Unexpected Java 6 vs 8 GC performance difference

[Jon Masamitsu]

Corbin,

Do you have a benchmark that will exhibit this problem?
If not, does you application just create lots of interned strings that 
are long
lived?

Thanks.

Jon

On 5/31/2016 3:36 PM, Hughes,Corbin wrote:
>
> After coming across -XX:+PrintGCTaskTimeStamps, I noticed that the 
> listed tasks don’t add up to the listed PSYoungGen times, so I ended 
> up adding more “printf” style timing outputs to the hotspot source, 
> and ended up tracking it down. Turns out it’s the interned strings. 
> The way they’re GC’d isn’t particularly efficient for large numbers of 
> strings: the call to 
> StringTable::unlink_or_oops_do(&_is_alive_closure, &root_closure); was 
> taking ~185 ms of the ~200 ms. I wish there were a flag to disable GC 
> of interned strings, but I think we’ll be able to get away with using 
> something like a Guava Interner instead of the built in StringTable, 
> so mystery solved.
>
> *From:*hotspot-gc-use [mailto:hotspot-gc-use-bounces at openjdk.java.net] 
> *On Behalf Of *Hughes,Corbin
> *Sent:* Friday, May 27, 2016 9:24 PM
> *To:* hotspot-gc-use at openjdk.java.net
> *Subject:* Re: Unexpected Java 6 vs 8 GC performance difference
>
> Jon,
>
> Thanks for the interest. Hopefully this log will be more helpful. This 
> is Java 7 with the –XX:-UseParallelOldGC flag. Perhaps notably, the 
> PSYoungGen collections are even slower in Java 7 (regardless of 
> whether or not the serial old collector is used).
>
> Thanks,
>
> Corbin
>
> {Heap before GC invocations=456 (full 0):
>
> PSYoungGen total 2091008K, used 2087456K [0x0000000780000000, 
> 0x0000000800000000, 0x0000000800000000)
>
> eden space 2084864K, 100% used 
> [0x0000000780000000,0x00000007ff400000,0x00000007ff400000)
>
> from space 6144K, 42% used 
> [0x00000007ff400000,0x00000007ff688000,0x00000007ffa00000)
>
> to   space 6144K, 0% used 
> [0x00000007ffa00000,0x00000007ffa00000,0x0000000800000000)
>
> PSOldGen total 3145728K, used 2267941K [0x00000006c0000000, 
> 0x0000000780000000, 0x0000000780000000)
>
> object space 3145728K, 72% used 
> [0x00000006c0000000,0x000000074a6c9508,0x0000000780000000)
>
> PSPermGen total 819200K, used 57100K [0x0000000680000000, 
> 0x00000006b2000000, 0x00000006c0000000)
>
> object space 819200K, 6% used 
> [0x0000000680000000,0x00000006837c3130,0x00000006b2000000)
>
> 1461.284: [GC
>
> Desired survivor size 6291456 bytes, new threshold 1 (max 15)
>
> [PSYoungGen: 2087456K->1482K(2091008K)] 4355397K->2269463K(5236736K), 
> 0.2986350 secs] [Times: user=0.34 sys=0.00, real=0.30 secs]
>
> Heap after GC invocations=456 (full 0):
>
> PSYoungGen total 2091008K, used 1482K [0x0000000780000000, 
> 0x0000000800000000, 0x0000000800000000)
>
> eden space 2084864K, 0% used 
> [0x0000000780000000,0x0000000780000000,0x00000007ff400000)
>
> from space 6144K, 24% used 
> [0x00000007ffa00000,0x00000007ffb72928,0x0000000800000000)
>
> to   space 6144K, 0% used 
> [0x00000007ff400000,0x00000007ff400000,0x00000007ffa00000)
>
> PSOldGen total 3145728K, used 2267981K [0x00000006c0000000, 
> 0x0000000780000000, 0x0000000780000000)
>
> object space 3145728K, 72% used 
> [0x00000006c0000000,0x000000074a6d3528,0x0000000780000000)
>
> PSPermGen total 819200K, used 57100K [0x0000000680000000, 
> 0x00000006b2000000, 0x00000006c0000000)
>
> object space 819200K, 6% used 
> [0x0000000680000000,0x00000006837c3130,0x00000006b2000000)
>
> }
>
> {Heap before GC invocations=457 (full 0):
>
> PSYoungGen total 2091008K, used 2086346K [0x0000000780000000, 
> 0x0000000800000000, 0x0000000800000000)
>
> eden space 2084864K, 100% used 
> [0x0000000780000000,0x00000007ff400000,0x00000007ff400000)
>
> from space 6144K, 24% used 
> [0x00000007ffa00000,0x00000007ffb72928,0x0000000800000000)
>
> to   space 6144K, 0% used 
> [0x00000007ff400000,0x00000007ff400000,0x00000007ffa00000)
>
> PSOldGen total 3145728K, used 2267981K [0x00000006c0000000, 
> 0x0000000780000000, 0x0000000780000000)
>
> object space 3145728K, 72% used 
> [0x00000006c0000000,0x000000074a6d3528,0x0000000780000000)
>
> PSPermGen total 819200K, used 57100K [0x0000000680000000, 
> 0x00000006b2000000, 0x00000006c0000000)
>
> object space 819200K, 6% used 
> [0x0000000680000000,0x00000006837c3130,0x00000006b2000000)
>
> 1463.560: [GC
>
> Desired survivor size 6291456 bytes, new threshold 1 (max 15)
>
> [PSYoungGen: 2086346K->3177K(2091008K)] 4354327K->2271183K(5236736K), 
> 0.2974600 secs] [Times: user=0.33 sys=0.00, real=0.30 secs]
>
> Heap after GC invocations=457 (full 0):
>
> PSYoungGen total 2091008K, used 3177K [0x0000000780000000, 
> 0x0000000800000000, 0x0000000800000000)
>
> eden space 2084864K, 0% used 
> [0x0000000780000000,0x0000000780000000,0x00000007ff400000)
>
> from space 6144K, 51% used 
> [0x00000007ff400000,0x00000007ff71a730,0x00000007ffa00000)
>
> to   space 6144K, 0% used 
> [0x00000007ffa00000,0x00000007ffa00000,0x0000000800000000)
>
> PSOldGen total 3145728K, used 2268005K [0x00000006c0000000, 
> 0x0000000780000000, 0x0000000780000000)
>
> object space 3145728K, 72% used 
> [0x00000006c0000000,0x000000074a6d9528,0x0000000780000000)
>
> PSPermGen total 819200K, used 57100K [0x0000000680000000, 
> 0x00000006b2000000, 0x00000006c0000000)
>
> object space 819200K, 6% used 
> [0x0000000680000000,0x00000006837c3130,0x00000006b2000000)
>
> }
>
> {Heap before GC invocations=458 (full 0):
>
> PSYoungGen total 2091008K, used 2088041K [0x0000000780000000, 
> 0x0000000800000000, 0x0000000800000000)
>
> eden space 2084864K, 100% used 
> [0x0000000780000000,0x00000007ff400000,0x00000007ff400000)
>
> from space 6144K, 51% used 
> [0x00000007ff400000,0x00000007ff71a730,0x00000007ffa00000)
>
> to   space 6144K, 0% used 
> [0x00000007ffa00000,0x00000007ffa00000,0x0000000800000000)
>
> PSOldGen total 3145728K, used 2268005K [0x00000006c0000000, 
> 0x0000000780000000, 0x0000000780000000)
>
> object space 3145728K, 72% used 
> [0x00000006c0000000,0x000000074a6d9528,0x0000000780000000)
>
> PSPermGen total 819200K, used 57100K [0x0000000680000000, 
> 0x00000006b2000000, 0x00000006c0000000)
>
> object space 819200K, 6% used 
> [0x0000000680000000,0x00000006837c31b0,0x00000006b2000000)
>
> 1465.998: [GC
>
> Desired survivor size 6291456 bytes, new threshold 1 (max 15)
>
> [PSYoungGen: 2088041K->1456K(2091008K)] 4356047K->2269485K(5236736K), 
> 0.2940590 secs] [Times: user=0.33 sys=0.00, real=0.29 secs]
>
> Heap after GC invocations=458 (full 0):
>
> PSYoungGen total 2091008K, used 1456K [0x0000000780000000, 
> 0x0000000800000000, 0x0000000800000000)
>
> eden space 2084864K, 0% used 
> [0x0000000780000000,0x0000000780000000,0x00000007ff400000)
>
> from space 6144K, 23% used 
> [0x00000007ffa00000,0x00000007ffb6c188,0x0000000800000000)
>
> to   space 6144K, 0% used 
> [0x00000007ff400000,0x00000007ff400000,0x00000007ffa00000)
>
> PSOldGen total 3145728K, used 2268029K [0x00000006c0000000, 
> 0x0000000780000000, 0x0000000780000000)
>
> object space 3145728K, 72% used 
> [0x00000006c0000000,0x000000074a6df528,0x0000000780000000)
>
> PSPermGen total 819200K, used 57100K [0x0000000680000000, 
> 0x00000006b2000000, 0x00000006c0000000)
>
> object space 819200K, 6% used 
> [0x0000000680000000,0x00000006837c31b0,0x00000006b2000000)
>
> }
>
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