[RFC -mm 0/6] sysv ipc: scale msgmnb with the number of cpus

[<Solofo.Ramangalahy@bull.net>]

The size in bytes of a SysV IPC message queue, msgmnb, is too small
for large machines, but we don't want to bloat small machines

Several methods are used already to modify (mainly increase) msgmnb:
. distribution specific patch
. system wide sysctl.conf
. application specific tuning via /proc/sys/kernel/msgmnb

Integrating this series would:
. reflect hardware and software evolutions and diversity,
. reduce configuration/tuning for the applications.

Here is the timeline of the evolution of MSG* #defines:
Year             1994         1999         1999          2008
Version           1.0       2.3.27       2.3.30        2.6.24
#define MSGMNI    128          128           16            16
#define MSGMAX   4056         8192         8192          8192
#define MSGMNB  16384        16384        16384         16384

This patch series scales msgmnb, with respect to the number of
cpus/cores for larger machines. For uniprocessor machines the value
does not increase.

This series is similar to (and depends on) the series which scales
msgmni, the number of IPC message queue identifiers, to the amount of
low memory.
While Nadia's previous series scaled msgmni along the memory axis,
hence the message pool (msgmni x msgmnb), this series uses a second
axis: the number of online CPUs.
As well as covering the (cpu,memory) space of machines size, this
reflects the parallelism allowed by lockless send/receive for
in-flight messages in queues (msgmnb / msgmax messages).

The initial scaling is done at initialization of the ipc namespace.
Furthermore, the value becomes dynamic with respect to cpu hotplug.

The msgmni and msgmnb values become dependent, as the value of msgmni
is computed with respect to the value of msgmnb.

The series is as follows:
. patch 1 introduces the scaling function 
. patch 2 deals with cpu hotplug
. patch 3 allows user space to disable the scaling mechanism
. patch 4 allows user space to reenable the scaling mechanism
. patch 5 finer grain disabling/reenabling scaling mechanism
          (disconnect msgmnb and msgmni)
. patch 6 adds documentation

--- 

The series applies to 2.6.26-rc2-mm1 + patch suppressing KERN_INFO
messages as discussed at:
http://article.gmane.org/gmane.linux.kernel/686229
"[PATCH 1/1] Only output msgmni value at boot time"
(in mmotm: ipc-only-output-msgmni-value-at-boot-time.patch)

The plan would be to have this ready for the 2.6.27 merge window if
there are no objections.

 Documentation/sysctl/kernel.txt |   27 ++++++++++++++++++++++
 include/linux/ipc_namespace.h   |    4 ++-
 include/linux/msg.h             |    5 ++++
 ipc/ipc_sysctl.c                |   48 ++++++++++++++++++++++++++++++----------
 ipc/ipcns_notifier.c            |   23 +++++++------------
 ipc/msg.c                       |   25 +++++++++++++++++---
 ipc/util.c                      |   28 +++++++++++++++++++++++
 ipc/util.h                      |    1
 8 files changed, 131 insertions(+), 30 deletions(-)

-- 
Solofo Ramangalahy
Bull SA.