Re: cas latch

Correct, HP doesn't do CAS.

There are some shared read latch operations that Oracle therefore implements through the CAS latch, which emulates a Compare and Swap operation used by sharable read latches on other platforms.

There is also a latch called 'RM cas latch' which is the resource manager CAS latch, which allows minimum blocking on updates to resource management totals for resource groups.

I'm no sure, though, which actions are most likely to result in significant loss of time on the CAS latch.

Regards

Jonathan Lewis
http://jonathanlewis.wordpress.com

Author: Cost Based Oracle: Fundamentals
http://www.jlcomp.demon.co.uk/cbo_book/ind_book.html

The Co-operative Oracle Users' FAQ
http://www.jlcomp.demon.co.uk/faq/ind_faq.html

• Original Message ----- From: "Bobak, Mark" <Mark.Bobak_at_il.proquest.com> To: "LS Cheng" <exriscer_at_gmail.com> Cc: <kaygopal_at_yahoo.com>; "oracle-l" <oracle-l_at_freelists.org> Sent: Monday, October 30, 2006 6:22 PM Subject: RE: cas latch

Why am I not surprised? If I recall correctly, the HP RISC arctitecture is one that does not support the CAS instruction, at least on some of the older CPUs. Not sure if that is still true for it's most recent CPU offerings.

-Mark

--

Mark J. Bobak

Senior Oracle Architect

ProQuest Information & Learning

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



________________________________

From: LS Cheng [mailto:exriscer_at_gmail.com]
Sent: Monday, October 30, 2006 1:18 PM
To: Bobak, Mark
Cc: kaygopal_at_yahoo.com; oracle-l
Subject: Re: cas latch


I saw quite a few cas latch waits today, Oracle 9.2.0.4 on HP-UX 11.11
PA-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_at_il.proquest.com> 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 roQ uest Information & Learning



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





________________________________

From: oracle-l-bounce_at_freelists.org [mailto:
oracle-l-bounce_at_freelists.org <mailto:oracle-l-bounce_at_freelists.org> ]
On Behalf Of K Gopalakrishnan
Sent: Monday, October 30, 2006 12:17 PM
To: exriscer_at_gmail.com; 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_at_gmail.com <mailto:exriscer_at_gmail.com>


>

To: oracle-l <oracle-l_at_freelists.org>
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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