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Threading and JVM

5 replies on 1 page. Most recent reply: Nov 14, 2002 9:36 AM by Matt Gerrans

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Roshan

Posts: 3
Nickname: jrosh
Registered: May, 2002

Threading and JVM

Posted: May 3, 2002 11:49 AM

hi,
As I understand the underlying oprating system is resposnsible for handling and executing java Threads. My question is, since the Bye code instructions are only understood by the Java VM, how can the OS deal with them (the byte code instructions). Does the OS have a seperate instance of VM for every thread that is invoked.

Thomas SMETS

Posts: 307
Nickname: tsmets
Registered: Apr, 2002

Re: Threading and JVM

Posted: May 3, 2002 3:33 PM

First of all the monthly magazine Dr Dobbs Journal (http://www.ddj.com) has a very interesting note on Threading (this month).

Not at all,
You question let me think you do not really see the difference between Threads & Processes.
Back to your question :
It's not technically required that the VM spawns (? dunno if that's proper english ?) a new thread in the underlying OS for each thread you create in the (java) app !
You therefore have the difference between green threads (which are native threads) & emulated threads which are only at VM level but for which there is not thread created in the underlying OS !
Hence for VM using the green threads model, you have the VM creating a thread for each thread ccreated at application level. In each of these threads a Byte code will be transalate into Machine code !

Hope this gives you a lead to further understand ... (?)

Rgds,

Thomas SMETS,
SCJP2 - Brussels

Dirk Froehner

Posts: 2
Nickname: froehner
Registered: Nov, 2002

Re: Threading and JVM

Posted: Nov 13, 2002 4:35 AM

Hi!

Roshan's question is actually not a proof for him not knowing the difference between threads and processes.

Considering e.g. Linux, the default behaviour is that Java spawns a new process for every thread you create in your application. If you look at the output of "ps" then, you see lots of processes instead of just one as you expected and you are very likely to be irritated about how your Java threads are actually implemented on lower levels.

Regards,
Dirk

Thomas SMETS

Posts: 307
Nickname: tsmets
Registered: Apr, 2002

Re: Threading and JVM

Posted: Nov 13, 2002 3:31 PM

Dirk,

Are you sure that under Linux it spawns a new process for each Thread ?
I thought the whole point of a good VM was to only spawn threads & definitavely not processes. Sun Solaris has also implemented a Light Weight thingie (i don't remember if it's process or thread, now), as advised in the Unix98 standart...

Can you confirm / infirm any ?

Tia,

Thomas,

Dirk Froehner

Posts: 2
Nickname: froehner
Registered: Nov, 2002

Re: Threading and JVM

Posted: Nov 14, 2002 4:20 AM

> Are you sure that under Linux it spawns a new
> process for each Thread ?
Yes, I am. That is the default case. You can see it in the "ps" output. Of course that is actually not what you want since the advantages of threads against processes are lost then. I'm not sure about the reasons, good explanations are hard to find in the forums, maybe one should ask one of the linux kernel developers and / or the JVM developers.

Matt Gerrans

Posts: 1153
Nickname: matt
Registered: Feb, 2002

Re: Threading and JVM

Posted: Nov 14, 2002 9:36 AM

I was wondering about this myself when I saw it on Linux. Someone told me it was a deficiency of Linux, or that all threads on Linux are processes. I'm not enough of a Linux buff to know if that is really the case...

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