Overhead of X?

[Bilderbeek, Manuel]

Hello all,

I have a simple general question about X.

I've been trying this on an 800x600x16bpp configured framebuffer: change
the screencolour with a memset. I can do this with about 90 fps. But
when I try this with a window in X, with the fbdev driver, I can get
only about 30fps. Is there so much overhead from X? 

Additional info: the X program was both a Java/Swing program and a Qt
program. Both give the same performance of 30 frames per second. This
conforms with the notion that all CPU cycles were eaten by the Xorg
process and not by java or the C++ binary. Whether I use the fbdev
driver or the vesa driver does not make a difference. 

So, can anyone explain to me where the overhead is coming from?

It seems there is some double buffering or other copying going on or so.
Would it be useful to disable e.g. the DBE?

If both tests would not be comparable, what would make a comparable
test? What kind of overhead should I expect then? (It is all on a local
system.)

Thank you in advance for your reply.

--
Kind regards,

Manuel Bilderbeek

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