msgmnb limits?

[Troels Arvin <troels.arvin@gmail.com>]

Hello,

On a server with lots of RAM (192GB), I ran into an application (DB2) 
which applies the following sysctl changes when it's started, if the 
values haven't already been set to - at least - the stated values:

  - kernel.msgmax = 65535
  - kernel.msgmnb = 65536

However, that doesn't work: Certain operations (LOAD and 
backups/restores) will fail with the following messages, unless 
kernel.msgmnb is increased:

DIA8557C No message was sent using the message queue.
CALLED  : OS, -, msgsnd                           OSERR: EAGAIN (11)
DATA #1 : system V message queue identifier., PD_TYPE_SYSV_QUEUE_ID


I tried setting kernel.msgmnb to 524288, and then things work. This 
value was chosen out of the blue, and I'd like to make a more informed 
choice:

  - Is there a limit to the size of msgmnb?
  - Can I expect the system to start thrashing if msgmnb is
    increased beyond a certain value?
  - Is my understanding correkt: msgmnb should always be at least
    as large as msgmax?

Thanks in advance.

-- 
Regards,
Troels Arvin