[Live-devel] Correct use of 'select' to avoid packet loss in Linux+BSD; correct use of WSAGetLastError and codes

[Stas Desyatnlkov]

Select is implemented on Windows differently. But on both systems it consumes too much of the CPU. I tried using live555 for the reception of 100 audio streams (g.711). It consumed about 10% CPU on my 3Ghz machine.
I end up rewriting it with a simple set of asynchronous sockets, the CPU consumption fell to 1% at peak levels on both Linux and Windows.

-----Original Message-----
From: live-devel-bounces at ns.live555.com [mailto:live-devel-bounces at ns.live555.com] On Behalf Of Aur?lien Nephtali
Sent: Wednesday, April 15, 2009 7:12 PM
To: LIVE555 Streaming Media - development & use
Subject: Re: [Live-devel] Correct use of 'select' to avoid packet loss in Linux+BSD; correct use of WSAGetLastError and codes

On Wed, Apr 15, 2009 at 4:45 PM, Ross Finlayson <finlayson at live555.com> wrote:
>> On Thu, Apr 9, 2009 at 9:52 AM, Ross Finlayson <finlayson at live555.com>
>> wrote:
>>>
>>>  Thanks for the suggestion; I like this.
>>>
>>>  Unless anyone sees a problem with this, I will include it in the next
>>>  release of the software.
>>
>> Hello,
>>
>> On Linux, select() is implemented above poll().
>
> What do you mean by this?
>

I mean that on Linux (and at least FreeBSD), select() is just a
wrapper around poll().

>
>>  In net/core/datagram.c
>> (linux 2.6.26 sources), in datagram_poll() you can see the lines that
>> check for readable event on the socket :
>>
>>       /* readable? */
>>        if (!skb_queue_empty(&sk->sk_receive_queue) ||
>>            (sk->sk_shutdown & RCV_SHUTDOWN))
>>                mask |= POLLIN | POLLRDNORM;
>>
>> The event is set if the socket input buffer is not empty.
>
> So what is your conclusion?  Are you implying that we should (on Unix
> systems) be using "poll()" instead of "select()", and that if we use
> "poll()", we won't need the optimization that Bryan Moore proposed? Or are
> you implying that we don't need this optimization even if we use
> "select()"??

I always use poll() on Linux because it is just a wrapper of select()
and because it does not have a limit on the number of sockets to
watch. The only problem is that it is not available on Windows.

In conclusion you should use poll() for Unix and select() for Windows.
The FIONREAD method is useless with poll()/select() under Unix because
there is no internal flag which tells the socket has events ; the
socket status is re-checked at each call.

I don't known how select() is implemented on Windows.

-- 
Aurélien Nephtali
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