1 ===================== 2 Scheduler Nice Design 3 ===================== 4 5 This document explains the thinking about the revamped and streamlined 6 nice-levels implementation in the new Linux scheduler. 7 8 Nice levels were always pretty weak under Linux and people continuously 9 pestered us to make nice +19 tasks use up much less CPU time. 10 11 Unfortunately that was not that easy to implement under the old 12 scheduler, (otherwise we'd have done it long ago) because nice level 13 support was historically coupled to timeslice length, and timeslice 14 units were driven by the HZ tick, so the smallest timeslice was 1/HZ. 15 16 In the O(1) scheduler (in 2003) we changed negative nice levels to be 17 much stronger than they were before in 2.4 (and people were happy about 18 that change), and we also intentionally calibrated the linear timeslice 19 rule so that nice +19 level would be _exactly_ 1 jiffy. To better 20 understand it, the timeslice graph went like this (cheesy ASCII art 21 alert!):: 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 39 So that if someone wanted to really renice tasks, +19 would give a much 40 bigger hit than the normal linear rule would do. (The solution of 41 changing the ABI to extend priorities was discarded early on.) 42 43 This approach worked to some degree for some time, but later on with 44 HZ=1000 it caused 1 jiffy to be 1 msec, which meant 0.1% CPU usage which 45 we felt to be a bit excessive. Excessive _not_ because it's too small of 46 a CPU utilization, but because it causes too frequent (once per 47 millisec) rescheduling. (and would thus trash the cache, etc. Remember, 48 this was long ago when hardware was weaker and caches were smaller, and 49 people were running number crunching apps at nice +19.) 50 51 So for HZ=1000 we changed nice +19 to 5msecs, because that felt like the 52 right minimal granularity - and this translates to 5% CPU utilization. 53 But the fundamental HZ-sensitive property for nice+19 still remained, 54 and we never got a single complaint about nice +19 being too _weak_ in 55 terms of CPU utilization, we only got complaints about it (still) being 56 too _strong_ :-) 57 58 To sum it up: we always wanted to make nice levels more consistent, but 59 within the constraints of HZ and jiffies and their nasty design level 60 coupling to timeslices and granularity it was not really viable. 61 62 The second (less frequent but still periodically occurring) complaint 63 about Linux's nice level support was its asymmetry around the origin 64 (which you can see demonstrated in the picture above), or more 65 accurately: the fact that nice level behavior depended on the _absolute_ 66 nice 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.) 74 Note that the 'inc' is relative to the current nice level. Tools like 75 bash's "nice" command mirror this relative API. 76 77 With the old scheduler, if you for example started a niced task with +1 78 and another task with +2, the CPU split between the two tasks would 79 depend on the nice level of the parent shell - if it was at nice -10 the 80 CPU split was different than if it was at +5 or +10. 81 82 A third complaint against Linux's nice level support was that negative 83 nice levels were not 'punchy enough', so lots of people had to resort to 84 run audio (and other multimedia) apps under RT priorities such as 85 SCHED_FIFO. But this caused other problems: SCHED_FIFO is not starvation 86 proof, and a buggy SCHED_FIFO app can also lock up the system for good. 87 88 The new scheduler in v2.6.23 addresses all three types of complaints: 89 90 To address the first complaint (of nice levels being not "punchy" 91 enough), the scheduler was decoupled from 'time slice' and HZ concepts 92 (and granularity was made a separate concept from nice levels) and thus 93 it was possible to implement better and more consistent nice +19 94 support: with the new scheduler nice +19 tasks get a HZ-independent 95 1.5%, instead of the variable 3%-5%-9% range they got in the old 96 scheduler. 97 98 To address the second complaint (of nice levels not being consistent), 99 the new scheduler makes nice(1) have the same CPU utilization effect on 100 tasks, regardless of their absolute nice levels. So on the new 101 scheduler, running a nice +10 and a nice 11 task has the same CPU 102 utilization "split" between them as running a nice -5 and a nice -4 103 task. (one will get 55% of the CPU, the other 45%.) That is why nice 104 levels were changed to be "multiplicative" (or exponential) - that way 105 it does not matter which nice level you start out from, the 'relative 106 result' will always be the same. 107 108 The third complaint (of negative nice levels not being "punchy" enough 109 and forcing audio apps to run under the more dangerous SCHED_FIFO 110 scheduling policy) is addressed by the new scheduler almost 111 automatically: stronger negative nice levels are an automatic 112 side-effect of the recalibrated dynamic range of nice levels.
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