[asterisk-biz] IAX Channels limit on Asterisk 1.4.17

[Trixter aka Bret McDanel]

On Wed, 2008-01-16 at 22:40 +0000, Tim H. Panton wrote:
> From what I heard at astri(dev?)con, this isn't quite right.
> On some OS's multiple threads can read from the same UDP port.
> These threads can be spread over multiple processors.
> 

They can in most infact, however the last thing you want is all of them
just willy nilly grabbing packets, you want some front end saying 'this
one is for this call, that one is for that call' and the overhead to
pass them to the correct thread is high enough that doing a front end
process like that, even with multiple threads is such that you dont want
to (I addressed this in a previous email).

You also dont want a ton of threads grabbing channels remotely and doing
stuff to them, that would be a locking nightmare and probably cause a
few channels to die mysterious deaths or just get into confused states
or have garbled audio on occasion.  Debugging why would also be
problematic.  The load of flipping locks on/off like that would also be
counterproductive.

It would be even worse if every thread read every packet and discarded
what is not for it (assuming you could do that, given that you commented
on it may not work in linux, it was sun guys ...)  I dont know if this
is what they were talking about, but if that happened there would be a
memcpy for each and every read (which means packets per second *
channels in use) from the kernel space network buffers to the userspace
buffers, there would be a context switch, software irq or sysenter,
stuff that would take a lot of cpu load.

There may be some other way that isnt addressed here that doesnt have
all of the negatives associated with it, but it would be very platform
specific if it did exist.  

Now I am working on a way to avoid a lot of the overhead associated with
a read, the memcpy, the context switch, etc are all removed.  This could
be adapted to asterisk, which would actually improve its performance
dramatically, although I really dont know exact numbers for asterisk, I
know with another softswitch that I am doing it for there is a 30-40%
speed boost with my additions on bridged calls.  
-- 
Trixter http://www.0xdecafbad.com     Bret McDanel
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