Re: [PATCHES] OS X shared memory documentation

[Chris Campbell]

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

[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);
            }
        }






Does that make sense?



(On 10.3.9 and later)


(On earlier systems)




Thanks!

- Chris


Referenced source code:

Mac OS X 10.4.4:

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

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