Re: XCB (Was Re: Window managers ( was Re: newbie question ))

[David Chisnall]

Hi Fred,

On 23 Apr 2009, at 16:12, Fred Kiefer wrote:

> could you please detail a bit on the benefits of an XCB backend  
> compared to an Xlib one?
> As you already pointed out newer versions of Xlib are internally  
> able to use XCB, so what can we gain by using it directly?

The big advantages (apart from linking against a much smaller library)  
are that XCB has a design which is trivial to integrate with a GNUstep  
runloop (i.e. it provides a file descriptor that you can add to the  
run loop and receive notifications when there are pending events,  
which is what I am doing) and that it is much better at latency  
hiding.  A lot of XLib calls attempt to provide a synchronous API on  
top of an asynchronous protocol, so the client (GNUstep app) is  
spending time waiting for the round trip to the server.  On a local  
machine this just adds a bit of overhead, but on a remote system even  
on a local network it is common for the GNUstep app to be spending  
more than 50% of its time in a blocking state waiting for data from  
the X server which is not really required immediately.  XCB is a pure  
asynchronous design, using futures, and lets you fire off messages to  
the X server and not block for receiving the replies until you  
actually need them.

David