int increment)
 72
 73(the first one is the glibc API, the second one is the syscall API.)
 74Note that the 'inc' is relative to the current nice level. Tools like
 75bash's "nice" command mirror this relative API.
 76
 77With the old scheduler, if you for example started a niced task with +1
 78and another task with +2, the CPU split between the two tasks would
 79depend on the nice level of the parent shell - if it was at nice -10 the
 80CPU split was different than if it was at +5 or +10.
 81
 82A third complaint against Linux's nice level support was that negative
 83nice levels were not 'punchy enough', so lots of people had to resort to
 84run audio (and other multimedia) apps under RT priorities such as
 85SCHED_FIFO. But this caused other problems: SCHED_FIFO is not starvation
 86proof, and a buggy SCHED_FIFO app can also lock up the system for good.
 87
 88The new scheduler in v2.6.23 addresses all three types of complaints:
 89
 90To address the first complaint (of nice levels being not "punchy"
 91enough), the scheduler was decoupled from 'time slice' and HZ concepts
 92(and granularity was made a separate concept from nice levels) and thus
 93it was possible to implement better and more consistent nice +19
 94support: with the new scheduler nice +19 tasks get a HZ-independent
 951.5%, instead of the variable 3%-5%-9% range they got in the old
 96scheduler.
 97
 98To address the second complaint (of nice levels not being consistent),
 99the new scheduler makes nice(1) have the same CPU utilization effect on
100tasks, regardless of their absolute nice levels. So on the new
101scheduler, running a nice +10 and a nice 11 task has the same CPU
102utilization "split" between them as running a nice -5 and a nice -4
103task. (one will get 55% of the CPU, the other 45%.) That is why nice
104levels were changed to be "multiplicative" (or exponential) - that way
105it does not matter which nice level you start out from, the 'relative
106result' will always be the same.
107
108The third complaint (of negative nice levels not being "punchy" enough
109and forcing audio apps to run under the more dangerous SCHED_FIFO
110scheduling policy) is addressed by the new scheduler almost
111automatically: stronger negative nice levels are an automatic
112side-effect of the recalibrated dynamic range of nice levels.