Insufficient Resource Error on msgsnd Call

[Dave Settle]

In article <1023 at dinl.mmc.UUCP> noren at dinl.UUCP (Chuck Noren) writes:
>We have been developing a application that uses System V message queues
>(perhaps thats the first mistake :-)) for interprocess communication.
>Everything has worked fine until we really wanted to stress test the
>application by sending it hundereds of messages at once.  The application
>chugs away nicely until it hangs.
>
>
>First a model of the application.  It consists of three processes (call them
>A, B, and C), and two message queues (call them 1 and 2).  The processes
>and queues are orgainized as:
>
>   +--------+                +--------+                +--------+
>   |        |      +-----+   |        |      +-----+   |        |
>   | Proc A |---->>| Q 1 |-->| Proc B |---->>| Q 2 |-->| Proc C |
>   |        |      +-----+   |        |      +-----+   |        |
>   +--------+                +--------+                +--------+
>
>Process A generates 200 messages in bursts of about 50 as fast as it
>can go (CPU bound) and puts it into Queue 1.  Process B reads the
>messages from Queue 1, processes them while looking things up in
>an Ingres database (we are using Ingres 5.0).  Process B sends even more
>messages to Queue 2 which is read by Process C.
>
>After Process A sends 150 messages (and Process B deleivers more messages),
>Process B tries to write to Queue 2 and hangs (using IPC_WAIT on the msgsnd
>call).  Queue 2 looks empty because Process C is blocking on it (using
>msgsnd with IPC_WAIT).  Queue 1 appears full because when I try to write
>to it with a no-wait (using diagnostic software), 
>it returns with an errno of 11 (the Sun 3 manual
>indicates this is caused by a fork with process limit exceeded or insufficent
>resources).  Trying to write to Queue 2 produces the same error.

The error EAGAIN, to which you refer here, is used in a specific manner by
the 'msgsnd' call to mean 'No more space available to store your message'

>Any suggestions of what could be happening?  Is Ingres using resources
>common to Message Queues?  Have I shown a misunderstanding of how message
>queues are to be used?
>

>From what you have written, I suspect that the problem is that you have
exceeded the GLOBAL message space buffer size with writes to Q1. P2 can
therefore not write to Q2 (no more message space), so nobody can proceed.

You have confused me by stating both that you cannot write to Q2, and that
P3 is sitting trying to read from it, and also that you can cure the problem
for a bit by READING from it - isn't P3 trying to do just that?

But generally, the system has a serious flaw: if Q1 causes the system message
space to fill up, then P2 is stuck - it can't write messages to Q2 'cos
the system is full, but on the other hand, it can't free some space until
it has disposed of the message and read Q1.

I think that you can configure the system (look in /etc/master) so that
you can force the individual queue to be full, before the whole system
is full. I've never done this, though, so I might be wrong.

You might also consider using 'crash' to examine the state of the kernel
message queue structures: it's very useful for things like this.

Cheers,
	Dave
-- 

Dave Settle, Universal (CMS) Ltd, Thames Tower, Burleys Way, Leicester, UK.

dave at ucms.co.uk	 (someday)		...!mcvax!ukc!nott-cs!ucms!dave
dave at ucms.uucp 	  (today)

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