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Block io priorities | ||
=================== | ||
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Intro | ||
----- | ||
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With the introduction of cfq v3 (aka cfq-ts or time sliced cfq), basic io | ||
priorities is supported for reads on files. This enables users to io nice | ||
processes or process groups, similar to what has been possible to cpu | ||
scheduling for ages. This document mainly details the current possibilites | ||
with cfq, other io schedulers do not support io priorities so far. | ||
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Scheduling classes | ||
------------------ | ||
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CFQ implements three generic scheduling classes that determine how io is | ||
served for a process. | ||
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IOPRIO_CLASS_RT: This is the realtime io class. This scheduling class is given | ||
higher priority than any other in the system, processes from this class are | ||
given first access to the disk every time. Thus it needs to be used with some | ||
care, one io RT process can starve the entire system. Within the RT class, | ||
there are 8 levels of class data that determine exactly how much time this | ||
process needs the disk for on each service. In the future this might change | ||
to be more directly mappable to performance, by passing in a wanted data | ||
rate instead. | ||
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IOPRIO_CLASS_BE: This is the best-effort scheduling class, which is the default | ||
for any process that hasn't set a specific io priority. The class data | ||
determines how much io bandwidth the process will get, it's directly mappable | ||
to the cpu nice levels just more coarsely implemented. 0 is the highest | ||
BE prio level, 7 is the lowest. The mapping between cpu nice level and io | ||
nice level is determined as: io_nice = (cpu_nice + 20) / 5. | ||
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IOPRIO_CLASS_IDLE: This is the idle scheduling class, processes running at this | ||
level only get io time when no one else needs the disk. The idle class has no | ||
class data, since it doesn't really apply here. | ||
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Tools | ||
----- | ||
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See below for a sample ionice tool. Usage: | ||
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# ionice -c<class> -n<level> -p<pid> | ||
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If pid isn't given, the current process is assumed. IO priority settings | ||
are inherited on fork, so you can use ionice to start the process at a given | ||
level: | ||
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# ionice -c2 -n0 /bin/ls | ||
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will run ls at the best-effort scheduling class at the highest priority. | ||
For a running process, you can give the pid instead: | ||
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# ionice -c1 -n2 -p100 | ||
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will change pid 100 to run at the realtime scheduling class, at priority 2. | ||
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---> snip ionice.c tool <--- | ||
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#include <stdio.h> | ||
#include <stdlib.h> | ||
#include <errno.h> | ||
#include <getopt.h> | ||
#include <unistd.h> | ||
#include <sys/ptrace.h> | ||
#include <asm/unistd.h> | ||
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extern int sys_ioprio_set(int, int, int); | ||
extern int sys_ioprio_get(int, int); | ||
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#if defined(__i386__) | ||
#define __NR_ioprio_set 289 | ||
#define __NR_ioprio_get 290 | ||
#elif defined(__ppc__) | ||
#define __NR_ioprio_set 273 | ||
#define __NR_ioprio_get 274 | ||
#elif defined(__x86_64__) | ||
#define __NR_ioprio_set 251 | ||
#define __NR_ioprio_get 252 | ||
#elif defined(__ia64__) | ||
#define __NR_ioprio_set 1274 | ||
#define __NR_ioprio_get 1275 | ||
#else | ||
#error "Unsupported arch" | ||
#endif | ||
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_syscall3(int, ioprio_set, int, which, int, who, int, ioprio); | ||
_syscall2(int, ioprio_get, int, which, int, who); | ||
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enum { | ||
IOPRIO_CLASS_NONE, | ||
IOPRIO_CLASS_RT, | ||
IOPRIO_CLASS_BE, | ||
IOPRIO_CLASS_IDLE, | ||
}; | ||
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enum { | ||
IOPRIO_WHO_PROCESS = 1, | ||
IOPRIO_WHO_PGRP, | ||
IOPRIO_WHO_USER, | ||
}; | ||
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#define IOPRIO_CLASS_SHIFT 13 | ||
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const char *to_prio[] = { "none", "realtime", "best-effort", "idle", }; | ||
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int main(int argc, char *argv[]) | ||
{ | ||
int ioprio = 4, set = 0, ioprio_class = IOPRIO_CLASS_BE; | ||
int c, pid = 0; | ||
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while ((c = getopt(argc, argv, "+n:c:p:")) != EOF) { | ||
switch (c) { | ||
case 'n': | ||
ioprio = strtol(optarg, NULL, 10); | ||
set = 1; | ||
break; | ||
case 'c': | ||
ioprio_class = strtol(optarg, NULL, 10); | ||
set = 1; | ||
break; | ||
case 'p': | ||
pid = strtol(optarg, NULL, 10); | ||
break; | ||
} | ||
} | ||
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switch (ioprio_class) { | ||
case IOPRIO_CLASS_NONE: | ||
ioprio_class = IOPRIO_CLASS_BE; | ||
break; | ||
case IOPRIO_CLASS_RT: | ||
case IOPRIO_CLASS_BE: | ||
break; | ||
case IOPRIO_CLASS_IDLE: | ||
ioprio = 7; | ||
break; | ||
default: | ||
printf("bad prio class %d\n", ioprio_class); | ||
return 1; | ||
} | ||
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if (!set) { | ||
if (!pid && argv[optind]) | ||
pid = strtol(argv[optind], NULL, 10); | ||
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ioprio = ioprio_get(IOPRIO_WHO_PROCESS, pid); | ||
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printf("pid=%d, %d\n", pid, ioprio); | ||
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if (ioprio == -1) | ||
perror("ioprio_get"); | ||
else { | ||
ioprio_class = ioprio >> IOPRIO_CLASS_SHIFT; | ||
ioprio = ioprio & 0xff; | ||
printf("%s: prio %d\n", to_prio[ioprio_class], ioprio); | ||
} | ||
} else { | ||
if (ioprio_set(IOPRIO_WHO_PROCESS, pid, ioprio | ioprio_class << IOPRIO_CLASS_SHIFT) == -1) { | ||
perror("ioprio_set"); | ||
return 1; | ||
} | ||
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if (argv[optind]) | ||
execvp(argv[optind], &argv[optind]); | ||
} | ||
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return 0; | ||
} | ||
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---> snip ionice.c tool <--- | ||
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March 11 2005, Jens Axboe <axboe@suse.de> |
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zreladdr-y := 0xf0008000 |
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