Re: stack depth limit exceeded problem.

[Thomas Hallgren <thhal(at)mailblocks(dot)com>]

Oliver Jowett wrote:

> Thomas Hallgren wrote:
>
>  
>
> > PL/Java runs a JVM. Since a JVM is multi threaded, PL/Java goes to
> > fairly  extreme measures to ensure that only one thread at a time can
> > access the backend. So far, this have worked well but there is one small
> > problem. [...]
> >    
>
> I assume this means you have a single lock serializing requests to the
> backend?
>  
Yes, of course. I also make sure that the main thread cannot return 
until another thread that is servicing a backend request has completed. 
There's absolutely no way two threads can execute backend code 
simultaniously.

> If you can't solve the depth checking problem (Tom doesn't seem to like
> the idea of multiple threads calling into the backend..), what about
> turning the original thread (i.e. the "main" backend thread) into a
> "backend interface thread" that does nothing but feed callbacks into the
> backend on request? Then run all the user code in a separate thread that
> passes backend requests to the interface thread rather than directly
> executing them. If it starts extra threads which makes DB requests, the
> mechanism stays the same..
>  
I though about that. The drawback is that each and every call must spawn 
a new thread, no matter how trivial that call might be. If you do a 
select from a table with 10,000 records and execute a function for each 
record, you get 20,000 context switches. Avoiding that kind of overhead 
is one of the motivating factors for keeping the VM in-process.

I don't rule out such a solution but I'd like to have a discussion with 
Tom and iron out what the problems are when one thread at a time is 
allowed to execute. Perhaps I can solve them.

Regards,
Thomas Hallgren