[asterisk-dev] [RFC] optimising RTP traffic?

[Roy Sigurd Karlsbakk]

> I'd like to offer my 2c on the issue:  Before you look really hard  
> at what it takes to try to move RTP forwarding up from the  
> application and into the kernel, it would make sense to:
>
> a) Profile the present situation:  You can use oprofile or other  
> tools to profile the entire system, and see where most of the time  
> is being spent.   If the goal is to optimize this process, this  
> would point out the places to look.  I don't know if this has been  
> done or not yet, but it seems the original idea was based on  
> intuition, not on actual results.

I've done so, which was the reason I started digging into this. I  
tried setting up two boxes sending SIP/RTP calls between them without  
reinvites, and with a single xeon ~3GHz, 500 calls kills the system.  
About 70% of the time is spent in kernel, as I can see from context  
switching between kernel- and userspace. This makes Asterisk unusable  
to do such bridging in large traffic amounts. I'm told, though, that  
this is not a problem since this is not what asterisk's built to do.  
However, I find it hard to beleive software shouldn't be allowed to  
get better.

> b) Build a system that you can at least use for profiling, which  
> works using sendfile() or similar, even if it doesn't actually work  
> properly, and benchmark it.  The point here is to just get a  
> ballpark of the improvement you expect to see, to determine if it's  
> worth actually making it all work.  You don't need to dot the i's  
> and cross the t's, just make it something you can set up calls  
> with, and benchmark.

It really is not necessary to do this. Looking at the oprofile output  
shows all the time is spent within readfrom/sendto and related calls  
and their respective switches of contexts. Sendfile is plain and  
doesn't involve such things. Doing the sendfile test will be a  
complete waste of time, beleive me. sendfile() just takes two file  
descriptors and connects them, without involving either userspace nor  
kernel checksumming, so i'd guess something like 60%+ of the time  
taken to bridge RTP will be gone.

roy
--
Roy Sigurd Karlsbakk
roy at karlsbakk.net
(+47) 98013356
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