Re: Getting rid of SHMMAX/SHMALL ?

["linux-os \(Dick Johnson\)" <linux-os@analogic.com>]

On Thu, 4 Aug 2005, Andi Kleen wrote:

>
> I noticed that even 64bit architectures have a ridiculously low
> max limit on shared memory segments by default:
>
> #define SHMMAX 0x2000000                 /* max shared seg size (bytes) */
> #define SHMMNI 4096                      /* max num of segs system wide */
> #define SHMALL (SHMMAX/PAGE_SIZE*(SHMMNI/16)) /* max shm system wide (pages) */
>
> Even on 32bit architectures it is far too small and doesn't
> make much sense. Does anybody remember why we even have this limit?
>
> IMHO per process shm mappings should just be controlled by the normal
> process and global mappings with the same heuristics as tmpfs
> (by default max memory / 2 or more if shmfs is mounted with more)
> Actually I suspect databases will usually want to use more
> so it might even make sense to support max memory - 1/8*max_memory
>
> I would propose to get rid of of shmmax completely
> and only keep the old shmall sysctl for compatibility.
>
> Comments?
>
> -Andi


It doesn't seem to be used very much. Here's the `grep` of the
entire 2.6.12 source-tree:


size_t	shm_ctlmax = SHMMAX;
./ipc/shm.c
                     (actually only bits 25..16 get used since SHMMAX is so low)
./include/asm-m68k/shm.h
 	KERN_SHMMAX=34,         /* long: Maximum shared memory segment */
./include/linux/sysctl.h
  * SHMMAX, SHMMNI and SHMALL are upper limits are defaults which can
#define SHMMAX 0x2000000		 /* max shared seg size (bytes) */
#define SHMALL (SHMMAX/PAGE_SIZE*(SHMMNI/16)) /* max shm system wide (pages) */
./include/linux/shm.h
                     (actually only bits 25..16 get used since SHMMAX is so low)
./include/asm-h8300/shm.h
#ifndef SHMMAX
#define SHMMAX          0x003fa000
./include/asm-arm26/shmparam.h
 		.ctl_name	= KERN_SHMMAX,
./kernel/sysctl.c



Cheers,
Dick Johnson
Penguin : Linux version 2.6.12 on an i686 machine (5537.79 BogoMips).
Warning : 98.36% of all statistics are fiction.
.
I apologize for the following. I tried to kill it with the above dot :

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