Re: Http Accept filters (accf_http)

On Thu, 24 Apr 2008, Antony Mawer wrote:

> Robert Watson wrote:
> ...
>> I'm aware of a few problems relating to accept filters, and "fixing" them 
>> has been on my todo list for several years.  Unfortunately, other things 
>> keep getting ahead of that in the todo list.  One known issue is that 
>> accept filters aren't entirely happy with the new multi-processor locking 
>> world order.
>
> Does this mean that on a multi-core system, one is better avoiding accept 
> filters and accepting a small latency hit in return for better SMP 
> scalability/reduced lock contention?

My intuition would have been to avoid it, except that I've had reports from 
some very high-end hosting providers (thousands of front-end systems) that it 
appears to work fine in practice.  Sometimes it's hard to say what problems 
will be visible, and what won't.  It's quite possible that since accept 
filters always run from the input path, they are always protected by &tcbinfo 
in 5.3-7.0, and as such the races aren't exercisable in practice, and they 
work just fine.  If you are running with accept filters and *are* running into 
problems, I can try and make some time to diagnose the problem, since if a 
problem does exist, I would like to fix it.  If tcbinfo is preventing it, we 
will need to fix them in 8.x, as we're eating away at that lock in order to 
improve input path parallelism.

Robert N M Watson
Computer Laboratory
University of Cambridge