Re: Calculating the required value of "ulimit -q"

[Eric Wong <normalperson@yhbt.net>]

Iñaki Baz Castillo <ibc@aliax.net> wrote:
> Hi, I create a mqueue with msgsize 1024 and maxmsg 5000, so in total:
> 
>   5120000 bytes
> 
> I set for root that value:
> 
>   ~# ulimit -q 5120000
> 
> However I get Errno::ENOMEM. In order it to work I must increase the ulimit to 
> a value closed to 5160000.
> 
> Could I know which is the exact algorithm to calculate the required "ulimit -
> q" value based on the desired msgsize and maxmsg?
> I would like my application to suggest that value if it cannot start due to 
> memory limitations when creating the mqueue.

Hi Iñaki,

Under Linux, "man 2 getrlimit" says this:

  RLIMIT_MSGQUEUE (Since Linux 2.6.8)
        Specifies the limit on the number of bytes that can be allocated
        for POSIX message queues for the real user  ID  of  the  calling
        process.	 This  limit is enforced for mq_open(3).  Each message
        queue that the user creates counts (until it is removed) against
        this limit according to the formula:

        bytes = attr.mq_maxmsg * sizeof(struct msg_msg *) +
                attr.mq_maxmsg * attr.mq_msgsize

        where  attr  is  the  mq_attr  structure specified as the fourth
        argument to mq_open(3).

        The first addend in the formula,	which  includes  sizeof(struct
        msg_msg *) (4 bytes on Linux/i386), ensures that the user cannot
        create an unlimited number of zero-length  messages  (such  mes
        sages nevertheless each consume some system memory for bookkeep
        ing overhead).

I'm not sure about other OSes (info/research/experiences would be
greatly appreciated!).

-- 
Eric Wong