Java:增加YoungGen大小以提高GC性能

Question

我正在阅读以下文章:http://java.sun.com/docs/hotspot/gc1.4.2/example.html并且无法理解以下几行:

Young generation size is too small

The young generation heap size in this first example is about 4 Mbytes with an overall heap size of about 32 Mbytes.

[GC [DefNew: 4032K->64K(4032K), 0.0429742 secs] 9350K->7748K(32704K), 0.0431096 secs]

[GC [DefNew: 4032K->64K(4032K), 0.0403446 secs] 11716K->10121K(32704K), 0.0404867 secs]

[GC [DefNew: 4032K->64K(4032K), 0.0443969 secs] 14089K->12562K(32704K), 0.0445251 secs]

This output seems reasonable from the point of view of the time spent in garbage collection but note that although the occupancy of the young generation is decreasing (e.g., in the first line from 4032 to 64k by 3968k) the occupancy of the entire heap is decreasing by considerably less (by 1602k from 9350k to 7748k). This indicates that only about 40% objects in the young generation were garbage and the rest survive the collection and are being promoted into the old generation.


Increasing the young generation size to increase the free space after the collection


The young generation heap size in this next run is increased to 8 Mbytes.


[GC [DefNew: 8128K->64K(8128K), 0.0453670 secs] 13000K->7427K(32704K), 0.0454906 secs]

[GC [DefNew: 8128K->64K(8128K), 0.0388632 secs] 15491K->9663K(32704K), 0.0390013 secs]

[GC [DefNew: 8128K->64K(8128K), 0.0388610 secs] 17727K->11829K(32704K), 0.0389919 secs]


With an 8 Mbyte size most of young generation is garbage at the time of the minor collection. In the first line the young generation heap decreases by 8064k from 8128k to 64k and the entire heap decreases by 5573k from 13000k to 7427k meaning about 68% of the young generation was garbage. As would be expected the size of the young generation does not change the amount of live objects that survive the minor collection (about 2.4M bytes in each case) but there are fewer minor collections and the cost of the collections in terms of the minor collection pause times are comparable (fractions of a second listed at the end of each line).

我的问题是,在这种情况下,增加YoungGen的大小如何帮助我们.应用程序在YoungGen中分配的对象总数将保持不变.

Answer 1

从YoungGen中提取一些东西是昂贵的,它会导致:

• 它能活得更久,浪费记忆力
• 未来的垃圾收集(每一个直到死亡)都要贵一些
• 它可以被YoungGen系列忽略,你必须等到你耗尽更宝贵的资源,并迫使更昂贵的收藏品.

通过增加YoungGen的大小,他们确保更多的对象被它收集,因此不会遇到上面的任何坏点.也就是说,对象快速死亡并且不会让任何人损失任何成本,所以一切都更快？

另外,你正在阅读的文档有一个更现代的版本,虽然它似乎缺乏你的例子:http: //www.oracle.com/technetwork/java/javase/gc-tuning-6-140523.html