Heap sizing problems

If the operation of the heap, using the default settings, does not give the best results for your application, there are actions that you can take.

For the majority of applications, the default settings work well. The heap expands until it reaches a steady state, then remains in that state, which should give a heap occupancy (the amount of live data on the heap at any given time) of 70%. At this level, the frequency and pause time of garbage collection should be acceptable.

For some applications, the default settings might not give the best results. Listed here are some problems that might occur, and some suggested actions that you can take. Use verbose:gc to help you monitor the heap.

The frequency of garbage collections is too high until the heap reaches a steady state.: Use verbose:gc to determine the size of the heap at a steady state and set -Xms to this value.
The heap is fully expanded and the occupancy level is greater than 70%.: Increase the -Xmx value so that the heap is not more than 70% occupied. The maximum heap size should, if possible, be able to be contained in physical memory to avoid paging. For the best performance, try to ensure that the heap never pages.
At 70% occupancy the frequency of garbage collections is too great.: Change the setting of -Xminf. The default is 0.3, which tries to maintain 30% free space by expanding the heap. A setting of 0.4, for example, increases this free space target to 40%, and reduces the frequency of garbage collections.
Pause times are too long.: If your application uses many short-lived objects, or is transaction-based (that is, objects in the transaction do not survive beyond the transaction commit), or if the heap space is fragmented, try using the -Xgcpolicy:gencon (gencon is the default) garbage collection policy. This policy treats short-lived objects differently from long-lived objects, and can reduce pause times and heap fragmentation.; In other situations, if a reduction in throughput is acceptable, try using the -Xgcpolicy:optavgpause policy. This policy reduces the pause times and makes them more consistent when the heap occupancy rises. It does, however, reduce throughput by approximately 5%, although this value varies with different applications.; If pause times are unacceptable during a global garbage collection, due to a large heap size, try using -Xgcpolicy:balanced. The balanced garbage collection policy can also address frequent class unloading issues, where many class loaders are being created, but require a global collection to unload. This policy is available for 64-bit platforms and must be used with the -Xcompressedrefs option. The policy is intended for environments where heap sizes are greater than 4 GB.

Here are some useful tips:

Ensure that the heap never pages; that is, the maximum heap size must be able to be contained in physical memory.
Avoid finalizers. You cannot guarantee when a finalizer will run, and often they cause problems. If you do use finalizers, try to avoid allocating objects in the finalizer method. A verbose:gc trace shows whether finalizers are being called.
Avoid compaction. A verbose:gc trace shows whether compaction is occurring. Compaction is usually caused by requests for large memory allocations. Analyze requests for large memory allocations and avoid them if possible. If they are large arrays, for example, try to split them into smaller arrays.