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

["Antonino A. Daplas" <adaplas@gmail.com>]

Jon Smirl wrote:
> On 7/29/05, James Simmons <jsimmons@infradead.org> wrote:
>>> 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 !!!!
>> Alot of drivers use memcpy. If memcpy sucks then drivers should be copying
>> byte by byte then. The question I have is this the case for non intel
>> platforms as well. Could someone run the numbers on other platforms?
> 
> memmove/memcpy is faster. memcpy is faster than memmove so use it if
> you can. But, there is a lower limit probably around 16 bytes or so
> where the loop becomes faster.  So if you know that you will always be
> copying small fragments use the loop.  The compiler can't decide

Yes, the loop copies each row of a font character.  For an 8x16 font
that's 1 byte. The maximum fontwidth is 32. A 12x22 font does not pass
through this function because the width is not a multiple of 8.  So,
currently, it's used mostly for 8x16 fonts. 

I already know people using 16x30 fonts. There are probably others bigger
than that. 

Of course, we can always use Duff's version to loop-unroll that particular
section, but even at 4 bytes, I don't know if it's worth the effort. Anyone
knows people using 32 wide fonts?

Tony



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