Re: Combined SGA size and Total Memory
- From: Mladen Gogala <gogala@xxxxxxxxxxxxx>
- To: oracle-l@xxxxxxxxxxxxx
- Date: Wed, 23 Jun 2004 20:33:31 -0400
On 06/23/2004 08:04:03 PM, Thomas Biju wrote:
> R3JhZW1lLA0KTm8sIEknbSBub3QgdGFsa2luZyBhYm91dCBpbmNyZWFzaW5nIHRoZSBzaXplIG9m
I hate when I have to decode email. It should be sent in ASCII format, no MIME
encoding. Nevertheless, you wrote:
******************************************************************************************
Graeme,
No, I'm not talking about increasing the size of SGA simply because we have
memory. The reason for the question was basically different. We
typically create new databases on the same HP or Sun server which currently
host anywhere from 6 to 23 databases... At what point should we
alert the management for a memory upgrade? My point was there should not be any
rule of thumb such as SGA should be less than 30% of total
RAM, more memory requirement should be based on the load on the server....
Thanks,
Biju
*******************************************************************************************
There shouldn't be a rule of thumb. There are utilities called top, sar,
iostat, vmstat and glance telling you how much time are CPUs spending in the
kernel mode. When CPU is in the kernel mode, it is executing kernel code, not
user code (unless your system has been badly hacked), which means that the
system is not performing work for the user. There are several things that are
done in kernel mode, most notable of which are servicing interrupts or paging.
System services interrupts for I/O. The architecture of a Unix minicomputer
(unless it's an HP SuperDome, in which case it's a completely different
situation) is such that there are several interrupts for each I/O request. When
you issue command like "commit", it is going to cause several interrupt
requests. Interrupts are not a problem. I used to work on HP 9000/N system with
16 CPUs, which could seamlessly perform up to 5000 I/O requests per second
against EMC Symmetrix storage array. When you start seeing 30% of your total
CPU power being utilized in the kernel mode, you are paging. It is at this
point that you should start tuning your virtual memory subsystem or start
considering adding memory. There is a very good book named "Optimizing Oracle
for Performance" in which authors introduce a creature called "performance
analyst" (I had a private email discussion about that with one of the authors),
who should be capable of both diagnosing performance problems and giving the
overview of the bottlenecks, in layman's terms, to the damagement. In my
opinion, an almost perfect person like that (another example of almost perfect
people was Mary Poppins) should also have extensive experience with diagnosing
Unix performance problems and know how to tune kernel and present the effects
of his or her work to the damagement. Mary Poppins is not to be confused with
Mary Bobbins, who once kept her watchful eye on little Lisa, Bart and baby
Simpson.
--
Mladen Gogala
Oracle DBA
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