G1 slower than default garbage collector on Java 7

Question

I have been trying to use G1 on my applications, and noticed it takes more time to start an application compared to default garbage collector on java 7u91. On the other hand, less collections were execute as G1 is supposed to be.

Is there any reason why G1 is slower? My application is using a minimum and maximum heap of 128mb, over Solaris 64bits version with no custom parameters to VM. (Java server edition)

Why would you expect them not to make the faster garbage collector the default? — Louis Wasserman, Feb 16 '16 at 00:34
Doesn't gc1 more efficient than the other garbage collectors? if not, why to migrate to it. — Carlos Alberto, Feb 16 '16 at 00:36
What makes you think that? If you were designing Java, why would you not have the best possible performance by default? I'd expect non-default garbage collectors to only be better in rare, very specific circumstances. — Louis Wasserman, Feb 16 '16 at 00:37
ok, thinking this way I can use CMS with best options to tune the application. In my opinion if java 9 tends to have gc1 as default for a good reason must be. — Carlos Alberto, Feb 16 '16 at 00:43
if java 9 has GC1 as a default that's probably because they've made GC1 better in Java 9. — Louis Wasserman, Feb 16 '16 at 00:43
Ok I agree that, but what i am trying to find out in this post is if anyone notice this question. Have you? — Carlos Alberto, Feb 16 '16 at 00:47
Post your VM parameters. Performance depends on parameters you set. Have a look at http://stackoverflow.com/questions/8111310/java-7-jdk-7-garbage-collection-and-documentation/34254605#34254605 — Ravindra babu, Feb 16 '16 at 01:55
I am using no-one parameters, just max and min heap equals with 128mb — Carlos Alberto, Feb 16 '16 at 01:57
Of course, with G1 took 136 seconds and using default took 112 seconds. It's not a big difference but it's slower... that's it!. — Carlos Alberto, Feb 16 '16 at 02:01
It takes >100 seconds to _start_ your application? This makes no sense. — bmargulies, Feb 16 '16 at 02:15
Yes, it's an old RISC server, and application starts many queues during startup. — Carlos Alberto, Feb 16 '16 at 02:17

the8472 · Accepted Answer · 2016-02-16T13:41:12.847

and noticed it takes more time to start an application

That has several reasons

First: In most cases the default collector is the parallel collector, also known as throughput collector. It is the most efficient one in terms of wall time spent in GC relative to wall time spent in application code.^[1]. It does not have to bear the extra costs of performing concurrent work.

G1 primarily optimizes for lower pause times on large heaps by expending additional CPU cycles on partially concurrent collections, which requires that you have CPU cycles to spare. Throughput is only a secondary goal.

Second: enabling G1 changes more than the algorithm used, many default settings are also changed.

diff <(java -XX:+UseG1GC -XX:+PrintFlagsFinal) <(java -XX:+PrintFlagsFinal) shows how other flags are changed

On java 8 there are the following differences that significantly affect GC behavior, among several others:

<     uintx GCTimeRatio                               = 9                                   {product}
>     uintx GCTimeRatio                               = 99                                  {product}
<     uintx MaxGCPauseMillis                          = 200                                 {product}
>     uintx MaxGCPauseMillis                          = 18446744073709551615                    {product}

Third: Application startup is not exactly a good way to measure anything since that's a transient event and the heap still has to settle into its final size. Collectors aim for steady-state operation and may handle such transients differently.

On an "old RISC server" and a heap size of 128 MB you definitely want to stick to either the parallel or serial collectors. Such a configuration does not benefit from what G1 has to offer.

^[1] For CPU cycles instead of wall time it would be the serial collector.

G1 slower than default garbage collector on Java 7

1 Answers1