€•®?      Œsphinx.addnodes”Œdocument”“”)”}”(Œ	rawsource”Œ ”Œchildren”]”(Œtranslations”ŒLanguagesNode”“”)”}”(hhh]”(h Œpending_xref”“”)”}”(hhh]”Œdocutils.nodes”ŒText”“”ŒChinese (Simplified)”…””}”Œparent”hsbaŒ
attributes”}”(Œids”]”Œclasses”]”Œnames”]”Œdupnames”]”Œbackrefs”]”Œ	refdomain”Œstd”Œreftype”Œdoc”Œ	reftarget”Œ//translations/zh_CN/scheduler/sched-nice-design”Œmodname”NŒ	classname”NŒrefexplicit”ˆuŒtagname”hhhubh)”}”(hhh]”hŒChinese (Traditional)”…””}”hh2sbah}”(h]”h ]”h"]”h$]”h&]”Œ	refdomain”h)Œreftype”h+Œ	reftarget”Œ//translations/zh_TW/scheduler/sched-nice-design”Œmodname”NŒ	classname”NŒrefexplicit”ˆuh1hhhubh)”}”(hhh]”hŒItalian”…””}”hhFsbah}”(h]”h ]”h"]”h$]”h&]”Œ	refdomain”h)Œreftype”h+Œ	reftarget”Œ//translations/it_IT/scheduler/sched-nice-design”Œmodname”NŒ	classname”NŒrefexplicit”ˆuh1hhhubh)”}”(hhh]”hŒJapanese”…””}”hhZsbah}”(h]”h ]”h"]”h$]”h&]”Œ	refdomain”h)Œreftype”h+Œ	reftarget”Œ//translations/ja_JP/scheduler/sched-nice-design”Œmodname”NŒ	classname”NŒrefexplicit”ˆuh1hhhubh)”}”(hhh]”hŒKorean”…””}”hhnsbah}”(h]”h ]”h"]”h$]”h&]”Œ	refdomain”h)Œreftype”h+Œ	reftarget”Œ//translations/ko_KR/scheduler/sched-nice-design”Œmodname”NŒ	classname”NŒrefexplicit”ˆuh1hhhubh)”}”(hhh]”hŒSpanish”…””}”hh‚sbah}”(h]”h ]”h"]”h$]”h&]”Œ	refdomain”h)Œreftype”h+Œ	reftarget”Œ//translations/sp_SP/scheduler/sched-nice-design”Œmodname”NŒ	classname”NŒrefexplicit”ˆuh1hhhubeh}”(h]”h ]”h"]”h$]”h&]”Œcurrent_language”ŒEnglish”uh1h
hhŒ	_document”hŒsource”NŒline”NubhŒsection”“”)”}”(hhh]”(hŒtitle”“”)”}”(hŒScheduler Nice Design”h]”hŒScheduler Nice Design”…””}”(hh¨hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h¦hh£hžhhŸŒI/var/lib/git/docbuild/linux/Documentation/scheduler/sched-nice-design.rst”h KubhŒ	paragraph”“”)”}”(hŒ}This document explains the thinking about the revamped and streamlined
nice-levels implementation in the new Linux scheduler.”h]”hŒ}This document explains the thinking about the revamped and streamlined
nice-levels implementation in the new Linux scheduler.”…””}”(hh¹hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h·hŸh¶h Khh£hžhubh¸)”}”(hŒ…Nice levels were always pretty weak under Linux and people continuously
pestered us to make nice +19 tasks use up much less CPU time.”h]”hŒ…Nice levels were always pretty weak under Linux and people continuously
pestered us to make nice +19 tasks use up much less CPU time.”…””}”(hhÇhžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h·hŸh¶h Khh£hžhubh¸)”}”(hX  Unfortunately that was not that easy to implement under the old
scheduler, (otherwise we'd have done it long ago) because nice level
support was historically coupled to timeslice length, and timeslice
units were driven by the HZ tick, so the smallest timeslice was 1/HZ.”h]”hX  Unfortunately that was not that easy to implement under the old
scheduler, (otherwise weâ€™d have done it long ago) because nice level
support was historically coupled to timeslice length, and timeslice
units were driven by the HZ tick, so the smallest timeslice was 1/HZ.”…””}”(hhÕhžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h·hŸh¶h Khh£hžhubh¸)”}”(hXe  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!)::”h]”hXd  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!):”…””}”(hhãhžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h·hŸh¶h Khh£hžhubhŒliteral_block”“”)”}”(hXŽ                    A
            \     | [timeslice length]
             \    |
              \   |
               \  |
                \ |
                 \|___100msecs
                  |^ . _
                  |      ^ . _
                  |            ^ . _
-*----------------------------------*-----> [nice level]
-20               |                +19
                  |
                  |”h]”hXŽ                    A
            \     | [timeslice length]
             \    |
              \   |
               \  |
                \ |
                 \|___100msecs
                  |^ . _
                  |      ^ . _
                  |            ^ . _
-*----------------------------------*-----> [nice level]
-20               |                +19
                  |
                  |”…””}”hhósbah}”(h]”h ]”h"]”h$]”h&]”Œ	xml:space”Œpreserve”uh1hñhŸh¶h Khh£hžhubh¸)”}”(hŒÈSo 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.)”h]”hŒÈSo 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.)”…””}”(hj  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h·hŸh¶h K'hh£hžhubh¸)”}”(hXÞ  This approach worked to some degree for some time, but later on with
HZ=1000 it caused 1 jiffy to be 1 msec, which meant 0.1% CPU usage which
we felt to be a bit excessive. Excessive _not_ because it's too small of
a CPU utilization, but because it causes too frequent (once per
millisec) rescheduling. (and would thus trash the cache, etc. Remember,
this was long ago when hardware was weaker and caches were smaller, and
people were running number crunching apps at nice +19.)”h]”hXà  This approach worked to some degree for some time, but later on with
HZ=1000 it caused 1 jiffy to be 1 msec, which meant 0.1% CPU usage which
we felt to be a bit excessive. Excessive _not_ because itâ€™s too small of
a CPU utilization, but because it causes too frequent (once per
millisec) rescheduling. (and would thus trash the cache, etc. Remember,
this was long ago when hardware was weaker and caches were smaller, and
people were running number crunching apps at nice +19.)”…””}”(hj  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h·hŸh¶h K+hh£hžhubh¸)”}”(hXu  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_ :-)”h]”hXu  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_ :-)”…””}”(hj  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h·hŸh¶h K3hh£hžhubh¸)”}”(hŒÎTo sum it up: we always wanted to make nice levels more consistent, but
within the constraints of HZ and jiffies and their nasty design level
coupling to timeslices and granularity it was not really viable.”h]”hŒÎTo sum it up: we always wanted to make nice levels more consistent, but
within the constraints of HZ and jiffies and their nasty design level
coupling to timeslices and granularity it was not really viable.”…””}”(hj-  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h·hŸh¶h K:hh£hžhubh¸)”}”(hX]  The second (less frequent but still periodically occurring) complaint
about Linux's nice level support was its asymmetry around the origin
(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":”h]”hXc  The second (less frequent but still periodically occurring) complaint
about Linuxâ€™s nice level support was its asymmetry around the origin
(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
â€œrelativeâ€:”…””}”(hj;  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h·hŸh¶h K>hh£hžhubhŒblock_quote”“”)”}”(hŒ<int nice(int inc);

asmlinkage long sys_nice(int increment)
”h]”(h¸)”}”(hŒint nice(int inc);”h]”hŒint nice(int inc);”…””}”(hjO  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h·hŸh¶h KEhjK  ubh¸)”}”(hŒ'asmlinkage long sys_nice(int increment)”h]”hŒ'asmlinkage long sys_nice(int increment)”…””}”(hj]  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h·hŸh¶h KGhjK  ubeh}”(h]”h ]”h"]”h$]”h&]”uh1jI  hŸh¶h KEhh£hžhubh¸)”}”(hŒº(the first one is the glibc API, the second one is the syscall API.)
Note that the 'inc' is relative to the current nice level. Tools like
bash's "nice" command mirror this relative API.”h]”hŒÄ(the first one is the glibc API, the second one is the syscall API.)
Note that the â€˜incâ€™ is relative to the current nice level. Tools like
bashâ€™s â€œniceâ€ command mirror this relative API.”…””}”(hjq  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h·hŸh¶h KIhh£hžhubh¸)”}”(hX	  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.”h]”hX	  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.”…””}”(hj  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h·hŸh¶h KMhh£hžhubh¸)”}”(hXb  A third complaint against Linux's nice level support was that negative
nice levels were not 'punchy enough', so lots of people had to resort to
run audio (and other multimedia) apps under RT priorities such as
SCHED_FIFO. But this caused other problems: SCHED_FIFO is not starvation
proof, and a buggy SCHED_FIFO app can also lock up the system for good.”h]”hXh  A third complaint against Linuxâ€™s nice level support was that negative
nice levels were not â€˜punchy enoughâ€™, so lots of people had to resort to
run audio (and other multimedia) apps under RT priorities such as
SCHED_FIFO. But this caused other problems: SCHED_FIFO is not starvation
proof, and a buggy SCHED_FIFO app can also lock up the system for good.”…””}”(hj  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h·hŸh¶h KRhh£hžhubh¸)”}”(hŒEThe new scheduler in v2.6.23 addresses all three types of complaints:”h]”hŒEThe new scheduler in v2.6.23 addresses all three types of complaints:”…””}”(hj›  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h·hŸh¶h KXhh£hžhubh¸)”}”(hX¡  To address the first complaint (of nice levels being not "punchy"
enough), the scheduler was decoupled from 'time slice' and HZ concepts
(and granularity was made a separate concept from nice levels) and thus
it was possible to implement better and more consistent nice +19
support: with the new scheduler nice +19 tasks get a HZ-independent
1.5%, instead of the variable 3%-5%-9% range they got in the old
scheduler.”h]”hX©  To address the first complaint (of nice levels being not â€œpunchyâ€
enough), the scheduler was decoupled from â€˜time sliceâ€™ and HZ concepts
(and granularity was made a separate concept from nice levels) and thus
it was possible to implement better and more consistent nice +19
support: with the new scheduler nice +19 tasks get a HZ-independent
1.5%, instead of the variable 3%-5%-9% range they got in the old
scheduler.”…””}”(hj©  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h·hŸh¶h KZhh£hžhubh¸)”}”(hXF  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.”h]”hXR  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 â€œsplitâ€ 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 â€œmultiplicativeâ€ (or exponential) - that way
it does not matter which nice level you start out from, the â€˜relative
resultâ€™ will always be the same.”…””}”(hj·  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h·hŸh¶h Kbhh£hžhubh¸)”}”(hX@  The third complaint (of negative nice levels not being "punchy" enough
and forcing audio apps to run under the more dangerous SCHED_FIFO
scheduling policy) is addressed by the new scheduler almost
automatically: stronger negative nice levels are an automatic
side-effect of the recalibrated dynamic range of nice levels.”h]”hXD  The third complaint (of negative nice levels not being â€œpunchyâ€ enough
and forcing audio apps to run under the more dangerous SCHED_FIFO
scheduling policy) is addressed by the new scheduler almost
automatically: stronger negative nice levels are an automatic
side-effect of the recalibrated dynamic range of nice levels.”…””}”(hjÅ  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1h·hŸh¶h Klhh£hžhubeh}”(h]”Œscheduler-nice-design”ah ]”h"]”Œscheduler nice design”ah$]”h&]”uh1h¡hhhžhhŸh¶h Kubeh}”(h]”h ]”h"]”h$]”h&]”Œsource”h¶uh1hŒcurrent_source”NŒcurrent_line”NŒsettings”Œdocutils.frontend”ŒValues”“”)”}”(h¦NŒ	generator”NŒ	datestamp”NŒsource_link”NŒ
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