Drawing optimization question

[Adrian Robert]

Hi,

I'm drawing directly to a view, either using [NSString -drawAtPoint..]  
or,
say, with NSRectFill() and friends.

I do something like:

    [view lockFocus]
      (draw comands ...)
    [view unlockFocus]
    [[view window] flushWindow]

The problem is high CPU consumption occurs for the X server (not
the app) when this happens repeatedly.  I'm guessing this is because
the entire window is being flushed, rather than just the changed portion
(which is usually small).  My questions:

- Is this a correct interpretation of what is happening?  NeXT's
  docs
   
(http://www.channelu.com/NeXT/NeXTStep/3.3/nd/Concepts/Performance/ 
D_ImprovingResponseTime.htmld/)
  say the Window Server maintains a "dirty rect" of what has been
  touched in the buffer and just flushes this, but is this true in
  GNUstep?

- Is there any way to explicitly flush just a portion of the window?

- Alternatively, could I just cause the drawing to go directly to X,
  and do away with the flush completely?
  I thought NSBackingStoreRetained would do this, but the results I
  get with it are not what I'd expect.

If there is a good RTFM reference somewhere on this that would be
helpful too; I haven't had success finding useful explanations in
Apple's docs.

Thanks for any help!