[Info] MaxPermSize and how it relates to the overall heap

MaxPermSize and how it relates to the overall heap

Many people have asked if the MaxPermSize value is a part of the overall -Xmx heap setting or additional to it. There is a GC document on the Sun website which is causing some confusion due to a somewhat vague explanation and an errant diagram. The more I look at this document, the more I think the original author has made a subtle mistake in describing -Xmx as it relates to the PermSize and MaxPermSize.

First, a quick definition of the "permanent generation".

"The permanent generation is used to hold reflective data of the VM itself such as class objects and method objects. These reflective objects are allocated directly into the permanent generation, and it is sized independently from the other generations." [ref]

Yes, PermSize is additional to the -Xmx value set by the user on the JVM options. But MaxPermSize allows for the JVM to be able to grow the PermSize to the amount specified. Initially when the VM is loaded, the MaxPermSize will still be the default value (32mb for -client and 64mb for -server) but will not actually take up that amount until it is needed. On the other hand, if you were to set BOTH PermSize and MaxPermSize to 256mb, you would notice that the overall heap has increased by 256mb additional to the -Xmx setting.

So for example, if you set your -Xmx to 256m and your -MaxPermSize to 256m, you could check with the Solaris 'pmap' command how much memory the resulting process is taking up.


$ uname -a
SunOS devnull 5.8 Generic_108528-27 sun4u sparc

$ java -version
java version "1.3.1_02"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.3.1_02-b02)
Java HotSpot(TM) Client VM (build 1.3.1_02-b02, mixed mode)

$ java -Xms256m -Xmx256m -XX:MaxPermSize=256m Hello &
$ pmap 6432
6432: /usr/java1.3.1/bin/../bin/sparc/native_threads/java -Xms256m -Xmx256m

total 288416K
Notice above that the overall heap is not 256m+256m yet? Why? We did not specify PermSize yet, only MaxPermSize.

$ java -Xms256m -Xmx256m -XX:PermSize=256m -XX:MaxPermSize=256m Hello &
$ pmap 6472
6472: /usr/java1.3.1/bin/../bin/sparc/native_threads/java -Xms256m -Xmx256m

total 550544K

Now we see the overall heap grow, -Xmx+PermSize. This shows conclusive proof that PermSize and MaxPermSize are additional to the -Xmx setting.



lucyfisher85397172 said...

Make no mistake: Our mission at Tip Top Equities is to sift through the thousands of underperforming companies out there to find the golden needle in the haystack. A stock worthy of your investment. A stock with the potential for big returns. More often than not, the stocks we profile show a significant increase in stock price, sometimes in days, not months or years. We have come across what we feel is one of those rare deals that the public has not heard about yet. Read on to find out more.

Nano Superlattice Technology Inc. (OTCBB Symbol: NSLT) is a nanotechnology company engaged in the coating of tools and components with nano structured PVD coatings for high-tech industries.

Nano utilizes Arc Bond Sputtering and Superlattice technology to apply multi-layers of super-hard elemental coatings on an array of precision products to achieve a variety of physical properties. The application of the coating on industrial products is designed to change their physical properties, improving a product's durability, resistance, chemical and physical characteristics as well as performance. Nano's super-hard alloy coating materials were especially developed for printed circuit board drills in response to special market requirements

The cutting of circuit boards causes severe wear on the cutting edge of drills and routers. With the increased miniaturization of personal electronics devices the dimensions of holes and cut aways are currently less than 0.2 mm. Nano coats tools with an ultra thin coating (only a few nanometers in thickness) of nitrides which can have a hardness of up to half that of diamond. This has proven to increase tool life by almost ten times. Nano plans to continue research and development into these techniques due to the vast application range for this type of nanotechnology

We believe that Nano is a company on the move. With today�s steady move towards miniaturization we feel that Nano is a company with the right product at the right time. It is our opinion that an investment in Nano will produce great returns for our readers.

Online Stock trading, in the New York Stock Exchange, and Toronto Stock Exchange, or any other stock market requires many hours of stock research. Always consult a stock broker for stock prices of penny stocks, and always seek proper free stock advice, as well as read a stock chart. This is not encouragement to buy stock, but merely a possible hot stock pick. Get a live stock market quote, before making a stock investment or participating in the stock market game or buying or selling a stock option.

johnbrinick2582 said...

I read over your blog, and i found it inquisitive, you may find My Blog interesting. My blog is just about my day to day life, as a park ranger. So please Click Here To Read My Blog