CODEWITHSUNDEEP
javagarbage-collection
Eddie
Eddie

Reputation: 61

Encouraging a major GC in Java (but not a STW GC)

I am exploring a tree of stuff in Java, but I am memory limited. I handle this by looking at occupancy stats, and when I'm over 80% (say) stop allocating new bits of the tree to avoid the OutOfMemory. And just compute using what I've got so far using the 20% headroom.

Then every now and again I decide to move down the tree. This should free up 90% of the tree I've allocated because I forget the old root and move to one of it's children. And it does free that memory, but only if I call System.gc(), which is evil and stops my world. But if I don't call gc() my limiting code stops me adding to my tree.

What I want to do is call out to the G1 or CMS collectors that I could really do with the old generation getting a clean, and carrying on computing and allocate new bits of the tree as the collector does it's job.

Any ideas how that can be accomplished? Or, equally helpful, how I could avoid this artificial 80% limit, that is the root of my problem.

Upvotes: 3

Views: 400

Answers (1)

the8472
the8472

Reputation: 43052

Setting -XX:+ExplicitGCInvokesConcurrent should trigger a concurrent cycle if you invoke System.gc()

Alternatively you could use -XX:CMSInitiatingOccupancyFraction=50 -XX:+UseCMSInitiatingOccupancyOnly for CMS which will initiate concurrent cycles once the heap reaches that threshold, since you're aiming for 80%+ occupancy that would basically continuously burn CPU time (possibly limited by GCTimeRatio, i'm not sure how those goals interact) to clean the old generation and thus free up memory swiftly.

The equivalent parameter for G1GC (InitiatingHeapOccupancyPercent) already defaults to 45%, so it should already be running mixed collection cycles if you have reached that point. You should inspect GC logs to verify.

Or, equally helpful, how I could avoid this artificial 80% limit, that is the root of my problem.

  • overprovision memory, RAM is often cheaper than developer hours and GCs need some breathing room to work efficiently anyway
  • consider using soft references to hold onto data that's useful but can be discarded by the GC if needed. They are not free though, so see if the extra costs are worth it.

Upvotes: 1

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