How to avoid fragmentation
Java SDK R1.3.1 SR7 provides command-line options for specifying the size of the JVM kCluster and pCluster regions.
| Region | Switch | Notes |
|---|---|---|
| kCluster | -Xk | |
| pCluster overflowsize | -Xp sz,ovfl | sz = pCluster size (Kb)
ovfl = overflow size (Kb) |
Set the initial sizes of the clusters large enough to help avoid fragmentation issues. It is not unusual in a large Java application that the default kCluster space is insufficient to allocate all class blocks.
For the -Xk parameter, IBM recommends adding 10% to the reported value. In the example below, the reported value is 3955.
<GC(VFY-SUM): pinned=4265(classes=3955/freeclasses=0) dosed=10388 movable=1233792 free=5658>
-Xk4200 is probably a reasonable setting.
The difference between pinned (=4265) and classes (=3955) provides a guide for the initial size of pCluster. However, because each object might be a different size, it is difficult to predict the requirements for the pCluster and pCluster overflow options.
