[轉帖]Unnecessary GCLocker-initiated young GCs

https://www.cnblogs.com/zhangshengdong/p/9196128.html

 

Details

• Type: Bug
• Resolution:Fixed
• Priority: P3
• Fix Version/s:14
• Affects Version/s:7u60, 8, 8u20, 8u40, 9, 11, 13
• Component/s:hotspot
• Labels:

  • 8u20-defer-SQE-OK
   
  • 8u20-defer-approved
   
  • azul-interest
   
  • bugdb_29836288
   
  • gc-locker
   
  • gc-pending-review
   
  • jdk11u-fix-request
   
  • jdk11u-fix-yes
   
  • jdk13u-fix-request
   
  • jdk13u-fix-yes
   
  • jdk8u-fix-request
   
  • jdk8u-fix-yes
   
  • jdk9-defer-yes
   
  • redhat-interest
   
  • sust:dcs

• Subcomponent:

  gc
• Resolved In Build:

  b08

Backports

Issue	Fix Version	Assignee	Priority	Status	Resolution	Resolved In Build
JDK-8245298	13.0.4	Kim Barrett	P3	Resolved	Fixed	b02
JDK-8229114	11.0.6-oracle	Fairoz Matte	P3	Resolved	Fixed	b01
JDK-8232099	11.0.6	Kim Barrett	P3	Resolved	Fixed	b01
JDK-8234690	openjdk8u242	Kim Barrett	P3	Resolved	Fixed	team
JDK-8235552	openjdk8u232	Kim Barrett	P3	Resolved	Fixed	master
JDK-8229021	8u241	Fairoz Matte	P3	Resolved	Fixed	b01
JDK-8231651	8u231	Fairoz Matte	P3	Resolved	Fixed	b31
JDK-8229410	8u221	Fairoz Matte	P3	Closed	Fixed	b35
JDK-8234991	emb-8u241	Fairoz Matte	P3	Resolved	Fixed	team

Description

We are seeing very strange behavior in some GC logs (and sometimes said strange behavior happens quite frequently). The common pattern is that a GC locker-initiated young GC happens immediately after another young GC. The GC locker-initiated one seems completely unnecessary given that the eden is either empty or has very low occupancy (<10%).

Here's an example of this (with ParNew):

{Heap before GC invocations=2 (full 0):
 par new generation total 471872K, used 433003K [0x00000007bae00000, 0x00000007dae00000, 0x00000007dae00000)
  eden space 419456K, 100% used [0x00000007bae00000, 0x00000007d47a0000, 0x00000007d47a0000)
  from space 52416K, 25% used [0x00000007d47a0000, 0x00000007d54dacb0, 0x00000007d7ad0000)
  to space 52416K, 0% used [0x00000007d7ad0000, 0x00000007d7ad0000, 0x00000007dae00000)
 tenured generation total 524288K, used 0K [0x00000007dae00000, 0x00000007fae00000, 0x00000007fae00000)
   the space 524288K, 0% used [0x00000007dae00000, 0x00000007dae00000, 0x00000007dae00200, 0x00000007fae00000)
 compacting perm gen total 21248K, used 2549K [0x00000007fae00000, 0x00000007fc2c0000, 0x0000000800000000)
   the space 21248K, 12% used [0x00000007fae00000, 0x00000007fb07d7a0, 0x00000007fb07d800, 0x00000007fc2c0000)
No shared spaces configured.
1.119: [GC (Allocation Failure)[ParNew: 433003K->15843K(471872K), 0.0103090 secs] 433003K->15843K(996160K), 0.0103320 secs] [Times: user=0.03 sys=0.00, real=0.01 secs]
Heap after GC invocations=3 (full 0):
 par new generation total 471872K, used 15843K [0x00000007bae00000, 0x00000007dae00000, 0x00000007dae00000)
  eden space 419456K, 0% used [0x00000007bae00000, 0x00000007bae00000, 0x00000007d47a0000)
  from space 52416K, 30% used [0x00000007d7ad0000, 0x00000007d8a48c88, 0x00000007dae00000)
  to space 52416K, 0% used [0x00000007d47a0000, 0x00000007d47a0000, 0x00000007d7ad0000)
 tenured generation total 524288K, used 0K [0x00000007dae00000, 0x00000007fae00000, 0x00000007fae00000)
   the space 524288K, 0% used [0x00000007dae00000, 0x00000007dae00000, 0x00000007dae00200, 0x00000007fae00000)
 compacting perm gen total 21248K, used 2549K [0x00000007fae00000, 0x00000007fc2c0000, 0x0000000800000000)
   the space 21248K, 12% used [0x00000007fae00000, 0x00000007fb07d7a0, 0x00000007fb07d800, 0x00000007fc2c0000)
No shared spaces configured.
}
{Heap before GC invocations=3 (full 0):
 par new generation total 471872K, used 24002K [0x00000007bae00000, 0x00000007dae00000, 0x00000007dae00000)
  eden space 419456K, 1% used [0x00000007bae00000, 0x00000007bb5f7c50, 0x00000007d47a0000)
  from space 52416K, 30% used [0x00000007d7ad0000, 0x00000007d8a48c88, 0x00000007dae00000)
  to space 52416K, 0% used [0x00000007d47a0000, 0x00000007d47a0000, 0x00000007d7ad0000)
 tenured generation total 524288K, used 0K [0x00000007dae00000, 0x00000007fae00000, 0x00000007fae00000)
   the space 524288K, 0% used [0x00000007dae00000, 0x00000007dae00000, 0x00000007dae00200, 0x00000007fae00000)
 compacting perm gen total 21248K, used 2549K [0x00000007fae00000, 0x00000007fc2c0000, 0x0000000800000000)
   the space 21248K, 12% used [0x00000007fae00000, 0x00000007fb07d7a0, 0x00000007fb07d800, 0x00000007fc2c0000)
No shared spaces configured.
1.130: [GC (GCLocker Initiated GC)[ParNew: 24002K->12748K(471872K), 0.0123930 secs] 24002K->12748K(996160K), 0.0124130 secs] [Times: user=0.04 sys=0.01, real=0.01 secs]
Heap after GC invocations=4 (full 0):
 par new generation total 471872K, used 12748K [0x00000007bae00000, 0x00000007dae00000, 0x00000007dae00000)
  eden space 419456K, 0% used [0x00000007bae00000, 0x00000007bae00000, 0x00000007d47a0000)
  from space 52416K, 24% used [0x00000007d47a0000, 0x00000007d5413320, 0x00000007d7ad0000)
  to space 52416K, 0% used [0x00000007d7ad0000, 0x00000007d7ad0000, 0x00000007dae00000)
 tenured generation total 524288K, used 0K [0x00000007dae00000, 0x00000007fae00000, 0x00000007fae00000)
   the space 524288K, 0% used [0x00000007dae00000, 0x00000007dae00000, 0x00000007dae00200, 0x00000007fae00000)
 compacting perm gen total 21248K, used 2549K [0x00000007fae00000, 0x00000007fc2c0000, 0x0000000800000000)
   the space 21248K, 12% used [0x00000007fae00000, 0x00000007fb07d7a0, 0x00000007fb07d800, 0x00000007fc2c0000)
No shared spaces configured.
}

Notice that:

* The timestamp of the second GC (1.130) is almost equal to the timestamp of the first GC plus the duration of the first GC (1.119 + 0.0103320 = 1.1293). In this test young GCs normally happen at a frequency of one every 100ms-110ms or so.
* The eden at the start of the second GC is almost empty (1% occupancy). We've also seen it very often with a completely empty eden.
* (the big hint) The second GC is GClocker-initiated.

This happens most often with ParNew (in some cases, more than 30% of the GCs are those unnecessary ones) but also happens with ParallelGC too but less frequently (maybe 1%-1.5% of the GCs are those unnecessary ones). I was unable to reproduce it with G1.

I can reproduce it with with latest JDK 7, JDK 8, and also the latest hotspot-gc/hotspot workspace. I haven't tried with JDK 6.