On Jan 8, 2009, at 13:52, Jay Ballinger wrote:

> Hi everyone,
> We're running Resin 3.0.18 Pro on a Windows 32-bit system and we're  
> looking to understand/tune our memory settings a bit.
> According to http://www.caucho.com/resin-3.0/performance/jvm-tuning.xtp 
>  the stack size is set to 2048k unless set otherwise. Is this true?  
> In using JConsole we can't see where any explicit -Xss setting is  
> passed and we can't readily determine what the current stack size is  
> set to.

I think Caucho tends to err on the side of caution in this regard, and  
is also (understandably) more *nix-central than Windows. So take that  
2048K as being "appropriate for most situations" with a grain of salt  
because situations can vary wildly.

> So, here's a short list of questions. And thanks for your help.
> - If JConsole doesn't show -Xss, is it really there for resin?

Whether it's explicitly set or not, there IS a stack size, which would  
end up being the default stack size of the operating system if the JVM  
itself doesn't set it (I do not think it does, but could be wrong and  
don't feel like looking it up now).

> - How was "2048k is an appropriate value for most situations."  
> determined?

See above.

> - Can resin/jvm/jconsole report the current stack memory setting?

I don't know if there's a way to query a running JVM for this  
information. It's best if you explicitly set the stack size using the  
JVM option -Xss (but see below).

> (general sun jvm on windows questions)
> - What is the default stack memory size?

That's too general of a question. It depends on the OS, and (perhaps)  
the version of the OS.

> - When a thread is created, does it grab all of the stack memory, or  
> can the stack memory start small and grow to the default or -Xss size?

Again, might depend on the operating system. Contrary to what Knut  
said, and to the best of my knowledge and in the testing that I did  
(it's been a while), all of the memory for the stack is pre-allocated  
(at least from the perspective of the JVM itself). But it's easy  
enough to write a program to check this.

Thread stacks, at least on Windows, are more complicated than just a  
single memory allocation (down at the Windows API level) because  
you've got both reserved and committed memory allocations, and one  
always has to be >= the other (I forget which), and there's a 64K  
granularity in sizing, and you're dealing with physical and virtual  
memory, and this is all set in the linker and therefore configurable,  
but can also be set programmatically on a per-thread basis. In other  
words, it's quite involved. See this page as an example of how complex  
this kind of memory management is when you go into how the underlying  
OS is actually managing all of this 

Remember, thread allocation/destruction as well as context switching  
and just about everything else is managed by the operating system, not  
Java (unless you're running "green" threads, which I don't think is  
even around anymore?). This is why the memory used by the thread's  
stack is not part of the Java heap. On a 32-bit machine, the maximum  
addressable space for an individual java program is 2G. So, for  
example, if you run java with these parameters on a 32-bit machine:

java -server -Xms1500M -Xmx1500M -Xss1M

then you'll leave enough room to run ~548 threads (2048M - 1500M =  
548M / 1M stack = 548 threads). Of course, that assumes that none of  
the 548M of non-java-heap memory is being used for anything else,  
which is a false assumption, perhaps hugely false  depending on what  
your java program is doing, e.g. graphics operations or the like.

Something I discovered while optimizing our Resin on Windows  
environments is that the version of Windows and Java we use (64-bit  
Windows 2003 Server with Java 1.5) will allow a minimum stack size of  
128K, and if you try to specify a stack size of 256K or 512K, Windows  
(or maybe the JVM, I don't know which one for sure) automatically  
jumps you up to 1M. So you have a choice between 128K, 1M, 2M, ??? (I  
didn't test above 2M).

Fwiw, we've got dozens of Windows servers running Resin Pro 3.0.x  
(upgrading to 3.1.x in the not-too-distant future), and we run all of  
them with a 128K stack size. In general, unless you're running code  
that's highly recursive, you don't need very large stack sizes in Java  


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