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Re: OS X shared memory documentation

From: "Jim C(dot) Nasby" <jnasby(at)pervasive(dot)com>
To: Chris Campbell <chris(at)bignerdranch(dot)com>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, pgsql-patches(at)postgresql(dot)org
Subject: Re: OS X shared memory documentation
Date: 2006-02-23 00:56:54
Message-ID: 20060223005654.GS86022@pervasive.com (view raw or flat)
Thread:
Lists: pgsql-patches
Outstanding writeup! Mind if I post it in our knowledgebase at
http://www.pervasivepostgres.com/instantkb13/? (Unless someone has a
better idea of where this could go...)

On Sun, Feb 12, 2006 at 04:33:27PM -0500, Chris Campbell wrote:
> 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
> 



-- 
Jim C. Nasby, Sr. Engineering Consultant      jnasby(at)pervasive(dot)com
Pervasive Software      http://pervasive.com    work: 512-231-6117
vcard: http://jim.nasby.net/pervasive.vcf       cell: 512-569-9461

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