Message Queues

[Kurt Wall]

On Fri, Feb 03, 2006 at 09:49:18PM -0500, Kurt Wall took 41 lines to write:
> On Wed, Feb 01, 2006 at 10:44:59PM -0700, Steve Jardine took 17 lines to write:
> > Heya All,
> > 
> >     I have a need to send a large amount of messages over a message queue. 
> > Currently, I max out at 400 entries on a specific queue. Does anyone have an idea
> > what system variable I can change to increase the number of entries on a single
> > queue?
> 
> Steve:
> 
> I believe the maximum number of messages is limited by the size of each 
> message.
> 
> $ sysctl -a | grep msg
> ...
> kernel.msgmnb = 16384   /* default size of a message queue */
> kernel.msgmni = 16      /* maximum number of message queue ids */
> kernel.msgmax = 8192    /* maximum size of a message */
> 
> So, if you have a message that is 4096 bytes, you can only get 4 of
> them onto a queue. Happily, kernel.msgmnb <= INT_MAX, which is
> 2147483647 on my system.
> 
> So, I believe what you need to do is increase kernel.msgmnb using
> sysctl. For example, you can say (as root):
> 
> # sysctl kernel.msgmnb=100000
> 
> This increases the message queue size to 100,000 bytes. You should then
> be able to stuff more messages into a single queue. I haven't tested
> this, of course. I'm doing that now.

I've established that this works as stated. The attached program
creates a message queue and adds messages to until msgsnd() fails.
The larger one makes kernel.msgmnb, the more messages it can add:

$ sysctl kernel.msgmnb
kernel.msgmnb = 16384
$ ./a.out
queue ID: 262146
message size is 17 bytes
strlen(buf.mtext) = 18
added 963 17-byte messages (16371 bytes)
$ sudo sysctl kernel.msgmnb=40000
kernel.msgmnb = 40000
$ ./a.out
queue ID: 294915
message size is 17 bytes
strlen(buf.mtext) = 19
added 2352 17-byte messages (39984 bytes)
$

Kurt
-- 
"If God lived on Earth, people would knock out all His windows."
		-- Yiddish saying
-------------- next part --------------
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/ipc.h>
#include <sys/msg.h>

#define MSG_TYPE 1

int main()
{
	int id, cnt, size, ret;
	struct msgbuf {
		long mtype;
		char *mtext;
	} buf;


	/* create a queue */
	if ((id = msgget(IPC_PRIVATE, 0666)) < 0) {
		perror("msgget");
		exit(EXIT_FAILURE);
	}
	printf("queue ID: %d\n", id);

	/* create a message */
	size = strlen("message number ") + 2;
	printf("message size is %d bytes\n", size);
	if ((buf.mtext = malloc(size)) == NULL) {
		perror("malloc");
		exit(EXIT_FAILURE);
	}

	/* initialize the message buffer */
	buf.mtype = MSG_TYPE;
	cnt = 0;
	sprintf(buf.mtext, "message number %d", cnt + 1);

	/* add messages to the queue until it fails */
	while ((ret = msgsnd(id, &buf, size, IPC_NOWAIT)) == 0) {
		cnt++;
		sprintf(buf.mtext, "message number %d", cnt + 1);
	}
	printf("strlen(buf.mtext) = %d\n", (int)strlen(buf.mtext));
	printf("added %d %d-byte messages (%d bytes)\n", cnt, size, cnt*size);

	/* clean up */
	free(buf.mtext);

	return 0;
}