Re: Asynchronous Distributed Object Calls

FROM : Jens Alfke
DATE : Sun Feb 10 22:51:00 2008

On 10 Feb '08, at 12:15 PM, Adam Thorsen wrote:

> Is there a limit to the number of oneway calls to a DO instance 
> before it starts blocking?  I've noticed that if I make about 6 
> async requests (none of which are completed by the time the last 
> call is made) the seventh call blocks in the caller.


IIRC, this is because the Mach message queue used by DO has a limited 
fixed size. To avoid overflowing the queue, it blocks until there's 
room.

We ran into this problem while implementing the first version of the 
iChatAgent; it could get stuck this way and hang if a client process 
also hung. I call this kind of spreading system freeze a "tar-baby". 
At the time, I believe the engineer who fixed the bug ended up having 
to implement a background thread pool for dispatching asynchronous DO 
messages. If no threads were ready to send another message, it would 
just create a new one.

(My personal opinion is that DO is kind of dangerous — an "attractive 
nuisance" in legalese. It gives the impression of making everything 
really easy, but there are nasty details like this that complicate 
things. I also kept running into strange and nasty ref-counting 
crashers that I was somehow never quite smart enough to figure out how 
to fix. But maybe that's just me.)

—Jens_______________________________________________

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