From owner-freebsd-current  Mon Apr 12  8: 8:17 1999
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From: Mikhail Teterin <mi@misha.cisco.com>
Message-Id: <199904121505.LAA70241@misha.cisco.com>
Subject: Re: swap-related problems
In-Reply-To: <199904120400.VAA15072@apollo.backplane.com> from Matthew Dillon at "Apr 11, 1999 09:00:24 pm"
To: dillon@apollo.backplane.com (Matthew Dillon)
Date: Mon, 12 Apr 1999 11:05:21 -0400 (EDT)
Cc: current@freebsd.org
Reply-To: mi@aldan.algebra.com
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Matthew Dillon once wrote:
 
>     If you unset the datasize limit and the program does not exceed
>     the maximum system-supported datasize limit, malloc() should not
>     return NULL even if the system is out of swap.

Can you explain why? Our malloc(3) is a little fuzzy as to the
circumstances under which malloc will fail, but on Solaris, it is
more explicit:

	RETURN VALUES
		If there is no available memory, malloc(), realloc(),
		memalign(), valloc(), and calloc() return a null pointer.

I consider being out-of-swap as having "no available memory". Wouldn't you?

Now, for better diagnostics and for a possibility of smarter applications,
they also have:

	ERRORS
	     If malloc(), calloc(), or realloc() returns  unsuccessfully,
	     errno will be set to indicate the following:

	     ENOMEM     size bytes of  memory  exceeds  the  physical
			limits  of  your  system, and cannot be allo-
			cated.

	     EAGAIN     There is not enough memory available AT  THIS
			POINT  IN  TIME  to  allocate  size  bytes of
			memory; but the application could  try  again
			later.

>     The correct solution is to set a maximum datasize limit. That is
>     what it is there for. I generally unlimit the datasize for root
>     and set it to around 64MB for all other users.

Wait, I want to give all the memory available to a program, but I
want it to be NOTIFIED when no more memory is left. True, I can
set the limit to the actual amount of my swap, but that would be
hard to maintain and still does not solve a problem of running
together with some other memory consuming application.

The existing situation is sort of like killing a program for trying
to write to disk in excess of the user's quota, or for being ready
to RUN when the load is 1.

	-mi


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