RE: RAC install on Linux

From: Ruel, Chris <Chris.Ruel_at_lfg.com>
Date: Thu, 22 Feb 2018 20:37:01 +0000
Message-ID: <MWHPR08MB289380E9EBD7941525E212619BCD0_at_MWHPR08MB2893.namprd08.prod.outlook.com>

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..

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Chris Ruel * Oracle Database Administrator * Lincoln Financial Group cruel_at_lfg.com * Desk:317.759.2172 * Cell 317.523.8482

-----Original Message-----
From: oracle-l-bounce_at_freelists.org [mailto:oracle-l-bounce_at_freelists.org] On Behalf Of Hameed, Amir Sent: Thursday, February 22, 2018 3:06 PM To: contact_at_soocs.de; neil_chandler_at_hotmail.com Cc: Oracle-L Freelists <oracle-l_at_freelists.org> 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_at_freelists.org [mailto:oracle-l-bounce_at_freelists.org] On Behalf Of Stefan Koehler Sent: Thursday, February 22, 2018 5:21 AM To: neil_chandler_at_hotmail.com
Cc: Oracle-L Freelists <oracle-l_at_freelists.org> 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: _at_OracleSK

> Neil Chandler <neil_chandler_at_hotmail.com> hat am 22. Februar 2018 um 09:47 geschrieben:
>
> Transparent Hugepages should be disabled, to prevent the Kernel “helping you out” and causing CPU spikes and unpredictable performance hits.
>
> You should always be using Hugepages.
>
> They give a minor performance improvement and a significant memory saving in terms of the amount of memory needed to handle the pages - less Transaction Lookaside Buffers, which also means less TLB misses (which are expensive).
>
> You are handling the memory chopped up into 2MB pieces instead of 4K. But you also have a single shared memory TLB for Hugepages.
>
> 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 overallocated them (I’ve seen this several times at clients so it’s not 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.
>
> As an added bonus, AMM can’t use Hugepages, so your are forced to use ASMM. AMM doesn’t work well and has been kind-of deprecated by oracle 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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��i��0���zX���+��n��{�+i�^ Received on Thu Feb 22 2018 - 21:37:01 CET