对于多线程应用程序，有用的JVM选项有哪些？

Question

I am working on an application that creates a lot of threads and relies heavily on String manipulation.
The application works for a good 24 hrs at a time and needs to be always very responsive.
I am trying to keep the creation of objects to a minimum. The application is doing well without any configuration at the moment.

But I was wondering for my own knowledge if there were any advantages (or disavantages) in using a specific JVM configuration?

Please bear with me, I am pretty new on on the subject of the JVM/GC configuration:

I was wondering if there were any JVM options I should absolutely use while working with multithreads?

Should I configure the heap?

Should I also configure the GC?

Should I keep the Garbage Collection to a minimum?

I started reading: http://www.oracle.com/technetwork/java/javase/tech/vmoptions-jsp-140102.html
Any tips on the subject would greatly be appreciated.

Thanks in advance,

Answer 1

Generally, the best intial advice concerning tweaking your JVM is don't . Unless you are experiencing specific JVM-related problems with the default settings, leave them alone.

If you do need to fiddle around with the settings, I would recommend you set up a representative testcase and use an advanced profiler such as JProfiler .

Furthermore, you should really read the technical documentation regarding the HotSpot VM, specifically the Memory Management Whitepaper, all of which you may find here .

Answer 2

If it is working fine then you should not do anything.

If your application is CPU bound you should not create Lot of threads. Reason is lot of time is wasted in context switching. String manipulation if it in memory then there should be only those threads which are required

NCPU = UCPU* (1+W/C)

Where  NCPU--> Number of CPU
UCPU--> Target CPU Utilization
W-->Wait time
C--> Compute time

So for CPU bound operations it should be max (Number of CPU +1) threads.

Also there are lot of test cases defined for concurrency applications in Java Concurrency in Practice. You may want to check those.

Answer 3

I was wondering if there were any JVM options I should absolutely use while working with multithreads?

No.

Should I configure the heap?

No, apart from setting the heap size to something reasonable (with -Xmx and -Xms)

Should I also configure the GC?

No, unless you have a particular need for "low-pause". The default throughput compiler is the best option if you are currently meeting your "responsiveness" goals. If you are not meeting those goals then you should consider CMS or G1 ... but beware that they reduce pauses but they also reduce throughput.

Should I keep the Garbage Collection to a minimum?

No. That is not a sensible goal. Your aim is to maximize throughput, and minimizing GC won't necessarily achieve that. In a lot of case, it is more efficient to generate garbage than to to have the application do extra work to avoid generating garbage. (And as Peter Lawrey pointed out, you've also got the extra developer effort in writing and maintaining mode complex code.)

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I would advise you to use a profiler to see if your application is spending a lot of time (CPU time or elapsed time) relative to doing other productive work. If not, or if the application is already running fast enough then don't fiddle with the JVM options.

If you are worried that your application won't cope with increased load in the future, then tweaking the GC doesn't scale. A better option is to investigate scaling up your hardware and/or figuring out how to do the work on multiple machines. In addition, tuning the GC to improve performance with current load may actually result in worse performance when the load increases. (Consider the problem that arises with CMS when it can't keep up and is forced to do a full stop-the-world collection to recover.)

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Finally, it is generally speak a bad idea to have lots of threads. It is better to use a small number of worker threads (roughly equal to the number of processors / cores) and feed them work via concurrent queues, etcetera.

Answer 4

I was wondering if there were any JVM options I should absolutely use while working with multithreads?

All the best options will be on by default. If you look at HotSpot VM Options you can see quite a few are -XX:+ which means they are on by default.

Should I configure the heap?

Possibly. But I would leave the default setting if you can.

Should I also configure the GC?

Possibly. But I would leave the default setting if you can.

Should I keep the Garbage Collection to a minimum?

Reducing the amount of garbage created takes effort. It provides some benefit up to a point. You have to decide what is the best use of your time and how much time to spend reducing the amount of garbage created.

I would always start with a memory profiler and find where you are creating the most garbage. Start from the top of the list rather than trying to tune everything as this ensures you will get the most benefit for the least amount of effort.

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BTW: I am an advocate of low garbage and off heap programs where it makes sense to do so. I have written trading systems which can run for a day without even a minor GC and programs which can load/use 500+ GB of data in off heap memory. However, you have to be able to demonstrate or quantify how much difference it will make to the end users or your business to determine whether it is really worth it.

Answer 5

In the past, I have faced the similar server application: lots of String manipulation, String creation, and needs to be always very responsive. The app worked fine with default configuration, until run into high-stress situation. You need to enable -XX:+UseConcMarkSweepGC for low pause, and fine tune other parameters to ensure the app behavior the way that you want. Here is the short list:

-XX:+CMSParallelRemarkEnabled
-XX:+CMSScavengeBeforeRemark
-XX:+UseCMSInitiatingOccupancyOnly
-XX:CMSInitiatingOccupancyFraction=nn
-XX:CMSWaitDuration=300000
-XX:GCTimeRatio=nn
-XX:+DisableExplicitGC