Why does G1GC log "humongous allocation request failed"?

Environment

• Java using G1GC

Issue

• We see the following messages in our error log followed by a Full GC:

 87080.686: [G1Ergonomics (Heap Sizing) attempt heap expansion, reason: humongous allocation request failed, allocation request: 531476496 bytes]
 87080.686: [G1Ergonomics (Heap Sizing) expand the heap, requested expansion amount: 318767104 bytes, attempted expansion amount: 318767104 bytes]
 87080.686: [G1Ergonomics (Heap Sizing) did not expand the heap, reason: heap expansion operation failed]

Root Cause

These messages indicate numerous allocation failures for humongous objects (objects in the heap that are larger than 50% of the G1 region size) eventually ending in a Full GC to free up a contiguous set of regions. From the message, we can see that the collector is requesting 531M of contiguous heap regions, but unable to get that requiring a Full GC to compact and rearrange the regions in the heap. Are objects of this side considered normal in the application? Is there a heap dump from the time of the issue that we can take a look at these large objects and try to see if we can reduce their size? If they are considered normal, consider raising the size of -XX:G1HeapRegionSize=16M to try and combat this.

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