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How to interpret GC logs when tuning GemFire

Applies to

GemFire 6 and later

Purpose

The purpose of this article is to describe the format seen in the GC logs so this can be used to tune the heap.

Description

The following is an example of a GC log entry:

325209.429: [GC 325209.429: [ParNew
Desired survivor size 4128768 bytes, new threshold 2 (max 2)
- age 1: 1812544 bytes, 1812544 total
- age 2: 305568 bytes, 2118112 total
: 1034582K->2103K(1040512K), 0.0116910 secs]
2598263K->1565814K(3137664K), 0.0118312 secs]

The GC log entry is comprised of the following elements:
{Time since VM started, in seconds}: [GC {Time since VM started}: [{GC type}
Desired survivor size {size of one survivor space} bytes, new threshold {y} (max {x}) <- how many collections objects can stay in the young gen.
- age 1: {a} bytes, {a} total <- How many objects have survived one collection. Next collection, any surviving objects will appear in age 2
- age 2: {b} bytes, {a+b} total
: {pre-collection young gen[5] usage}K->{post-collection young gen usage}K({Total young gen size}K), {young gen time} secs]
{pre-collection heap size}K->{post-collection heap size}K({total heap size}K), {total collection time} secs]

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