frequent major gc but not free heap?

[yuanjun Li]

After running several hours, My http server begin frequenly major gc, but
no heap was freed.

several times major gc later, promotion failed and concurrent mode
failure occured,
then heap was freed. My gc log is below :

{Heap before GC invocations=7172 (full 720):
 par new generation   total 737280K, used 667492K [0x000000076b800000,
0x000000079d800000, 0x000000079d800000)
  eden space 655360K, 100% used [0x000000076b800000,
0x0000000793800000, 0x0000000793800000)
  from space 81920K,  14% used [0x0000000793800000,
0x00000007943d91d0, 0x0000000798800000)
  to   space 81920K,   0% used [0x0000000798800000,
0x0000000798800000, 0x000000079d800000)
 concurrent mark-sweep generation total 1482752K, used 1479471K
[0x000000079d800000, 0x00000007f8000000, 0x00000007f8000000)
 concurrent-mark-sweep perm gen total 131072K, used 58091K
[0x00000007f8000000, 0x0000000800000000,
0x0000000800000000)2015-11-19T21:50:02.692+0800: 113963.532:
[GC2015-11-19T21:50:02.692+0800: 113963.532: [ParNew (promotion
failed)Desired survivor size 41943040 bytes, new threshold 15 (max
15)- age   1:    3826144 bytes,    3826144 total- age   2:     305696
bytes,    4131840 total- age   3:     181416 bytes,    4313256 total-
age   4:     940632 bytes,    5253888 total- age   5:      88368
bytes,    5342256 total- age   6:     159840 bytes,    5502096 total-
age   7:     733856 bytes,    6235952 total- age   8:      64712
bytes,    6300664 total- age   9:     314304 bytes,    6614968 total-
age  10:     587160 bytes,    7202128 total- age  11:      38728
bytes,    7240856 total- age  12:     221160 bytes,    7462016 total-
age  13:     648376 bytes,    8110392 total- age  14:      33296
bytes,    8143688 total- age  15:     380768 bytes,    8524456 total:
667492K->665908K(737280K), 0.7665810
secs]2015-11-19T21:50:03.459+0800: 113964.299:
[CMS2015-11-19T21:50:05.161+0800: 113966.001: [CMS-concurrent-mark:
3.579/4.747 secs] [Times: user=13.41 sys=0.35, rea
l=4.75 secs]
 (concurrent mode failure): 1479910K->44010K(1482752K), 4.7267420
secs] 2146964K->44010K(2220032K), [CMS Perm :
58091K->57795K(131072K)], 5.4939440 secs] [Times: user=9.07 sys=0.13,
real=5.49 secs] Heap after GC invocations=7173 (full 721):
 par new generation   total 737280K, used 0K [0x000000076b800000,
0x000000079d800000, 0x000000079d800000)
  eden space 655360K,   0% used [0x000000076b800000,
0x000000076b800000, 0x0000000793800000)
  from space 81920K,   0% used [0x0000000798800000,
0x0000000798800000, 0x000000079d800000)
  to   space 81920K,   0% used [0x0000000793800000,
0x0000000793800000, 0x0000000798800000)
 concurrent mark-sweep generation total 1482752K, used 44010K
[0x000000079d800000, 0x00000007f8000000, 0x00000007f8000000)
 concurrent-mark-sweep perm gen total 131072K, used 57795K
[0x00000007f8000000, 0x0000000800000000, 0x0000000800000000)}

It seems the CMS GC doesn't make any sense. Could you please explain to me ?

This is my gc config:

-server -Xms2248m -Xmx2248m -Xmn800m -XX:PermSize=128m
-XX:MaxPermSize=128m -XX:MaxTenuringThreshold=15
-XX:+UseCMSCompactAtFullCollection -XX:CMSFullGCsBeforeCompaction=0
-XX:+UseConcMarkSweepGC -XX:+PrintGCDetails -XX:+PrintGCTimeStamps
-XX:+PrintGCDateStamps -Xloggc:gc.log -XX:+PrintHeapAtGC
-XX:+PrintTenuringDistribution -XX:+UseFastAccessorMethods
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