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Exadata memory configuration

Read this post if your Exadata compute nodes have 512/768GB of RAM or you plan to upgrade to the same.

There has been a lot of information about hugepages and I wouldn’t go into too much details. For efficiency, the (x86) CPU allocates RAM by chunks (pages) of 4K bytes and those pages can be swapped to disk. For example, if your SGA allocates 32GB this will take 8388608 pages and given that Page Table Entry consume 8bytes that’s 64MB to look-up. Hugepages, on the other hand, are 2M. Pages that are used as huge pages are reserved inside the kernel and cannot be used for other purposes.  Huge pages cannot be swapped out under memory pressure, obviously there is decreased page table overhead and page lookups are not required since the pages are not subject to replacement. The bottom line is that you need to use them, especially now with the amount of RAM we get nowadays.

For every new Exadata deployment I usually set the amount of hugepages to 60% of the physical RAM:
256GB RAM = 150 GB (75k pages)
512GB RAM = 300 GB (150k pages)
768GB RAM = 460 GB (230k pages)

This allows databases to allocate SGA from the hugepages. If you want to allocate the exact number of hugepages that you need, Oracle has a script which will walk through all instances and give you the number of hugepages you need to set on the system, you can find the Doc ID in the reference below.

This also brings important point – to make sure your databases don’t allocate from both 4K and 2M pages make sure the parameter use_large_pages is set to ONLY for all databases. Starting with 11.2.0.3 (I think) you’ll find hugepages information in the alertlog when database starts:

************************ Large Pages Information *******************
Per process system memlock (soft) limit = 681 GB

Total Shared Global Region in Large Pages = 2050 MB (100%)

Large Pages used by this instance: 1025 (2050 MB)
Large Pages unused system wide = 202863 (396 GB)
Large Pages configured system wide = 230000 (449 GB)
Large Page size = 2048 KB
********************************************************************

 

Now there is one more parameter you need to change if you deploy or upgrade Exadata with 512/768GB of RAM. That is the total amount of shared memory, in pages, that the system can use at one time or kernel.shmall. On Exadata, this parameter is set to 214G by default which is enough if your compute nodes have only 256GB of RAM. If the sum of all databases SGA memory is less than 214GB that’s ok but the moment you try to start another database you’ll get the following error:

Linux-x86_64 Error: 28: No space left on device

For that reason, if you deploy or upgrade Exadata with 512G/768GB of physical RAM make sure you upgrade kernel.shmall  too!

Some Oracle docs suggest this parameter should be set to the half of the physical memory, other suggest it should be set to the all available memory. Here’s how to calculate it:

kernel.shmall = physical RAM size / pagesize

To get the pagesize run getconf PAGE_SIZE on the command prompt. You need to set shmall to at least match the size of the hugepages – because that’s where we’d allocate SGA memory from. So if you run Exadata with 768G of RAM and have 460 GB of hugepages you’ll set shmall to 120586240 (460GB / 4K pagesize).

Using HUGEPAGES does not alter the calculation for configuring shmall!

Reference:
HugePages on Linux: What It Is… and What It Is Not… (Doc ID 361323.1)
Upon startup of Linux database get ORA-27102: out of memory Linux-X86_64 Error: 28: No space left on device (Doc ID 301830.1)

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