🚀 go-pugleaf

I'm posting this for someone with no net access.

Posted-Date: Mon, 25 Sep 89 11:17:24 -0600
To: bywater!scifi!njs (Nick Simicich)
Subject: overcommitting the page space on RT
Date: Mon, 25 Sep 89 11:17:24 -0600
From: "James Moody" <uunet!cs.utexas.edu!auschs!moody.austin.ibm.com!moody>


Nick,
	Recently, Mark Spotswood posted a news article (I believe it
was comp.sys.ibm.pc.rt) asking for help with a problem on
overcommitting the page space on the RT.  I am most familiar with this
problem since I was/am a development programmer on the VRM.  One of
our vendors was the first we knew of to have the problem and I
developed a solution for them.  Mark's article reminded me that this
is always going to be a problem for large, memory consuming
applications that were originally written for a BSD system.  My
solution is an easy way for developers to port these kind of apps to a
system that uses delayed allocation of paging slots (like AIX version
2).

	I don't have the capability to post articles to the outside
world but I believe this may be an article worth posting.  I mailed
Mark a copy of this and he replied saying it was exactly what he
wanted.  If you could post this for me I would appreciate it.  Thanks
in advance.

/* ----------------------- cut here --------------------------------*/
#define PAGESIZE 2048
#define NULL 0
#include <sys/signal.h>
volatile int dangerflag;
void free();
int main()
{	int size, *ptr;
	
	void handler();
	char *MALLOC();
	
	/* ensure this program catches sigdanger */
	signal(SIGDANGER,handler);

	
	/*************************************************************
	 your favorite memory hogging program which uses MALLOC to
	 allocate storage (not malloc)
	*************************************************************/
	size = ????;

	ptr = MALLOC(size);
	....
	....

}

void handler()
{
	/* sigdanger is sent when the number of paging slots drops
	below the pswarn threshold (see /etc/master) */
	dangerflag = 1;
}
#define MAXMEMSIZE	0x1000000

/* MALLOC is used to overcome the possibility of overcommitting the
   page space */

char *MALLOC(size)
unsigned int size;

{
	char	*malloc();
	char	*p,*q;
	int	i;
	volatile int	*numps; /* number of paging slots from low memory */

	/* Make sure dangerflag is initialized */
	dangerflag = 0;

	/* point to number of paging slots in AIX low memory */
	numps = (int *)0xb8;

	printf("slots,size is %d %d",*numps,size);

	/* ensure there is enough backing storage to back all of
	   memory (note MAXMEMSIZE is as good as I can get here: that
	   is, a lessor value wouldn't work on machines with less
	   memory */
	if ((*numps * PAGESIZE) < (size + MAXMEMSIZE)) return((char*)NULL);


	/* call the real malloc to get the storage */
	q = p = malloc(size);
	if (p == (char *)NULL) return(p);

	for(i = 0; i < size ; i += PAGESIZE)
	{
		/* touch and dirty the next page */
		*q = 0;

		/* Get out if we went below the pswarn threshold */
		if (dangerflag) goto getout;

		/* bump to the next page (note: must be careful to 
		   touch on the next page boundary and not in the
		   middle of the page */
		q = (char *)(((int)q & (~(PAGESIZE-1))) + PAGESIZE); 
	}
	printf("slots,size is %d %d",*numps,size);

	/* ensure there is still enough backing storage */
	if ((*numps * PAGESIZE) < (size + MAXMEMSIZE)) goto getout;
	return(p);


	getout: /* There isn't enough space available */
	printf("slots,size is %d %d",*numps,size);
	free(p);
	return((char *)NULL);
}

/* ------------------------- cut here -----------------------------*/


Zippy says:
..  the MYSTERIANS are in here with my CORDUROY SOAP DISH!!

James Moody	Adv Workstation Div	
		IBM Austin, 2502	  
		aesnet: moody@moody.austin.ibm.com
		vnet: MOODY at AUSVM6
		[512] 823-3743		  t/l   793-3743
[external mail here--->] cs.utexas.edu!ibmaus!auschs!moody!moody


P.S.  I'm not sure the return mail address is correct because I'm
something of a novice at this.
-- 
Nick Simicich --- uunet!bywater!scifi!njs --- njs@ibm.com (Internet)