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

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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@xxxxxxx> 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@xxxxxxxxxxxxxx

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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