X[Shm]GetImage is slow; what can I do about this?

Joel Dice dicej at mailsnare.net
Mon Apr 4 16:24:54 PDT 2005

On Mon, 4 Apr 2005, Joel Dice wrote:

> On Mon, 4 Apr 2005, Jonathan Lennox wrote:
>> As I've mentioned before on this list, I'm working on an X-based server for
>> the protocol defined in the Internet-Draft
>> <http://www.ietf.org/internet-drafts/draft-lennox-avt-app-sharing-00.txt>.
>> It's like VNC, but optionally window-based, so you can share individual
>> applications -- e.g., just a slideshow, as part of a multimedia conference
>> -- instead of your whole desktop.
>> My current implementation is based on using XDamage to track changes to
>> windows or the whole screen, and then using XShmGetImage to retrieve screen
>> images to transmit them over the network.
>> However, I've found that XShmGetImage is remarkably slow, to the point of
>> probably being very painful for interactive work.  Retrieving an image of 
>> my
>> whole 1280x1024 desktop takes about 1.5 seconds, and even retrieving the
>> contents of a just-scrolled 512x342 XTerm takes 150 ms.
>> Some Googling seems to indicate that XGetImage slowness comes about because
>> the X server keeps screen images in video RAM, not main memory, and video
>> cards don't tend to be optimized for uploading images to the CPU.
>> So my questions are:
>> 1.  Is this true?  And is my experience typical?
>> 2.  Is there anything that can be done about this, for instance some way to
>> convince the X server to keep a mirrored copy of a window in main memory?
>> (Could XComposite be of use here?)
> May I assume you're using the shared-memory extension (extensions/XShm.h) 
> when it's available?  That's good for a speed boost.  Also, I noticed a 
> dramatic improvement in XGetImage speed when switching from the free nv 
> driver to the proprietary nvidia driver.  On my nVidia 6600GT under Linux, 
> the performance is excellent.

Sorry, I guess you are using XShm - I only noticed the [Shm] in your 
subject after sending my last mail. :)

> On my ATI Rage Mobility -based laptop, using the free ati driver results in 
> performance comparible to our MS Windows implementation on the same hardware. 
> Are you developing for Windows also?  If so, have you tried doing 
> head-to-head benchmarks?
>> For reference's sake, I'm using X.Org 6.8.2 on FreeBSD 5.3; my video card 
>> is
>> an nVIDIA GeForce2 MX400.
>> -- 
>> Jonathan Lennox
>> lennox at cs dot columbia dot edu
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