"This behavior is a direct result of the instance parameter setting
*'pre_page_sga
= true'*, which causes oracle processes to touch every single page in the
SGA during process initialization. When not using huge pages the use of
this parameter puts a lot of strain on the memory management of the OS when
using large SGAs. "
This happens when huge pages are not used.
That is because of the large number of buffers required with 4K pagesize.
Probably not going to happen with a 2M pagesize unless the SGA is
incredibly large.
I don't have time to do the math right now. :)
Jared Still
Certifiable Oracle DBA and Part Time Perl Evangelist
Principal Consultant at Pythian
Pythian Blog http://www.pythian.com/blog/author/still/
Github: https://github.com/jkstill
On Thu, Feb 22, 2018 at 1:37 PM, Ruel, Chris <Chris.Ruel@xxxxxxx> wrote:
Someone correct me if I am wrong but HP's do not lock memory. They just
use a larger page size. In fact some people had problems locking memory
with HP's enabled. I don't know if Oracle ever admitted to this being a
bug. This note is a little older but I know it affected us as late as
11.2.0.4:
Oracle Support Document 1276966.1 (Rac Nodes Frequently Evicted
(Rebooted) By CRS When PRE_PAGE_SGA is Set to True) can be found at:
https://support.oracle.com/epmos/faces/DocumentDisplay?id=1276966.1
Also, looking at the thread below, this is news to me that you must use
ASMM with HP's. We use HP's with standard memory management in some cases
and it seems to work fine. I know that AMM is a no-no.
Chris..
_____________________________________________________________________
Chris Ruel * Oracle Database Administrator * Lincoln Financial Group
cruel@xxxxxxx * Desk:317.759.2172 * Cell 317.523.8482
-----Original Message-----
From: oracle-l-bounce@xxxxxxxxxxxxx [mailto:oracle-l-bounce@xxxxxxxxxxxxx]
On Behalf Of Hameed, Amir
Sent: Thursday, February 22, 2018 3:06 PM
To: contact@xxxxxxxx; neil_chandler@xxxxxxxxxxx
Cc: Oracle-L Freelists <oracle-l@xxxxxxxxxxxxx>
Subject: RE: RAC install on Linux
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In those VM shops where memory is over-subscribed, how do Huge Pages work
in terms of locking memory pages so that they cannot be swapped?
Thanks
-----Original Message-----
From: oracle-l-bounce@xxxxxxxxxxxxx [mailto:oracle-l-bounce@xxxxxxxxxxxxx]
On Behalf Of Stefan Koehler
Sent: Thursday, February 22, 2018 5:21 AM
To: neil_chandler@xxxxxxxxxxx
Cc: Oracle-L Freelists <oracle-l@xxxxxxxxxxxxx>
Subject: Re: RAC install on Linux
Hello Neil,
You also have contiguous memory allocation and it can’t be swapped.
Hugepages can also be non-contiguous aka. fragmented - called external
fragmentation (https://lwn.net/Articles/376606/). AFAIK this is also the
reason why Oracle has implemented enhancement request #14500387 and you get
an "ORA-27107: AUTO value for USE_LARGE_PAGES parameter is no longer
supported" (https://docs.oracle.com/database/121/ERRMG/ORA-24280.
htm#GUID-7B2814DB-F946-4EB1-80B5-219B256E90C0__GUID-
E1254AED-AB39-4D52-97E2-5D6011E161E7) nowadays for dynamic huge pages
allocation via oradism.
Best Regards
Stefan Koehler
Independent Oracle performance consultant and researcher
Website: http://www.soocs.de
Twitter: @OracleSK
Neil Chandler <neil_chandler@xxxxxxxxxxx> hat am 22. Februar 2018 um09:47 geschrieben:
“helping you out” and causing CPU spikes and unpredictable performance hits.
Transparent Hugepages should be disabled, to prevent the Kernel
saving in terms of the amount of memory needed to handle the pages - less
You should always be using Hugepages.
They give a minor performance improvement and a significant memory
Transaction Lookaside Buffers, which also means less TLB misses (which are
expensive).
But you also have a single shared memory TLB for Hugepages.
You are handling the memory chopped up into 2MB pieces instead of 4K.
overallocated them (I’ve seen this several times at clients so it’s not
The kernel has less work to do, bookkeeping fewer pointers in the TLB.
You also have contiguous memory allocation and it can’t be swapped.
If you are having problems with Hugepages, you have probably
uncommon). Hugepages can *only* by used for your SGA’s. All of your SGA’s
should fit into the Hugepages and that should generally be no more than
about 60% of the total server memory (but there are exceptions), leaving
plenty of “normal” memory (small pages) for PGA , O/S and other stuff like
monitoring agendas.
ASMM. AMM doesn’t work well and has been kind-of deprecated by oracle
As an added bonus, AMM can’t use Hugepages, so your are forced to use
anyway - dbca won’t let you setup AMM if the server has more than 4GB of
memory.
--
Neil.
sent from my phone
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