[Bug 26982] drm-next/linux-next glxgears CPU usage doubles for 32bit, doesn't on 64bit

Fri Jan 21 12:13:59 PST 2011

https://bugzilla.kernel.org/show_bug.cgi?id=26982

--- Comment #2 from Anonymous Emailer <anonymous at kernel-bugs.osdl.org>  2011-01-21 20:13:57 ---
Reply-To: garry.hurley.jr at gmail.com

Silly question, and probably not relevant with modern chipsets, but could
this be a problem with hardware acceleration versus software acceleration?
Or maybe caching video RAM into System RAM?  I know the former used to be a
big factor in the earlier GPUs (3dfx Voodoo 2 and so forth) and for a while,
the best solution was to disable hardware acceleration in Linux.  Maybe
there is a problem on Chris's card that the hardware acceleration is not
working properly?  The latter would only affect CPU cycles during the cache
update cycles, but if the cache was being refreshed at a fairly fast rate,
it could cause some serious performance lags.

Just a couple of thoughts from the far left of the peanut gallery.

On Fri, Jan 21, 2011 at 2:19 PM, <bugzilla-daemon at bugzilla.kernel.org>wrote:

> https://bugzilla.kernel.org/show_bug.cgi?id=26982
>
>
> Mario Kleiner <mario.kleiner at tuebingen.mpg.de> changed:
>
>           What    |Removed                     |Added
>
> ----------------------------------------------------------------------------
>                 CC|                            |mario.kleiner at tuebingen.mpg
>                   |                            |.de
>
>
>
>
> --- Comment #1 from Mario Kleiner <mario.kleiner at tuebingen.mpg.de>
>  2011-01-21 19:19:45 ---
> Not sure what the reason is, but it's probably not the vblank timestamp or
> pageflipping stuff (Disclaimer: I'm the author of the vblank patches). I
> just
> used ftrace on two 32-bit kernel versions:
>
> * The 2.6.37-rc5 kernel (from drm-next) i used to develop and test the
> patches
> - includes the vblank timestamping and radeon kms pageflip patches applied,
> but
> nothing much further.
>
> * The current drm-next kernel snapshot from 10.1.2011, as packaged by
> Ubuntu
> for my Ubuntu 10.10 test system.
>
> I traced with and without pageflipped fullscreen swap, with windowed swap
> (no
> pageflipping) and with graphics idle, but vblank irq's still on.
>
> Test system was a Core2Duo 2.2 Ghz (Apple MacBookPro 2,1) with ATI Mobility
> X1600 (RV530). The kms driver takes basically the same path of execution
> for my
> gpu and yours when handling vblank's or pageflips.
>
> Basically all the work of those patches and of the pageflip patches (which
> doesn't involve execution of loops or other things with possibly unbounded
> execution time), is done in the vblank irq handler, so excessive time spent
> in
> those handlers shouldn't show up in cpu usage of your graphics app, but
> rather
> affect whatever is running on the system. The only interaction with
> userspace
> when scheduling and completing swaps is with the x-server, not direclty
> with
> the client app, so that bits should only show up in X's cpu usage.
>
> ftrace'ing the execution time of the functions in the irq handlers shows
> less
> than 0.1 msecs of time spent per page-flipped swap or vblank irq under
> graphics
> load, less than 0.05 msecs for interaction with userspace (X-server) during
> preparation of a pageflip. If you assume a 60 Hz refresh/swap this will end
> up
> using less than 1% cpu on one core for swaps, less than 0.5% when idle with
> vblanks turned on.
>
> You could play with various things to see if it changes something:
>
> 1. Running the app non-fullscreen (and without desktop compositor), so it
> doesn't use pageflipping. Or disable pageflipping in xorg.conf via driver
> option: Option "EnablePageFlip" "no", to rule out pageflip related
> problems.
>
> 2. echo 0 > /sys/modules/drm/parameters/timestamp_precision_usec
> This will disable almost all cleverness in the vblank timestamping, see if
> it
> makes a change.
>
> Or using ftrace:
>
> cd /sys/kernel/debug/tracing/
>
> echo radeon_crtc_page_flip > set_graph_function
> echo function_graph > current_tracer
> cat trace_pipe > /tmp/trace.out &
> sleep 5 ; echo 1 > tracing_enabled ; sleep 20 ; echo 0 > tracing_enabled
> more /tmp/trace.out
>
> -> Gives you execution time of the pageflip ioctl() -- what should account
> against x-server's cpu usage.
>
> echo radeon_crtc_handle_flip > set_graph_function
> -> Gives you time spent in irq handler for performing the page flip.
>
> echo drm_handle_vblank > set_graph_function
> -> Gives you time spent in vblank irq handler for vblank timestamping.
>
> This should all report fairly low numbers (not much more than maybe 100-200
> usecs, not the multi-millisecond range needed to create the cpu load you
> report).
>
> I don't observe increased cpu load for my test apps, but my x-server,
> libdrm,
> mesa is not at the latest git versions, but somewhere at the state of
> november/december last year iirc, so there could be some source of bad
> interaction with latest kernel?
>
> thanks,
> -mario
>
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