xsrc/40509: X11 performance is very poor

[kivinen%iki.fi@localhost]

>Number:         40509
>Category:       xsrc
>Synopsis:       X11 performance is very poor after upgrading
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    xsrc-manager
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Wed Jan 28 20:45:00 +0000 2009
>Originator:     Tero Kivinen
>Release:        NetBSD 5.0_BETA
>Organization:
>Environment:
System: NetBSD haste.i.kivinen.iki.fi 5.0_BETA NetBSD 5.0_BETA (GENERIC) #0: 
Sat Jan 24 19:01:52 EET 2009 root@:/usr/obj/sys/arch/i386/compile/GENERIC i386
Architecture: i386
Machine: i386

Graphics card: Matrox Graphics, Inc. MGA G400/G450 rev 130
Graphics card memory: 32 MB
Graphics resolution: 2048x1536 @ 71Hz

>Description:

        I upgraded my NetBSD 3.99.15 version using XFree86 Version
        4.5.0 to NetBSD 5.0 Beta using X.Org X Server 1.4.2 and
        immediately noticed that X11 performance was very poor. The
        xterm scrolling speed was so slow, that it was almost
        impossible to use.

        x11perf -scroll500 returned 4.6 operations / second with X.org
        X server.

        It seems the DRI cannot be enabled at the given resolution as
        there was not enough graphics memory, but even when I lowered
        resolution to 1920x1440, the DRI could not be enabled (I
        assume there is no kernel support for the mga drivers). After
        playing around with few options I noticed that if I enable
        ShadowFB which will cause acceleration to be disabled, I
        suddenly get 28x boost to the performance and x11perf
        -scroll500 will now return 131 operations / second. This also
        make xterm and so on usable.

        I think this is related to the xorg X server, as I seem to
        have seen very poor performance on every platform where I had
        been forced to use xorg X server.

>How-To-Repeat:


>Fix:

        Perhaps switching back to XFree86 X server would help, or at
        least provide documentation that

        Option     "ShadowFB"           "on"

        can have huge performance boosts on some platforms.