Eclipse OpenJ9: A Java Virtual Machine for OpenJDK that's optimized for small footprint, fast start-up, and high throughput. Builds on Eclipse OMR (https://github.com/eclipse/omr) and combines with the Extensions for OpenJDK for OpenJ9 repo.
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Make `-Xmnx` proportional to `-Xsoftmx` in Balanced GC #13543
With the existing balanced GC eden sizing logic (and even with changes introduced in https://github.com/eclipse-openj9/openj9/pull/12043) , if a user specifies -Xmx10G -Xmnx5G -Xsoftmx2G, then the -xmnx being used by the eden sizing logic will be 5G.
In Gencon, as of https://github.com/eclipse/omr/pull/6141, this behaviour has been modified, such that the actual maximum amount of space used by nursery, is proportional to -Xmnx/-Xmx - That is, (using the example above), nursery will be allowed to use 50% (5G / 10G) of 2G softmx limit - or in other words, nursery can grow up to 1G. If softmx were to dynamically change to 8G, then nursery can grow up to 4G (again, 50% of softmx value).
In a more mathematical sense, actual max nursery size = (-xmnx/-xmx) * -xsoftmx
The behaviour in Balanced GC should be made consistent with the behaviour of Gencon, such that Eden size is proportional to the softmx in use.
Please note the following:
If -Xmnx or -Xmn have been specified, the the formula given above should be used. If -Xmnx or -Xmnhave not been specified, the formula may still be used, but 0.75 should be used instead of (-xmnx/-xmx)
Balanced GC default maximum and minimum values for eden size (as of https://github.com/eclipse-openj9/openj9/pull/12043), are drastically different than gencon. By default, eden can be anywhere between 1 region, and 75% of the max heap size in use. If no -Xmnx/-Xmn value has been specified, then eden should be allowed to use up to 75% of softmx value, and continue to be 75% of the softmx value if softmx changes dynamically.
The max/min eden size calculation in https://github.com/eclipse-openj9/openj9/pull/12043 , is done in MM_SchedulingDelegate::initialize(), and values _minEdenRegionCount and _maxEdenRegionCount are later consumed in MM_SchedulingDelegate::adjustIdealEdenRegionCount()
The documentation changes related to the new gencon and balanced behaviours are found in the following doc issue: https://github.com/eclipse-openj9/openj9-docs/issues/834 . The documentation here should be changed appropriately once the new behaviour for balanced GC is implemented.
cedrichansen
commented
3 years ago
With the existing balanced GC eden sizing logic (and even with changes introduced in https://github.com/eclipse-openj9/openj9/pull/12043) , if a user specifies
-Xmx10G -Xmnx5G -Xsoftmx2G, then the-xmnxbeing used by the eden sizing logic will be5G.In Gencon, as of https://github.com/eclipse/omr/pull/6141, this behaviour has been modified, such that the actual maximum amount of space used by nursery, is proportional to
-Xmnx/-Xmx- That is, (using the example above), nursery will be allowed to use 50% (5G / 10G) of 2G softmx limit - or in other words, nursery can grow up to 1G. If softmx were to dynamically change to 8G, then nursery can grow up to 4G (again, 50% of softmx value). In a more mathematical sense,actual max nursery size = (-xmnx/-xmx) * -xsoftmxThe behaviour in Balanced GC should be made consistent with the behaviour of Gencon, such that Eden size is proportional to the softmx in use.
Please note the following:
-Xmnxor-Xmnhave been specified, the the formula given above should be used. If-Xmnxor-Xmnhave not been specified, the formula may still be used, but 0.75 should be used instead of(-xmnx/-xmx)-Xmnx/-Xmnvalue has been specified, then eden should be allowed to use up to 75% ofsoftmxvalue, and continue to be 75% of the softmx value if softmx changes dynamically.MM_SchedulingDelegate::initialize(), and values_minEdenRegionCountand_maxEdenRegionCountare later consumed inMM_SchedulingDelegate::adjustIdealEdenRegionCount()