[PATCH 2/2] fbdev: Don't sort deferred-I/O pages by default

[Thomas Zimmermann]

Hi

Am 14.02.22 um 09:05 schrieb Geert Uytterhoeven:
> Hi Thomas,
> 
> On Thu, Feb 10, 2022 at 4:24 PM Thomas Zimmermann <tzimmermann at suse.de> wrote:
>> Fbdev's deferred I/O sorts all dirty pages by default, which incurs a
>> significant overhead. Make the sorting step optional and update the few
>> drivers that require it. Use a FIFO list by default.
>>
>> Sorting pages by memory offset for deferred I/O performs an implicit
>> bubble-sort step on the list of dirty pages. The algorithm goes through
>> the list of dirty pages and inserts each new page according to its
>> index field. Even worse, list traversal always starts at the first
>> entry. As video memory is most likely updated scanline by scanline, the
>> algorithm traverses through the complete list for each updated page.
>>
>> For example, with 1024x768x32bpp a page covers exactly one scanline.
>> Writing a single screen update from top to bottom requires updating
>> 768 pages. With an average list length of 384 entries, a screen update
>> creates (768 * 384 =) 294912 compare operation.
> 
> What about using folios?
> If consecutive pages are merged into a single entry, there's much less
> (or nothing in the example above) to sort.

How would the code know that? Calls to page_mkwrite happen 
pagefault-by-pagefault in any order AFAICT.

Best regards
Thomas

[1] 
https://elixir.bootlin.com/linux/v5.16.9/source/include/linux/mm_types.h#L258


> 
> Gr{oetje,eeting}s,
> 
>                          Geert
> 
> --
> Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert at linux-m68k.org
> 
> In personal conversations with technical people, I call myself a hacker. But
> when I'm talking to journalists I just say "programmer" or something like that.
>                                  -- Linus Torvalds

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Ivo Totev
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