shmmax calculation

[Ridzuan Zakaria]

Hello,

How big is my shmmax if the value is 0x4000000?

How do I convert it to Mbytes?

Thanks.

[James R. Ferguson · Accepted solution]

Hi:

One way is with 'bc':

echo "ibase=16;4000000;obase=10"|bc

This will convert base-16 input to base-10 output.

BTW, a megabyte = (1024*1024) bytes.

Regards!

...JRF...

[James R. Ferguson]

Hi (again):

BTW, 0x4000000 is the maximum shared memory segment size (in bytes) for a 32-bit kernel.

Regards!

...JRF...

[James Murtagh]

Hi,

Another way is with adb:

# echo 0x4000000=D | adb

This will give the answer in bytes. You will see it is 67108864, or ~64MB.

Regards,

James.

[Con O'Kelly]

Hi

I also use the Windows calculator!!
Select View -> Scientific.
Select Hex, type in the hex number, then select decimal.
So for your shmmax this gives 67108864 bytes (64MB).
As JRF said this is the maximum shared memory segment size, so if you're installing Oracle you'll need to increase it!

Cheers
Con

[Steven E. Protter]

If you are changing this value, know this little rule.

Your system will ignore any shmmax setting greater than 25% of available memory.

Memory is defined as physical memory PLUS swap.

swapinfo -tam

SEP

[CG UNIX SysAdmin]

I'm curious where you came up with the "little rule" that "...Your system will ignore any shmmax setting greater than 25% of available memory...."?

According to the HP documentation availble to me,
http://docs.hp.com/hpux/onlinedocs/939/KCParms/KCparam.ShMmax.html

"shmmax...The value used cannot exceed maximum available swap space..."

and there is no mention that I can find about the system ignoring anything, however "...Any shmget() system call that requests a segment larger than this limit returns an error (see HP-UX Reference entry shmget(2))."

Is there some updated documentation we can refer to?

Thanks.

Carl Muellner
cpm@capgroup.com

[Alzhy]

This equates to 67108864 bytes (~ 64 Megs.) As the other have stated -- there are a million ways to convert this.

A word about SHHMAX -- if you're running 32-bit Oracle or apps, set this to 4GB (or 0x100000000). For 64-bit Oracle - ask your DBA -- will there ever be a SGA size greater than 4GB? If there is then set it to whatever likely the biggest SGA or SHM reqs there could be. Ideally, Oracle SGA or apps SHM reqs. must FIT in one SHM segment for a server to perform optimally....

HTH.

[Alzhy]

This equates to 67108864 bytes (~ 64 Megs.) As the others have stated -- there are a million ways to convert this.

A word about SHHMAX -- if you're running 32-bit Oracle or apps, set this to 4GB (or 0x100000000). For 64-bit Oracle - ask your DBA -- will there ever be a SGA size greater than 4GB? If there is then set it to whatever likely the biggest SGA or SHM reqs there could be. Ideally, Oracle SGA or apps SHM reqs. must FIT in one SHM segment for a server to perform optimally....

HTH.

[patrick_119]

was just going thru the document from oracle for Ora Apps 11i {11.5.9] on HP UX 11i. They say that the param SHMMAX should be equal to available physical memory, large enough to hold the entire SGA in one shared memory segment. a lower value will result in spreading the SGA over several segments which will cause performance degradation.

[RAC_1]

As all gurus have put, by now you must have learned how to convery hex value to decimal.

Another command for that

printf "%d\n" 0x4000000


Now to the oracle part. If your server is running only that, and you need not woorl abt anything else, I would try to max memory setting for shmmax. I will also try setting shmmax to that of oracle sga.

Not e you have limitationm of 1.7 gb for 32 bit oracle.