mirror of https://github.com/mkerrisk/man-pages
ioprio_set.2: Document meaning of ioprio==0
Reported-by: Jens Axboe <axboe@fb.com> Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
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@ -137,7 +137,10 @@ value), this macro returns its priority
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component.
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.PP
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See the NOTES section for more
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information on scheduling classes and priorities.
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information on scheduling classes and priorities,
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as well as the meaning of specifying
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.I ioprio
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as 0.
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I/O priorities are supported for reads and for synchronous
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.RB ( O_DIRECT ,
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@ -222,6 +225,19 @@ These system calls have an effect only when used
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in conjunction with an I/O scheduler that supports I/O priorities.
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As at kernel 2.6.17 the only such scheduler is the Completely Fair Queuing
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(CFQ) I/O scheduler.
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If no I/O scheduler has been set for a thread,
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then by default the I/O priority will follow the CPU nice value
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.RB ( setpriority (2)).
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In Linux kernels before version 2.6.24,
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once an I/O priority had been set using
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.BR ioprio_set (),
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there was no way to reset the I/O scheduling behavior to the default.
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Since Linux 2.6.24,
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.\" commit 8ec680e4c3ec818efd1652f15199ed1c216ab550
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specifying
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.I ioprio
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as 0 can be used to reset to the default I/O scheduling behavior.
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.SS Selecting an I/O scheduler
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I/O schedulers are selected on a per-device basis via the special
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file
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