G1 Garbage Collector: Why survivor space is always full?

Question

Here is the output of jmap -heap command:

Survivor Space:
   regions  = 52
   capacity = 54525952 (52.0MB)
   used     = 54525952 (52.0MB)
   free     = 0 (0.0MB)
   100.0% used

I've executed it many times and I found that the value of capacity is alway equal to used.

My question is why survivor space is alway full(and so small)? I've specified -Xmx2200m -Xms2200m -Xmn1100m. (I expect the survivor space should be 220M, which means there should be more space for survivor region)

-- update--

Full output of jheap:

Garbage-First (G1) GC with 2 thread(s)

Heap Configuration:
   MinHeapFreeRatio         = 40
   MaxHeapFreeRatio         = 70
   MaxHeapSize              = 2306867200 (2200.0MB)
   NewSize                  = 1153433600 (1100.0MB)
   MaxNewSize               = 1153433600 (1100.0MB)
   OldSize                  = 4194304 (4.0MB)
   NewRatio                 = 2
   SurvivorRatio            = 8
   MetaspaceSize            = 16777216 (16.0MB)
   CompressedClassSpaceSize = 1073741824 (1024.0MB)
   MaxMetaspaceSize         = 4294963200 (4095.99609375MB)
   G1HeapRegionSize         = 1048576 (1.0MB)

Heap Usage:
G1 Heap:
   regions  = 14488
   capacity = 15191769088 (14488.0MB)
   used     = 1083703304 (1033.5000076293945MB)
   free     = 14108065784 (13454.499992370605MB)
   7.13348983730946% used
G1 Young Generation:
Eden Space:
   regions  = 988
   capacity = 1163919360 (1110.0MB)
   used     = 1035993088 (988.0MB)
   free     = 127926272 (122.0MB)
   89.009009009009% used
Survivor Space:
   regions  = 45
   capacity = 47185920 (45.0MB)
   used     = 47185920 (45.0MB)
   free     = 0 (0.0MB)
   100.0% used
G1 Old Generation:
   regions  = 2
   capacity = 1095761920 (1045.0MB)
   used     = 524296 (0.5000076293945312MB)
   free     = 1095237624 (1044.4999923706055MB)
   0.04784762003775419% used

30424 interned Strings occupying 3027304 bytes.

Answer 1

My observations.

1. G1GC is efficient when you stop customizing some of the parameters. I suggest to remove Xmn ( young gen) setting.

2. Set different values of -Xms and -Xmx

3. Region size should be configured as Maximum heap memory size / 2048. For 4G heap, 2 MB should be region size & for 2 GB heap, 1 MB should be ideal region size.

4. key parameters to be configured : -XX:G1HeapRegionSize=n, XX:MaxGCPauseMillis=m, -XX:ParallelGCThreads=n, -XX:ConcGCThreads=n apart from -Xms and -Xmx

Have a look at this SE post and infoQ article for more details.

Recommendation from Oracle

When you evaluate and fine-tune G1 GC, keep the following recommendations in mind:

Young Generation Size: Avoid explicitly setting young generation size with the -Xmn option or any or other related option such as -XX:NewRatio. Fixing the size of the young generation overrides the target pause-time goal.

Pause Time Goals: When you evaluate or tune any garbage collection, there is always a latency versus throughput trade-off.