GC settings and buffer memory sizing
Due to the bad garbage collection performance in some cases, it was decided to change the settings for the garbage collection and to change the way how the initial buffer size is calculated.
Memory pols and GC settings
The first thing was to define a new ratio of the old and new generation space in heap memory. It was concluded that the default values for the memory pools depend if the CMR is run on the 32bit or 64bit machine. The settings are following:
|
Heap size (initial & max) |
New generation |
Old generation |
Perm size (init/max) |
|---|---|---|---|---|
32bit |
1024MB |
386MB |
638MB |
128MB/128MB |
64bit |
1536MB |
512MB |
1024MB |
128MB/192MB |
The new GC settings included the definition that only after the 80% of the old generation space is full, garbage collection is activated. This percentage is very important for the buffer memory size calculation, because the buffer size can not be above 80% of old generation. Otherwise, it could happen that the garbage collection is always active. JVM parameters added for this purpose are:
XX:CMSInitiatingOccupancyFraction=80 -XX:+UseCMSInitiatingOccupancyOnly
Buffer memory sizing
After the changes of heap memory pool sizes and GC settings, it was clear that the calculation of the buffer size can not reside only on the total heap size, but has to be based on the size of old generation memory pool. It was decided that the buffer will, by default, have the size between 50-65% of old generation space, where bigger old generation size will enable bigger occupancy percentage for the buffer.
These default settings can be changed in config/buffer.properties file:
# Size of old space occupancy till which min occupancy will be active - 384MB buffer.minOldSpaceOccupancyActiveTillOldGenSize = 402653184 # Size of old space occupancy from which max occupancy will be active - 2GB buffer.maxOldSpaceOccupancyActiveFromOldGenSize = 2147483648 # percentage of the min old generation heap space buffer is occupying buffer.minOldSpaceOccupancy = 0.5 # percentage of the max old generation heap space buffer is occupying buffer.maxOldSpaceOccupancy = 0.65
CMR must run with Sun JVM
Due to the different way of naming the memory pools in different Java Virtual Machines, it is now necessary that CMR runs with Sun JVM. Otherwise the old generation pool size would not be found, and buffer would not be properly initialized, which would result in a CMR startup failure. However, CMR is already provided to the users with a JVM, thus this should be a problem from a user perspective.
Additional info
Additional info about heap size influence to the buffer size can also be found on public wiki page: Working with the in-memory storage.