Java Reference
In-Depth Information
if ((96 MB * MinRAMFraction) > Physical Memory) {
Default Xmx = Physical Memory / MinRAMFraction;
The initial heap size choice is similar, though it has fewer complications. The initial heap
size value is determined like this:
Default Xms = MaxRAM / InitialRAMFraction
As can be concluded from the default minimum heap sizes, the default value of the Ini-
tialRAMFraction flag is 64. The one caveat here occurs if that value is less than 5 MB—or,
strictly speaking, less than the values specified by -XX:OldSize= N (which defaults to 4 MB)
plus -XX:NewSize= N (which defaults to 1 MB). In that case, the sum of the old and new sizes
is used as the initial heap size.
1. The calculations for the default initial and maximum heap sizes are fairly straight-
forward on most machines.
2. Around the edges, these calculations can be quite involved.
The past two chapters have spent a lot of time delving into the deep details of how GC (and
its various algorithms) work. If GC is taking longer than you'd like, knowing how all of that
works should aid you in taking the necessary steps to improve things.
Now that we understand all the details, let's take a step back to determine an approach to
choosing and tuning a garbage collector. Here's a quick set of questions to ask yourself to
help put everything in context:
Can your application tolerate some full GC pauses?
If so, then the throughput collector will likely offer the best performance, while using
less CPU and a smaller heap than other collectors. If not, then choose a concurrent col-
lector—for smaller heaps, either CMS or G1. For larger heaps, G1.
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