Re: [drlvm] Class unloading support

[Geir Magnusson Jr.]

Xiao-Feng Li wrote:
On 10/28/06, Geir Magnusson Jr. <[EMAIL PROTECTED]> wrote:


Xiao-Feng Li wrote:
> All, I think the problem now is mainly about the class unloading
> design not about whether class unloading happens in server
> environment.

I think the problem is if class unloading happens cleanly.  Period.  The
fact that it most often happens in a server environment is irrelevant, 
IMO.

Yes. That's also my opinion. The _design_ of class unloading is the
focus of the discussion.

> Class unloading is definitely a feature required in future; but with
> the significance of the required modifications in JVM by this class
> unloading design 2 (using Java object for Vtable), it is probably
> safer to move this work into a branch at the moment until all other
> components are ready for it, and after we have thorough evaluation on
> it since there are still issues to be resolved or discussed.

I don't really agree.  It could be because of my background, but my
experience with java is in long-running, server-side processes, and
clean class-unloading is important.

I don't know what you "don't really agree". :-) 

Ah. Sorry - you said "it's required in future".  I think that it's 
required now, but that isn't actually at odds with what you said :)


My comment was that we
need thorough study to enable the special design so that the ongoing
development in other components are not impacted too much, and there
are still some issues to be discussed and resolved in this design.

Agreed.


> Or we can keep it in JIRA and keep the discussion and evaluation going
> on before we decide to support the special design (Java Vtable) in
> other components.

That's a different story :)  I'm not advocating one design over another,
but we have to be able to dump classloaders w/o leaking memory.

Agree again. :-)

So we're in violent agreement :)

geir


geir

>
> How about it?
>
> Thanks,
> xiaofeng
>
> On 10/27/06, Geir Magnusson Jr. <[EMAIL PROTECTED]> wrote:
>>
>>
>> Rana Dasgupta wrote:
>> > My knowledge in this area is limited. But my understanding was 
that web
>> > servers and other similar hosts recycled processes periodically as
>> > standard procedure, thereby tearing down all associated resources.
>>
>> Yes, but that has nothing to do with what would be happening in the 
app
>> server the web server talked to, if one had an architecture where a 
web
>> server "fronted" for the app server.
>>
>> > So
>> > classes loaded, but not used for a while went away anyway;
>>
>> Nope - they aren't loaded in the context of the webserver (when using
>> httpd).
>>
>> > this level of
>> > resource management was not really urgent. I know that IIS does 
this, I
>> > am not sure about httpd. I am not sure about other host 
environments.
>>
>> But a process fork model (or thread model) of a webserver has 
nothing to
>> do with what's going on in the VM.
>>
>> I'm talking about servlet engines and app servers like Tomcat and
>> Geronimo which have nothing to do with httpd.  Architecturally, 
they are
>> separated from the web server (unless you don't use an external
>> webserver, and just use the httpd connector in tomcat) and are 
separate,
>> independent processes.
>>
>>      httpd  <------>  Tomcat
>>
>> The java-based app servers are long running processes, running for 
weeks
>> or months.  We need to do clean class unloading.
>>
>> geir
>>
>>
>> >
>> >
>> > On 10/27/06, *Geir Magnusson Jr.* <[EMAIL PROTECTED]
>> > <mailto:[EMAIL PROTECTED]>> wrote:
>> >
>> >
>> >
>> >     Rana Dasgupta wrote:
>> >      > Aleksey,
>> >      >   I had a couple of questions.
>> >      >   You state that DRLVM does not implement the class unloading
>> >     optimization,
>> >      > and this may create memory pressure on some applications that
>> >     load many
>> >      > classes. Do we have a real case / example where an
>> application is
>> >     stuck for
>> >      > insufficient memory because it uses a lot of classes 
initially
>> >     and then
>> >      > stops using them, but these are not unloaded? One can 
imagine a
>> >     web browser
>> >      > doing something like this. Is a web browser a typical use 
case
>> >     for the
>> >      > Harmony JVM?
>> >      >
>> >
>> >     If I understand what you're asking correctly, you'll find this
>> pattern
>> >     in servlet engines or J2EE servers, where deployed apps can be
>> dumped
>> >     and reloaded repeatedly either during development or during
>> production
>> >     deployment, w/o taking the server down.
>> >
>> >     geir
>> >
>> >
>>
>