Neo Neo - 6 months ago 114
Java Question

G1 Garbage Collector: Why survivor space is always full?

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

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.