Re: shmmax sizing recommendations
Date: Thu, 19 Nov 2009 12:39:17 -0600
Message-ID: <94e86aed0911191039n785ee473x787dede2efab223_at_mail.gmail.com>
- Yes memory_target=0. I'm aware of the issue with AMM and HugePages.
- Even if no harm, I'd feel better setting the shm* to values that the host would be able to handle.
- Thanks for that note. That seems to be the general concensus re multiple fragments.
Don.
On Thu, Nov 19, 2009 at 12:13 PM, Yong Huang <yong321_at_yahoo.com> wrote:
>> While troubleshooting an ORA-04030 issue, Oracle Support also took
> ...
>> Oracle 11.1.0.7 64-bit on OEL 5.3. The server has 64gb of physical
> ...
>> SGA sizes of ~42 gb and HugePages configured for 44gb. The largest
>
> I highlight three points in your message: ORA-4030, 11g, HugePages. You
> probably already know that to use HugePages in 11g you need to turn off
> AMM by setting memory_target to 0. Can you double check? Make sure SGA
> *is* using HugePages, otherwise you would squeeze the memory available
> for not only SGA but also PGA to a small number.
>
> I have a note about HugePages:
>
> http://yong321.freeshell.org/oranotes/HugePages.txt
>
>
>> In the past I have set SHMMAX to either 1) the physical ram or 2)
>> theoretical limit. Since this is simply an upper limit, it really
>> has no performance "hit" for setting it too high.
>
> I remember Jim Mauro in his "Solaris Internals" 1st ed says shmmax is
> just a number checked to see if you attempt to create a shared memory
> segment larger than that. There's no other use. So there's no danger
> setting it too large. I think it's the same on Linux. Here's my setting:
>
> [oracle_at_dctrpcora1a ~]$ sysctl kernel.shmmax
> kernel.shmmax = 4294967296
> [oracle_at_dctrpcora1a ~]$ ssh dctrpcora1b /sbin/sysctl kernel.shmmax
> kernel.shmmax = 68719476736
>
> On both RHEL boxes, I have only 4 GB physical RAM. But I tested setting
> shmmax to 68 GB on one of them without any issue.
>
>
>> There is some overhead in having multiple shared memory segments vs.
>> a single one (for any given instance) but I dont have any numbers
>> in my pocket to qualify that.
>
> Steve Adams in his O'Reilly book says having multiple fragments in a
> shared memory segment slightly slows down instance startup and server
> process creation, with no other negative impact. But if you have very
> frequent process creations, it's better to have one big shared memory
> segment.
>
> Yong Huang
>
>
>
>
-- Don Seiler http://seilerwerks.wordpress.com ultimate: http://www.mufc.us -- http://www.freelists.org/webpage/oracle-lReceived on Thu Nov 19 2009 - 12:39:17 CST