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
