[cairo] basic question?

[Jamey Sharp]

On Wed, Aug 19, 2009 at 9:53 AM, Bill Spitzak<spitzak at gmail.com> wrote:
> For the normal displays Cairo has to be able to get the display to be
> up to date after every single cairo call.

Is that true? I don't see documentation calling for XFlush when using
the Xlib backend, but there are no calls to XFlush in the cairo source,
and the only calls to XSync are during connection close. That seems
appropriate to me: only the application can know when it cares about
having its drawing happen.

> It could also be used to queue up driver requests locally.

This is, of course, exactly what Xlib and XCB do for you already, and as
you suggest, that's all about avoiding syscalls and context switches.

> Tying this to the xlib flush is problematic as there are far too many
> xlib calls that do it and that programs find it hard to avoid.

Perhaps I'm mis-reading your comment. Xlib only flushes its buffer when
the buffer is full and you try to send more through it, or under various
circumstances around reading server responses. Those flushes are
generally necessary to accomplish what you asked Xlib to do, and in a
drawing-intensive setting it's the full-buffer case that's going to
trigger flush most of the time. You really can't get around that.

Looking at it the other way, the buffering is only an optimization.
Forcing a flush early may be necessary for correctness, but delaying one
never is. And it isn't even obvious that it's a good optimization any
more: if some of your CPU cores or your GPU are idle, then you should be
handing work off to them as early as possible instead of buffering them
longer.

Jamey