Re: Fw: framebuffer blitting performance loss 2.6.12 -> 2.6.13-rc3

[Luca <kronos@people.it>]

Il Fri, Jul 29, 2005 at 08:51:34PM +0100, James Simmons ha scritto: 
> > Thank you for your persistence.  I think I know the culprit.  Someone
> > insisted on using memcpy in fb_pad_aligned_buffer().  I have already
> > fixed this before, but apparently, the memcpy was brought back.  Try
> > the attached patch and let me know.
> 
> Yipes, I did that. The memcpy function is suppose to be optimized for the 
> platform. See string.h in the include/asm directory. I seen for example 
> the Athlon would use the 3DNow instruction set to copy data. Something 
> is really wrong with memcpy if moving byte by byte is faster !!!! 

For small copies MMX/3DNow are not used at all. In current kernel
MMX/3DNow memcpy is used only when data size is greater than 512bytes.
Remember that MMX/3DNow uses FPU so the kernel must save/restore state
and this overhead would make the copy slow for small chunks.

Luca
-- 
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d'acqua.
Proverbio yddish


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