Matthew Garrett on the race to idle [LWN.net]

Matthew Garrett on the race to idle

Posted May 13, 2008 13:41 UTC (Tue) by daenzer (✭ supporter ✭, #7050)
In reply to: Matthew Garrett on the race to idle by farnz
Parent article: Matthew Garrett on the race to idle

> We're not using timers at all - we use the DRM to wait for VBlank.

But then use X11 for rendering? If so, part of the reason for the latency being too high could
be the several context switches between the interrupt and the rendering operation taking
place. By using OpenGL with current Mesa and a current drm Git snapshot, it would be possible
to have the DRM emit the buffer swap operation from a tasklet triggered by the interrupt, and
the application could generate frames ahead of time and transparently sleep when it's too far
ahead.

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Matthew Garrett on the race to idle

Posted May 13, 2008 14:46 UTC (Tue) by farnz (guest, #17727) [Link]

We do use X11 for rendering, but it's definitely the move from a low C-state that kills us. We can easily measure the latency of the rendering operation; in the worst case we've seen thus far (not on an Intel platform), we see around 0.03 milliseconds between the wait for VBlank syscall returning, and us being about to enter the wait again. Note that we sync the X stream at this point, so we don't re-enter the wait until the server has finished drawing.

Note that we don't draw by blitting a new frame from scratch; we delta the existing on-screen frame instead, which minimises the work involved.