Linked by Thom Holwerda on Sat 28th Jun 2008 22:09 UTC, submitted by diegocg
X11, Window Managers "Maybe I'm just naive, but designing a graphics API such that all image data had to be sent over a socket to another process every time the image needed to be drawn seems like complete idiocy. Unfortunately, that is precisely what the X Window System forces a program to do, and exactly what Cairo does when drawing images in Linux - a full copy of the image data, send to another process, no less, every time it is drawn. One would think there would be some room for improvement. Unsurprisingly, others felt the same way about X, and decided to write an extension, Xlib Shm or XShm for short, that allows images to placed in a shared memory segment from which the X server reads which allows the program to avoid the memory copy. GTK already makes use of the XShm extension, and it seems like a good idea to see if Gecko couldn't do the same."
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Comment by SomeGuy
by SomeGuy on Sun 29th Jun 2008 17:58 UTC
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Sockets are essentially a memcpy() in kernel. They're quite cheap when amortized over blitting the same image to screen repeatedly

Shm leads to hard synchronization issues, which could lead to messy pixmap stomping if the client/X server isn't careful.

But more importantly, the SHM data *must* be kept in the shared buffer. This doesn't sound so important until you realize that the image *doesn't* want to live in the shared buffer when it's being accelerated. what does this mean?

Well, if you're using the shm extension, you're essentially forcing the X server to draw in software (not that it's a change from the poorly accelerated proprietary drivers he was using, anyways.)

If you upload images and paint them repeatedly, the good old upload-over-a-socket allows X to put the pixmaps wherever it wants to get best performance. This is really why XSHM is discouraged these days.

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