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+Block io priorities
+===================
+
+
+Intro
+-----
+
+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.
+
+Scheduling classes
+------------------
+
+CFQ implements three generic scheduling classes that determine how io is
+served for a process.
+
+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.
+
+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.
+
+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.
+
+Tools
+-----
+
+See below for a sample ionice tool. Usage:
+
+# ionice -c<class> -n<level> -p<pid>
+
+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:
+
+# ionice -c2 -n0 /bin/ls
+
+will run ls at the best-effort scheduling class at the highest priority.
+For a running process, you can give the pid instead:
+
+# ionice -c1 -n2 -p100
+
+will change pid 100 to run at the realtime scheduling class, at priority 2.
+
+---> snip ionice.c tool <---
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <errno.h>
+#include <getopt.h>
+#include <unistd.h>
+#include <sys/ptrace.h>
+#include <asm/unistd.h>
+
+extern int sys_ioprio_set(int, int, int);
+extern int sys_ioprio_get(int, int);
+
+#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
+
+_syscall3(int, ioprio_set, int, which, int, who, int, ioprio);
+_syscall2(int, ioprio_get, int, which, int, who);
+
+enum {
+        IOPRIO_CLASS_NONE,
+        IOPRIO_CLASS_RT,
+        IOPRIO_CLASS_BE,
+        IOPRIO_CLASS_IDLE,
+};
+
+enum {
+        IOPRIO_WHO_PROCESS = 1,
+        IOPRIO_WHO_PGRP,
+        IOPRIO_WHO_USER,
+};
+
+#define IOPRIO_CLASS_SHIFT      13
+
+const char *to_prio[] = { "none", "realtime", "best-effort", "idle", };
+
+int main(int argc, char *argv[])
+{
+        int ioprio = 4, set = 0, ioprio_class = IOPRIO_CLASS_BE;
+        int c, pid = 0;
+
+        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;
+                }
+        }
+
+        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;
+        }
+
+        if (!set) {
+                if (!pid && argv[optind])
+                        pid = strtol(argv[optind], NULL, 10);
+
+                ioprio = ioprio_get(IOPRIO_WHO_PROCESS, pid);
+
+                printf("pid=%d, %d\n", pid, ioprio);
+
+                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;
+                }
+
+                if (argv[optind])
+                        execvp(argv[optind], &argv[optind]);
+        }
+
+        return 0;
+}
+
+---> snip ionice.c tool <---
+
+
+March 11 2005, Jens Axboe <axboe@suse.de>