Nehalem EX

In my recent post entitled Exadata Database Machine X2-2 or X2-8? Sure! Why Not? Part I, I started to address the many questions folks are sending my way about what factors to consider when choosing between Exadata Database Machine X2-8 versus Exadata Database Machine X2-2. This post continues that thread.

As my friend Greg Rahn points out in his recent post about Exadata, the latest Exadata Storage Server is based on Intel Xeon 5600 (Westmere EP) processors. The Exadata Storage Server is the same whether the database grid is X2-2 or X2-8. The X2-2 database hosts are also based on Intel Xeon 5600. On the other hand, the X2-8 database hosts are based on Intel Xeon 7500 (Nehalem EX). This is a relevant distinction when thinking about database encryption.

Allocating hugepages for Oracle Database on Linux can be tricky. The following is a short list of some of the common problems associated with faulty attempts to get things properly configured:

1. Insufficient Hugepages. You can be short just a single 2MB hugepage at instance startup and Oracle will silently fall back to no hugepages. For instance, if an instance needs 10,000 hugepages but there are only 9,999 available at startup Oracle will create non-hugepages IPC shared memory and the 9,999 (x 2MB) is just wasted memory.
   1. Insufficient hugepages is an even more difficult situation when booting with _enable_NUMA_support=TRUE as partial hugepages backing is possible.
2. Improper Permissions. Both limits.conf(5) memlock and the shell ulimit –l must accommodate the desired amount of locked memory.

In general, list item 1 above has historically been the most difficult to deal with—especially on systems hosting several instances of Oracle. Since there is no way to determine whether an existing segment of shared memory is backed with hugepages, diagnostics are in short supply.

Oracle Database 11g Release 2 (11.2.0.2)
The fix for Oracle bugs 9195408 (unpublished) and 9931916 (published) is available in 11.2.0.2. In a sort of fast forward to the past, the Linux port now supports an initialization parameter to force the instance to use hugepages for all segments or fail to boot. I recall initialization parameters on Unix ports back in the early 1990s that did just that.

The initialization parameter is called use_large_pages and setting it to “only” results in the all or none scenario. This, by the way, addresses list item 1.1 above. That is, setting use_large_pages=only ensures an instance will not have some NUMA segments backed with hugepages and others without.

Last year I posted a blog entry entitled Linux Thinks It’s a CPU, But What Is It Really – Part I. Mapping Xeon 5500 (Nehalem) Processor Threads to Linux OS CPUs where I discussed the Intel CPU Topology Tool. The topology tool is most helpful when trying to quickly map Linux OS processors to physical processor [...]