Severe performance regression was observed on X Server 1.18 with older graphics cards

[Mark Kettenis]

> From: "Kevin Brace" <kevinbrace at gmx.com>
> Date: Mon, 31 Jul 2017 08:47:37 +0200
> 
> Hi,
> 
> I am hoping someone can track down a bad commit that led to severe
> performance regression likely caused by X Server 1.18.
> The other day, I was trying to figure out why a computer with
> Xubuntu 16.04.2 was running so slowly.
> The thing is, as long as I used Lubuntu 12.04 or Xubuntu 14.04, I
> was getting the performance I was expecting (fairly fast).
> Here is the spec of the computer.
> 
> CPU: AMD Opteron 165 (1.8 GHz, 1 MB L2 cache, dual core) 
> Mainboard: ASRock 939Dual-SATA2
> RAM: 2.25 GB
> Graphics: SiS 305 (AGP, 32 MB)
> 
> First, I noticed severe performance regression on Xubuntu 16.04.2.
> Xubuntu 16.04.2 uses Linux 4.10 and X Server 1.18.
> Xubuntu 14.04 (stock) uses Linux 3.13 and X Server 1.15.
> Lubuntu 12.04 (stock) uses Linux 3.2 and X Server 1.11.
> 
> Please note that all version used xf86-video-sis (it is renamed as
> xserver-xorg-video-sis for Debian / Ubuntu), and they all used the
> version from the upstream freedesktop.org repository.

This is almost certainly because the XAA acceleration support was
removed from the X server.  The driver will now use EXA, but EXA never
really worked properly for a lot of the "legacy" drivers.

> For Xubuntu 16.04, Canonical no longer builds
> xserver-xorg-video-sis, so I compiled the upstream code and
> installed it.

Try compiling the driver without EXA support.  The easiest way is
probably to disable the SIS_USE_EXA #define in src/sis.h by changing
the line above from #if 1 into #if 0.  That way you'll still have the
userland modesetting support but the driver will use shadowfb for
rendering which is what the xf86-video-vesa driver uses as well.

Cheers,

Mark