Re: cas latch

I saw quite a few cas latch waits today, Oracle 9.2.0.4 on HP-UX
11.11PA-RiSC. Do these CPUs support CAS instructions?

I am not sure why I am having it. I will read about wikipedia and see what
it is all about.

Thanks!


On 10/30/06, Bobak, Mark <Mark.Bobak@xxxxxxxxxxxxxxx> wrote: 

 As K Gopalakrishnan mentioned, CAS latch is "Compare and Swap".  My
understanding is that, on *most* modern CPUs, the CAS instruction is
built-in to the instruction set as an atomic operation.  In those cases, you
shouldn't see CAS latch used at all, cause Oracle can directly use the CAS
instruction itself.  In cases where the CPU does not support a CAS
instruction, Oracle has implemented the CAS functionality in software.  To
support the atomicity of the CAS functionality in software, the CAS latch is
utilized.

-Mark


***--*

*Mark J. Bobak*

*Senior Oracle Architect*

***P****ro****Q****uest Information & Learning*

There is nothing so useless as doing efficiently that which shouldn't be
done at all.  –Peter F. Drucker, 1909-2005



 ------------------------------
*From:* oracle-l-bounce@xxxxxxxxxxxxx [mailto:
oracle-l-bounce@xxxxxxxxxxxxx] *On Behalf Of *K Gopalakrishnan
*Sent:* Monday, October 30, 2006 12:17 PM
*To:* exriscer@xxxxxxxxx; oracle-l
*Subject:* Re: cas latch


 Cheng,

Do you see any contention for this latch?

CAS is Compare and Select. Check
http://en.wikipedia.org/wiki/Compare_and_swap for some details on CAS.



Have a nice day !!
------------------------------------------------------------
Best Regards,
K Gopalakrishnan,
Co-Author: Oracle Wait Interface: Oracle Press 2004.
http://www.amazon.com/gp/product/007222729X/

Author: Oracle Database 10g RAC Handbook, Oracle Press 2006
http://www.amazon.com/gp/product/007146509X/

----- Original Message ----
From: LS Cheng <exriscer@xxxxxxxxx>
To: oracle-l <oracle-l@xxxxxxxxxxxxx>
Sent: Monday, 30 October, 2006 8:43:47 AM
Subject: cas latch

Hi

I was wondering if anyone know what does cas latch do? I know is a shared
latch but not sure for what part of SGA?

Thanks






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