IBM Power System POWER7 Series, Optimization And Tuning Manual

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Chapter 7. Java
127
The Power Architecture does not require running applications in 64-bit mode to achieve best
performance because 32-bit and 64-bit modes have the same number of processor registers.
Consider the following items:
򐂰 Applications with a small memory requirement typically run faster as 32-bit applications
than as 64-bit applications.
򐂰 64-bit applications have a larger demand on memory because of the larger data types,
such as pointers being 64-bit instead of 32-bit, which leads to the following circumstances:
– The memory foot print increases because of the larger data types.
– The memory alignment of application data contributes to memory demand.
– More memory bandwidth is required.
For best performance, use 32-bit Java unless the memory requirement of the application
requires running in 64-bit mode.
For more information about this topic, see 7.6, “Related publications” on page 136.
7.3 Memory and page size considerations
IBM Java can take advantage of medium (64 KB) and large (16 MB) page sizes that are
supported by the current AIX versions and POWER processors. Using medium or large
pages instead of the default 4 KB page size can improve application performance. The
performance improvement of using medium or large pages is a result of a more efficient use
of the hardware translation caches, which are used when you translate application page
addresses to physical page addresses. Applications that are frequently accessing a vast
amount of memory benefit most from using pages sizes that are larger than 4 KB.
Table 7-1 shows the hardware and software requirements for 4 KB, 64 KB, and 16 MB pages:
Table 7-1 Page sizes that are supported by AIX and POWER processors
7.3.1 Medium and large pages for Java heap and code cache
Medium and large pages can be enabled for the Java heap and JIT code cache
independently of other memory areas. The JVM supports at least three page sizes,
depending on the platform:
򐂰 4 KB (default)
򐂰 64 KB
򐂰 16 MB
Page size Platform AIX version Requires user
configuration
4 KB All All No
64 KB or later AIX 5L V5.3 and later No
16 MB POWER4 or later AIX 5L V5.3 and later Yes