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

[Geert Uytterhoeven]

Hi Thomas,

On Mon, Feb 14, 2022 at 9:28 AM Thomas Zimmermann <tzimmermann at suse.de> wrote:
> Am 14.02.22 um 09:05 schrieb Geert Uytterhoeven:
> > 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.

fb_deferred_io_mkwrite() would still be called for a page, but an
adjacent page can be merged with an existing entry while adding it
to the list.

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