Re: OS X shared memory documentation

The definitive answer can be found in the source code for the Darwin
kernel, xnu (links are below):

1. The shmmax, shmmin, shmmni, shmseg, and shmall settings cannot be
changed after the shared memory system is initialized

2. The shared memory system is initialized immediately after all 5
settings have been configured

3. The shmall setting must be a multiple of the page size (on both
10.3 and 10.4)

Here are excerpts from the sysctl_shminfo() function found in
sysv_shm.c:

[1]
/* Set the values only if shared memory is not initialised */
if (!shm_inited) {
if ((error = SYSCTL_IN(req, arg1, sizeof(user_ssize_t)))
!= 0) {
sysctl_shminfo_ret = error;
goto sysctl_shminfo_out;
}

[2]
/* Initialize only when all values are set */
if ((shminfo.shmmax != (user_ssize_t)-1) &&
(shminfo.shmmin != (user_ssize_t)-1) &&
(shminfo.shmmni != (user_ssize_t)-1) &&
(shminfo.shmseg != (user_ssize_t)-1) &&
(shminfo.shmall != (user_ssize_t)-1)) {
shminit(NULL);
}

[3]
(10.3)
if (arg1 == &shminfo.shmmax) {
if (shminfo.shmmax & PAGE_MASK) {
shminfo.shmmax = -1;
return(EINVAL);
}
}

(10.4)
if (arg1 == &shminfo.shmmax) {
if (shminfo.shmmax & PAGE_MASK_64) {
shminfo.shmmax = -1;
return(EINVAL);
}
}

PAGE_MASK is "(PAGE_SIZE - 1)", and PAGE_MASK_64 is simply "(unsigned
long long)PAGE_MASK"

/etc/rc contains commands to set all 5 of the parameters. In 10.3.9
and later, you can place your own customized settings inside /etc/
sysctl.conf that will be executed before /etc/rc's commands. But if
you don't set them all, the shared memory system isn't initialized,
and /etc/rc will overwrite them.

Does that make sense?

In a nutshell, if you want to customize the shared memory settings,
you must:

(On 10.3.9 and later)
Set ALL 5 settings inside /etc/sysctl.conf, and make sure that shmmax
is a multiple of the page size (4096?). The page size is a runtime
variable that's set by kernel bootstrap mechanisms calling
vm_set_page_size() (I haven't tracked those down yet).

(On earlier systems)
Modify the desired settings inside /etc/rc, and be aware that OS
updates will overwrite your changes. If you upgrade to 10.3.9,
migrate to using /etc/sysctl.conf.

In my own OS X products that use a PostgreSQL server, I install an /
etc/sysctl.conf file on all servers. If installing on anything
earlier than 10.3, my installer modifies /etc/rc. That way, when the
customer upgrades their server to 10.3.9 and the /etc/rc file is
overwritten, the /etc/sysctl.conf file is already in place.

Thanks!

- Chris

Referenced source code:

Mac OS X 10.4.4:
http://www.opensource.apple.com/darwinsource/10.4.4.ppc/
xnu-792.6.56/bsd/kern/sysv_shm.c

Mac OS X 10.3.9:
http://www.opensource.apple.com/darwinsource/10.3.9/xnu-517.12.7/
bsd/kern/sysv_shm.c

Mac OS X 10.3:
http://www.opensource.apple.com/darwinsource/10.3/xnu-517/bsd/
kern/sysv_shm.c