Re: Why Oracle creates two shared memory segments instead of one?

On Oct 25, 3:44 pm, Vsevolod Afanassiev <vafanass..._at_yahoo.com> wrote:
> I noticed that on some platforms Oracle always creates two shared
> memory
> segments instead of one, even when both segments are much smaller than
> the
> limit.
>
> For example, on this Tru64 box the limit is set to 2 GB:
>
> oracle > sysconfig -q ipc
> ipc:
> msg_max = 8192
> msg_mnb = 16384
> msg_mni = 64
> msg_tql = 40
> shm_max = 2147483640 <<<---- Limit set to 2 GB
> shm_min = 1
> shm_mni = 1024
>
> Still Oracle creates two segments: one 8 MB, second 900 MB
>
> oracle> ipcs -am
>
> Shared Memory:
> T ID KEY MODE OWNER GROUP CREATOR CGROUP
> NATTCH SEGSZ CPID LPID ATIME DTIME CTIME
> m 1024 0x1e34 --rw-rw-rw- root daemon root
> daemon 4 524288 3679586 4104730 11:32:05 11:32:04
> 21:14:45
> m 2 0 --rw-r----- oracle dba oracle
> dba 9 8388608 4090358 4115445 8:40:03 8:40:13
> 10:58:11
> m 3 0x1c6492d0 --rw-r----- oracle dba oracle
> dba 9 947912704 4090358 4115445 8:40:03 8:40:13
> 10:58:11

Just guessing here, but since Oracle doesn't use semaphores on Tru64 ( http://www.oracle.com/technology/products/oracle9i/pdf/Oracle_9i_on_Tru64_UNIX.pdf metalink Note:153961.1 etc.) , I'll bet the little shared memory area is how they do interprocess communication. Try varying the size of things if you can, my prediction is the big one will go up and down, but the little one will stay the same. Of course, they may do everything with sockets since they have to support RAC and clusters, I dunno.

jg

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