[SOLVED] Can create only 9 POSIX message queues

nerdoctopus · 12-16-2011, 08:41 AM

Hi all,

I have developed an application on Fedora 14 in C++. I need POSIX message queues for IPC. I need multiple message queues. I create a message queue as below:

MessageQueue = mq_open("/AppMessageQueue", O_RDWR | O_CREAT, 0666, NULL);

From a non-root user, the application can only create 9 message queues. On creation of the 10th message queue, mq_open() generates error ENOMEM. No other application can create a new queue as long as 9 queues are open.

From a root user, the application can create more than 9 queues.

The system variables in /proc are as below:

/proc/sys/fs/mqueue/msg_max - 10
/proc/sys/fs/mqueue/msgsize_max - 8192
/proc/sys/fs/mqueue/queues_max - 256

Why can only 9 queues be created from non-root user?

dwhitney67 · 12-16-2011, 10:08 AM

Run the following command:

Code:

ulimit -q

This should list how many bytes can be allocated towards the creation of message queues. If the number is too small for your needs, you can try to increase it using a similar command, but in this case, specifying the desired size:

Code:

ulimit -q <size>

You also may have better success if you specify the (max) size of the data object you plan to send through your queues. You can do this by specifying the attributes for the queue; for example:

Code:

struct mq_attr attr;
attr.mq_flags   = 0;
attr.mq_maxmsg  = 100;
attr.ms_msgsize = 1000;   /* bytes */
attr.mq_curmsgs = 0;

mqd_t qid = mq_open(name, flags, mode, &attr);

P.S. On my Kubuntu 11.10 system, the result of running 'ulimit -q' is 819200.

nerdoctopus · 12-16-2011, 11:13 AM

Thank you for your reply.

From non-root user:

Code:

$ ulimit -q
819200

$ ulimit -q 1638400
bash: ulimit: POSIX message queues: cannot modify limit: Operation not permitted

Since I need to run the application from non-root user, I need to set the limit in non-root user. However, I get 'Operation not permitted' error.

I will try changing the message size and see if that helps.

nerdoctopus · 12-17-2011, 02:39 AM

I have found the solution.

The process max POSIX queue data size is 819200 bytes. Since I used default attributes in mq_open(), the queue was created with space for 10 messages each message of 8192 bytes. This way, for 10 queues the size limit of 819200 bytes would be reached. So the application was able to create only 9 queues.

I added below lines in /etc/security/limits.conf:

Code:

*       hard    msgqueue        8192000
*       soft    msgqueue        8192000

Before launching the application I do:

Code:

ulimit -q 8192000

This time I do not get "Operation not permitted" error. I launch the application and it can create more than 9 queues!

Thank you for your suggestions. I really appreciate it.

dwhitney67 · 12-17-2011, 07:47 AM

Does your application really need to send messages that are 8192 bytes in size? If you know that your messages will be smaller, then you should specify that size and the max number of messages that can be queued, as I demonstrated earlier.

Otherwise, as you found out, the message queue is created with the default message size (8192 bytes) and a depth of 10 messages.

nerdoctopus · 12-17-2011, 12:42 PM

A message can be maximum 2048 bytes in size. So changing the message size and reducing the number of messages in the queue will help in creating more message queues. I will create the messages queues with attributes as you have pointed.

Thank you for your kind support. It was really helpful.