RE: OT - Blog entry on hugepages

From: Yong Huang <yong321@xxxxxxxxx>
To: Jed_Walker@xxxxxxxxxxxxxxxxx
Date: Fri, 6 Apr 2012 08:44:38 -0700 (PDT)
> >ipcs
> ------ Shared Memory Segments --------
> key        shmid      owner     perms      bytes       nattch     status
> 0x00000000 29097988   oracle    640        4096       0
> 0x00000000 29130757   oracle    640        4096       0
> 0x0e2f3c64 29163526   oracle    640        4096       0
> 0x00000000 29655047   oracle    640        268435456  59
> 0x00000000 29687816   oracle    640        51271172096 59
> 0x2894b058 29720585   oracle    640        2097152    59
> 
> I could be wrong, but 51271172096/1024/1024/1000H (since pages are 
> in kbytes, this would seem to be my 48G SGA). I'm not sure what the 
> one above it is and it is big enough to make me wonder.

Jed,

Your "one above" (the one with 268435456 bytes) and the one below (2097152 
bytes) are all shared memory segments used by the same instance. The clue 
is the same number of attachments (very unlikely they all happen to have 
59 processes) and the fact that two of them have all-zero keys. Your 
51271172096/1024/1024/1024 is 47.75 GB and 268435456/1024/1024/1024 is 
0.25 GB. It always seems to have an extra 2MB segment.

Here's what I have (on my 64-bit Red Hat Linux 6 sandbox, uncertified!):

$ ipcs -m

------ Shared Memory Segments --------
key        shmid      owner      perms      bytes      nattch     status
0x00000000 229377     oracle     640        4096       0
0x00000000 262146     oracle     640        4096       0
0xb260fc88 294915     oracle     640        4096       0
0x00000000 1933316    oracle     640        16777216   57
0x00000000 1966085    oracle     640        1174405120 57
0xb82bfec8 1998854    oracle     640        2097152    57

$ ps -ef | grep pmon
oracle   11895     1  0 Apr05 ?        00:00:25 ora_pmon_sanora1
oracle   13195     1  0 Apr04 ?        00:00:31 asm_pmon_+ASM1

Now check the shared memory segments in the process memory map:

# egrep '1933316|1966085|1998854' /proc/11895/maps
60000000-61000000 rwxs 00000000 00:04 1933316                            
/SYSV00000000 (deleted)
61000000-a7000000 rwxs 00000000 00:04 1966085                            
/SYSV00000000 (deleted)
a7000000-a7200000 rwxs 00000000 00:04 1998854                            
/SYSVb82bfec8 (deleted)

(My ASM is using /dev/shm so ignore that.) You see all the 3 so-called 
SystemV shared memory segments. And the sizes are

60000000-61000000 -> 16777216 bytes
61000000-a7000000 -> 1174405120 bytes 
a7000000-a7200000 -> 2097152 bytes

The above check can be done regardless HugePages usage. In fact, my Oracle 
on Red Hat 6 was just installed and I haven't configured HugePages yet.

Yong Huang
--
http://www.freelists.org/webpage/oracle-l
RE: OT - Blog entry on hugepages

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