Resident memory set size

[Michael Kelly <mike@weatherwax.co.uk>]

I've been looking at why the swap appears not to be working at the 
moment. I've identified a small set of alterations to the code in 
vm_page_seg_evict that have been introduced relatively recently that if 
reverted restores swap functionality but I don't have sufficient 
knowledge to propose a fix yet.

As part of this research I have also come across another feature that 
arises when running low on memory. I have a virtual machine with 2GB of 
RAM and a very simple test program to gobble 400M of memory per instance 
(below). I run 2 of these and as expected each process has a virtual 
size of ~550M and an RSS of 400M. I then run a 3rd instance to consume a 
further 400M. This causes memory from the 1st of my test programs to 
have some of its memory paged out and its RSS drops to about 150M.

That all seems quite reasonable until you discover that no memory is 
actually being sent to swap as there are no messages sent to the pager.  
I was so surprised by this that I actually commented out all of the code 
in vm_pageout_scan except for the page balancing but it still did the 
same. It is the page balancing that is reducing the resident memory 
total for my first process. Each page that is moved gets marked as 
VM_PROT_NONE and the task's (pmap) resident_count decremented 
accordingly. There doesn't seem to be any equivalent increment 
associated with the page that it is moved to.

I'm very aware that I have a very limited knowledge of the gnumach 
kernel and I could be very mistaken in my analysis here. I was rather 
hoping for some input to this scenario to either correct my 
misunderstanding or else guide me into further research.

Regards,

Mike.

Test program:

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>

#define SZ 100000

int
main(int argc, const char *argv[])
{
        printf("Usemem: %d\n", getpid());
        void* mem = malloc(SZ * 4096);
        memset(mem, 127,  SZ * 4096);
        sleep(3600);
    free(mem);

    return 0;
}