[RFC -mm 6/6] sysv ipc: documentation for msgmnb scaling wrt. cpus

[<Solofo.Ramangalahy@bull.net>]

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From: Solofo Ramangalahy <Solofo.Ramangalahy@bull.net>

Add documentation to explain how to disable and reenable the
computation mechanism.

Signed-off-by: Solofo Ramangalahy <Solofo.Ramangalahy@bull.net>

---

 Documentation/sysctl/kernel.txt |   27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

Index: b/Documentation/sysctl/kernel.txt
===================================================================
--- a/Documentation/sysctl/kernel.txt
+++ b/Documentation/sysctl/kernel.txt
@@ -179,6 +179,33 @@ kernel stack.
 
 ==============================================================
 
+msgmnb
+
+Maximum size in bytes, not in message count, of a single SystemV IPC
+message queue (b stands for bytes).
+
+This value is dynamic and depends on the online cpu count of the
+machine (taking cpu hotplug into account).
+
+Computed values are between MSGMNB and MSGMNB*MSG_CPU_SCALE #define
+constants (currently [16384,65536]).
+
+The exact value is automatically (re)computed, but:
+. If the value is positioned from user space (via procfs or sysctl()),
+  to a positive value then the automatic recomputation is
+  disabled. This leaves control to user space. E.g.
+
+  # echo 16384 > /proc/sys/kernel/msgmnb
+
+. If the value is positioned from user space to a negative value, then
+  the computation is reenabled. E.g.
+
+  # echo -1 > /proc/sys/kernel/msgmnb
+
+See recompute_msgmnb() function in ipc/ directory for details.
+
+==============================================================
+
 osrelease, ostype & version:
 
 # cat osrelease

-- 
Solofo Ramangalahy
Bull SA.