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VM strict overcommit on RedHat AS 2.1 kernel 2.4.18-e12 ?

Linux-ia64 Bill Nottingham 2 participants 2 messages Jan 27, 2003

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Bill Nottingham

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Quoted 4 lines · Thierry

I know that Alan Cox has a patch and that a lot of job is currently done in kernel 2.5 .
This patch is partially integrated in the kernel 2.4.18-e12 delivered with the RedHat Advanced Server 2.1.
But there is a difference between the documentation about overcommit_memory in sysctl/vm.txt and the code really implemented in mm/mmap.c Is there any reason for that ?

What differences are you seeing between the documentation and the code?

Bill

#1 lore raw

Hi,

in the file : Documentation/vm/overcommit-accounting :

The Linux kernel supports four overcommit handling modes

0 - Heuristic overcommit handling. Obvious overcommits of
address space are refused. Used for a typical system. It ensures a seriously wild allocation fails while allowing overcommit to reduce swap usage

1 - No overcommit handling. Appropriate for some scientific
applications

2 - (NEW) strict overcommit. The total address space commit
for the system is not permitted to exceed swap + half ram. In almost all situations this means a process will not be killed while accessing pages but only by malloc failures that are reported back by the kernel mmap/brk code.

3 - (NEW) paranoid overcommit The total address space commit
for the system is not permitted to exceed swap. The machine will never kill a process accessing pages it has mapped except due to a bug (ie report it!)

in the source code : mm/mmap.c
int vm_enough_memory(long pages, int charge)

{
        /* Stupid algorithm to decide if we have enough memory: while
         * simple, it hopefully works in most obvious cases.. Easy to
         * fool it, but this should catch most mistakes.
         *
         * 23/11/98 NJC: Somewhat less stupid version of algorithm,
         * which tries to do "TheRightThing".  Instead of using half of
         * (buffers+cache), use the minimum values.  Allow an extra 2%
         * of num_physpages for safety margin.
         *
         * 2002/02/26 Alan Cox: Added two new modes that do real accounting
         */
        unsigned long free, allowed;
        struct sysinfo i;

        if(charge)
                atomic_add(pages, &vm_committed_space);

        /* Sometimes we want to use more memory than we have. */
        if (sysctl_overcommit_memory == 1)
            return 1;
        if (sysctl_overcommit_memory == 0)
        {
          .....
        }
        if(sysctl_overcommit_memory == 2)
        {
                /* FIXME - need to add arch hooks to get the bits we need
                   without the higher overhead crap */
                si_meminfo(&i);
                allowed += i.totalram >> 1;
        }
        if(atomic_read(&vm_committed_space) < allowed)
                return 1;
        if(charge)
                atomic_sub(pages, &vm_committed_space);
        return 0;

}
Regards,
Thierry
PS: Unfortunatelly I will be out of office until the 3rd Febuary with no access to my mail box.

-----Message d'origine-----
De : Bill Nottingham [mailto:[email protected]]
Envoyé : lundi 27 janvier 2003 17:36
À : Carron, Thierry
Cc : [email protected]
Objet : [Linux-ia64] Re: VM strict overcommit on RedHat AS 2.1 kernel 2.4.18-e12 ?

Quoted 4 lines · Thierry

I know that Alan Cox has a patch and that a lot of job is currently done in kernel 2.5 .
This patch is partially integrated in the kernel 2.4.18-e12 delivered with the RedHat Advanced Server 2.1.
But there is a difference between the documentation about overcommit_memory in sysctl/vm.txt and the code really implemented in mm/mmap.c Is there any reason for that ?

What differences are you seeing between the documentation and the code?

Bill

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