mmap improvements: zero-copy, avoid wrap-around, memcopy, mmap.ACCESS_COPY

[Antoine Martin]

mmap is fast but can be made faster:

• rather than extracting the pixel data from the mmap area, use it in place by creating a pixel buffer pointing to the mmap location
• avoid wrapping around the end of the mmap area (split buffer) when we have enough free area: this avoids having to re-construct the buffer from 2 non-contiguous chunks and allows us to write it more quickly too
• set the mmap area as mmap.ACCESS_COPY so the OS does not try to keep it in sync - this may require us to flush it, may not work/be worthwhile
• use memcopy to copy the pixbuf pixels into the mmap area?
• probably not do-able: create a pixbuf (or even the pixmap?) backed by an mmap area so we don't need to call get_pixels() at all

[Antoine Martin]

patch for measuring the improvement using zerocopy

[Antoine Martin]

Part 1: zero-copy
The patch above allows us to measure the zero-copy case against the standard code (one must add and False to make it go through the non zero-copy case for non-split areas).
Some numbers with a 1024x1024 glxgears on a fast system (ignoring small screen updates which skew the number due to the high overhead of method calls compared to the actual time spent preparing the data - which means the benefit is less, almost negligible, when used on small pixel buffers):

• in all cases, the actual draw_rgb_image takes from 1ms to 10ms (average around 5ms)
• with "directly from mmap", creating the pixel data reference takes about 0.01ms
• with the regular copy case, creating the pixel data buffer takes about 1ms

So the benefit is in the 10% to 50% range, average around 20%.

Merged in r400

[Antoine Martin]

Part 2: avoid wrapping around
Done in r401, which also adds diagrams to show how the mmap transfers work.

[Antoine Martin]

This is good enough for now, the only features left to consider are:

• set the mmap area as mmap.ACCESS_COPY so the OS does not try to keep it in sync - this may require us to flush it, may not work/be worthwhile - does not seem to work!
• use memcopy to copy the pixbuf pixels into the mmap area
• probably not do-able: create a pixbuf (or even the pixmap / cairo draw) backed by an mmap area so we don't need to call get_pixels() at all?

[Antoine Martin]

We now use XShm (see #345).

[Antoine Martin]

(setting correct milestone the work was completed in)

[migration script]

this ticket has been moved to: ​https://github.com/Xpra-org/xpra/issues/67