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Re: [Qemu-devel] Reducing X communication bandwidth

From: Fabrice Bellard
Subject: Re: [Qemu-devel] Reducing X communication bandwidth
Date: Sat, 10 Jul 2004 15:53:10 +0200
User-agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.4) Gecko/20030624

Julian Seward wrote:
Anyway, QEMU from cvs is unusable like this, at least when
running WinXP or Win2K.  The SDL layer (qemu/sdl.c) blasts
huge numbers of pixels across the network in response to even
the simplest graphical operations.  Notably, moving the mouse
pointer is appalling, with an update rate of about twice per
second, which is hopeless.

The attached patch against sdl.c fixes this.  It keeps a shadow
copy of video memory.  When a request arrives at sdl_update()
to redraw an area, the area is compared against the shadow copy,
and only the parts that have really changed are passed to
SDL_UpdateRect().  The comparison is done at a granularity of
32x32 chunks of pixels.

OK for the feature.

I just want to be sure it does not slow down the case where QEMU runs locally (comparisons may be slower than just copying). So there should be a specific support to detect if shared memory is used by SDL.

Another point is that it may not be necessary to add a second frame buffer. By modifying the vga core a single line of memory should be necessary to do the comparison.


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