Huge Pages

Jonathan Lewis's picture

A useful quick summary from Neil Chandler replying to a thread on Oracle-L:

Topic: RAC install on Linux

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* be 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.

There are a few follow-up emails after Neil’s; particularly helpful are two from Stefan Koehler, here and here.