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Java

A Work in Progress

When you execute "java ..." you get some number of garbage collection (GC) threads. If you want to know how many GC threads, you can executejava...

When you execute "java ..." you get some number of garbage collection (GC) threads. If you want to know how many GC threads, you can executejava -XX:+PrintFlagsFinal -versionand look for ParallelGCThreads in the output (of which there will be plenty). You will find something like uintx ParallelGCThreads := 23 {product} If the number of GC threads you see looks way too big to you, this note may be of some interest.That number of GC threads is some fraction of the number of...

Java

The Unspoken - The Why of GC Ergonomics

Do you use GC ergonomics, -XX:+UseAdaptiveSizePolicy,with the UseParallelGC collector? The jist of GC ergonomics for that collector isthat it tries to grow or...

Do you use GC ergonomics, -XX:+UseAdaptiveSizePolicy,with the UseParallelGC collector? The jist of GC ergonomics for that collector isthat it tries to grow or shrink the heap to meet a specified goal.The goals that you can choose are maximum pause time and/or throughput.Don't get too excited there. I'm speaking about UseParallelGC (thethroughput collector) so there are definite limits to what pausegoals can be achieved. When you say out loud "I don't care about pause times,...

Java

The Unspoken - Application Times

Sometimes it is interesting to know how long your applicationruns between garbage collections. That can be calculated from theGC logs but a convenient way to...

Sometimes it is interesting to know how long your applicationruns between garbage collections. That can be calculated from theGC logs but a convenient way to see that information is throughthe command line flags -XX:+PrintGCApplicationStoppedTime and-XX:+PrintGCApplicationConcurrentTime. Adding these to yourcommand line produces output such as this.Application time: 0.2863875 secondsTotal time for which application threads were stopped: 0.0225087 secondsApplication time:...

Java

The Unspoken - CMS and PrintGCDetails

What follows is an example of the GC logging output with CMS (UseConcMarkSweepGC) and PrintGCDetails plus some explanation of the output. The "CMS-initial-mark"...

What follows is an example of the GC logging output with CMS (UseConcMarkSweepGC) and PrintGCDetails plus some explanation of the output. The "CMS-initial-mark" indicates the start of a CMS concurrent collection.[GC [1 CMS-initial-mark: 463236K(515960K)] 464178K(522488K), 0.0018216 secs] [Times: user=0.01 sys=0.00, real=0.00 secs]463236K above is the space occupied by objects in the old (CMS) generation at the start of the collection. Not all those objects are necessarily...

Java

The Unspoken - Phases of CMS

CMS (-XX:+UseConcMarkSweepGC) or the concurrent mark sweep GC could have been called the mostly concurrent mark sweep GC and here's why.These are the phases of...

CMS (-XX:+UseConcMarkSweepGC) or the concurrent mark sweep GC could have been called the mostly concurrent mark sweep GC and here's why.These are the phases of a CMS concurrent collection.1. Initial mark. This is the the start of CMS collection. This is a stop-the-world phase. All the applications threads are stopped and CMS scans the application threads (e.g. the thread stacks) for object references.2. Concurrent mark. This is a concurrent phase. CMS restarts all the...

Java

Really? iCMS? Really?

When I use the term iCMS, I'm referring to theincremental mode of CMS (-XX:+UseConcMarkSweepGC-XX:+CMSIncrementalMode). This is a mode ofCMS where the...

When I use the term iCMS, I'm referring to theincremental mode of CMS (-XX:+UseConcMarkSweepGC-XX:+CMSIncrementalMode). This is a mode ofCMS where the concurrent phases (concurrent markingand concurrent sweeping) are run inshort increments (does some work, yields andsleeps for a while, then runs again). It wasimplemented in CMS primarily for use on platformswith a single hardware thread. The concurrent phasesare typically long (think seconds and not milliseconds).If CMS...

Java

Our Collectors

I drew this diagram on a white board for some customers recently. They seemed tolike it (or were just being very polite) so I thought I redraw it for...

I drew this diagram on a white board for some customers recently. They seemed tolike it (or were just being very polite) so I thought I redraw it for youramusement.Each blue box represents a collector that is used to collect a generation. The young generation is collected by the blue boxes in the yellow region andthe tenured generation is collected by the blue boxes in the gray region."Serial" is a stop-the-world, copying collector which uses a single GC thread."ParNew" is a...

Java

GC Errata 2

I know that none of you are still making the transition fromjdk1.4.2 to jdk5 (aka jdk1.5.0, yah, we changed the numbering) but if you were, I'd want you to know...

I know that none of you are still making the transition fromjdk1.4.2 to jdk5 (aka jdk1.5.0, yah, we changed the numbering) but if you were, I'd want you to know that you might need a larger permanent generation. In jdk5 class data sharing wasimplemented (the -Xshare option, see the "java -X" output)when the client VM (-client) is being used.The implementation of class data sharing split some of theinformation about classes into two parts (the part thatcan be shared and the...

Java

Did You Know ...

These are a few esoteric factoids that I never expected users toneed, but which have actually come up recently. Most of thetext is just background information....

These are a few esoteric factoids that I never expected users toneed, but which have actually come up recently. Most of thetext is just background information. If you already recognize the command line flags that I've bold'ed, you probably already knowmore than is good for you. ParallelCMSThreads The low-pause collector (UseConcMarkSweepGC) does parts of the collectionof the tenured generation concurrently with the execution of the application(i.e., not during a...

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