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Re: i915 performance, master, i915tex & gem

From:  Keith Packard <>
To:  Keith Whitwell <>
Subject:  Re: i915 performance, master, i915tex & gem
Date:  Mon, 19 May 2008 11:00:50 -0700
Message-ID:  <>
Cc:  Dave Airlie <>, Ian Romanick <>, DRI <>
Archive-link:  Article

On Mon, 2008-05-19 at 05:09 -0700, Keith Whitwell wrote:

> I
> think the latter is the significant result -- none of these experiments
> in memory management significantly change the command stream the
> hardware has to operate on, so what we're varying essentially is the
> CPU behaviour to acheive that command stream.  And it is in CPU usage
> where GEM (and Keith/Eric's now-abandoned TTM driver) do significantly
> dissapoint.

Your GEM results do not match mine; perhaps we're running different
kernels? Anything older than 2.6.24 won't be using clflush and will
instead use wbinvd, a significant performance impact.  Profiling would
show whether this is the case.

I did some fairly simple measurements using openarena and enemy
territory. Kernel version 2.6.25, CPU 1.3GHz Pentium M, 915GMS with the
slowest possible memory. I'm afraid I don't have a working TTM
environment at present; I will try to get that working so I can do more
complete comparisons.
				fps	real	user	kernel
glxgears classic:		665
glxgears GEM:			889
openareana classic:		17.1	 59.19	 37.13	 1.80
openarena GEM:			24.6	 44.06	 25.52	 5.29
enemy territory classic:	 9.0	382.13	226.38	11.51	
enemy territory GEM:		15.7	212.80	121.72	40.50

> Or to put it another way, GEM & master/TTM seem to burn huge
> amounts
> of CPU just running the memory manager.

I'm not seeing that in these demos; actual allocation is costing about
3% of the CPU time. Of course, for this hardware, the obvious solution
of re-using batch buffers would eliminate that cost entirely.

It would be nice to see the kernel time reduced further, but it's not
terrible so far.


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