[ds6-devel] io_stream scheduling refactor

[Chris Leishman]

Hi all,

I've just commited another minor refactoring to the branch 
"chris_refactor_291202".
The major points are:

1)
io_streams have been modified to handle their own buffers and 
scheduling.  When an io_stream is initialized, it is given a circular 
buffer to read into and write out from.  Methods have also been added 
so that the io_stream can report whether it needs to schedule itself 
for read or write.

This model has the advantage of removing all the buffer management and 
scheduling nuances out of the main select loop (readwrite).  It now 
allows for the scheduling algorithm to vary as the needs of the 
io_stream vary - allowing for mtu's and nru's to be heeded.

It also negates the need for the use of temporary buffers for the 
receipt of UDP packets.  By using the NRU (miNimum Receive Unit), we 
can specify the amount of data that must be able to be received in 
order to allow reading to occur.  The default NRU for UDP sockets is 
65536 - the maximum MTU a remote host can use.

I have chosen a very simple scheduling model for the current 
implementation.  At the top of each select loop (before calling select) 
each IOS is checked to see if it should be scheduled for read or write 
(using ios_schedule_(read|write)).  If this returns a valid 
filedescriptor, then that is used in the call to select - and if it is 
ready then the appropriate ios_(read|write) method is called for that 
IOS.  The ios_(read|write) method takes care of socktype specific 
notions (eg. using read v's recv or write v's send).

This approach is simple, but could be argued to be more expensive 
(though it isn't really that bad).  An alternative would be to provide 
each IOS with a callback by which it can change its scheduling 
requirements when (and only when) it needs to.  This would be more 
efficient than checking the requirements in every loop, but 
significantly more complex code wise.  For now I didn't think it was 
worth it, but comments would be appreciated!

2)
I've modified the connection_attributes to also contain the buffers 
that are used for local and remote.  This has the advantage of being 
able to configure their size before calling readwrite (and is also 
needed in order to init io_streams).

The buffer size for the remote buffer can now be set using the "-b 
size" option.

3)
The IOS now specifically contains the MTU and NRU of the connection, 
and these are heeded.  The MTU can be set via the "-m size" option, and 
the NRU via the "-r size" option.  I'm not sure if these are the best 
mnemonics - thoughts?

This will also allow for easy integration of PMTU discovery.


Anyway, give your comments.  I'd like to roll this into the HEAD, but I 
figure you should all get a look first.

Regards,
Chris