ionice: Extend the man page to explain the "none" class and cpu-nice inheritance

Extend the ionice man page to explain the "none" class and how the
cpu-nice => io-priority inheritance works.

Signed-off-by: Jakob Unterwurzacher <jakobunt@gmail.com>

1 files changed, 12 insertions, 4 deletions

diff --git a/schedutils/ionice.1 b/schedutils/ionice.1
index 2eca2b8b1..aef40c38e 100644
--- a/schedutils/ionice.1
+++ b/schedutils/ionice.1
@@ -11,7 +11,7 @@ This program sets or gets the io scheduling class and priority for a program.
 If no arguments or just \fI-p\fR is given, \fIionice\fR will query the current
 io scheduling class and priority for that process.
 
-As of this writing, Linux supports 3 scheduling classes:
+As of this writing, a process can be in one of four scheduling classes:
 
 .IP "\fBIdle\fP"
 A program running with idle io priority will only get disk time when no other
@@ -21,9 +21,9 @@ class does not take a priority argument. Presently, this scheduling class
 is permitted for an ordinary user (since kernel 2.6.25).
 
 .IP "\fBBest effort\fP"
-This is the default scheduling class for any process that hasn't asked for
-a specific io priority. Programs inherit the CPU nice setting for io
-priorities. This class takes a priority argument from \fI0-7\fR, with lower
+This is the effective scheduling class for any process that hasn't asked for
+a specific io priority.
+This class takes a priority argument from \fI0-7\fR, with lower
 number being higher priority. Programs running at the same best effort
 priority are served in a round-robin fashion.
 
@@ -35,6 +35,14 @@ some care, as it can starve other processes. As with the best effort class,
 will receive on each scheduling window. This scheduling class is not
 permitted for an ordinary (i.e., non-root) user.
 
+.IP "\fBNone\fP"
+A process that that hasn't asked for a specific io priority will formally be
+in this class. The io scheduler will treat a process that is in this class as
+if it were in the best effort class. The priority within the best effort
+class will be dynamically derived from the cpu nice level of the process:
+io_priority = (cpu_nice + 20) / 5
+
+
 .SH OPTIONS
 .IP "\fB-c \fIclass\fP"
 The scheduling class. \fI1\fR for real time, \fI2\fR for best-effort, \fI3\fR for idle.