Re: [RFC PATCH v3 1/5] grub-mkconfig linux: Fix quadratic algorithm for sorting menu items

[Daniel Kiper]

On Fri, May 20, 2022 at 10:37:37AM -0400, Mathieu Desnoyers wrote:
> The current implementation of the 10_linux script implements its menu
> items sorting in bash with a quadratic algorithm, calling "sed", "sort",
> "head", and "grep" to compare versions between individual lines, which
> is annoyingly slow for kernel developers who can easily end up with
> 50-100 kernels in /boot.
>
> As an example, on a Intel(R) Core(TM) i7-8650U CPU @ 1.90GHz, running:
>
>   /usr/sbin/grub-mkconfig > /dev/null
>
> With 44 kernels in /boot, this command takes 10-15 seconds to complete.
> After this fix, the same command runs in 5 seconds.
>
> With 116 kernels in /boot, this command takes 40 seconds to complete.
> After this fix, the same command runs in 8 seconds.
>
> For reference, the quadratic algorithm here is:
>
> while [ "x$list" != "x" ] ; do      <--- outer loop
>   linux=`version_find_latest $list`
>     version_find_latest()
>       for i in "$@" ; do            <--- inner loop
>         version_test_gt()
>           fork+exec sed
>             version_test_numeric()
>               version_sort
>                 fork+exec sort
>               fork+exec head -n 1
>               fork+exec grep
>   list=`echo $list | tr ' ' '\n' | fgrep -vx "$linux" | tr '\n' ' '`
>     tr
>     fgrep
>     tr
>
> So all commands executed under version_test_gt() are executed
> O(n^2) times where n is the number of kernel images in /boot.
>
> Here is the improved algorithm proposed:
>
> - Prepare a list with all the relevant information for ordering by a single
>   sort(1) execution. This is done by renaming ".old" suffixes by " 1" and
>   by suffixing all other files with " 2", thus making sure the ".old" entries
>   will follow the non-old entries in reverse-sorted-order.
> - Call version_reverse_sort on the list (sort -r -V): A single execution of
>   sort(1). For instance, GNU coreutils' sort will reverse-sort the list in
>   O(n*log(n)) with a merge sort.
> - Replace the " 1" suffixes by ".old", and remove the " 2" suffixes.
> - Iterate on the reverse-sorted list to output each menu entry item.
>
> Therefore, the algorithm proposed has O(n*log(n)) complexity with GNU
> coreutils' sort compared to the prior O(n^2) complexity. Moreover, the
> constant time required for each list entry is much less because sorting
> is done within a single execution of sort(1) rather than requiring
> O(n^2) executions of sed(1), sort(1), head(1), and grep(1) in
> sub-shells.
>
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> ---
> Changes since v1:
> - Escape the dot from .old in the sed match pattern, thus ensuring it
>   matches ".old" rather than "[any character]old".
> - Use "sed" rather than "sed -e" everywhere for consistency.
> - Document the new algorithm in the commit message.
>
> Changes since v2:
> - Rename version_reverse_sort_sort_has_v to version_sort_sort_has_v,
> - Combine multiple sed executions into a single sed -e ... -e ...
>
> Changes since v3:
> - Modify version_sort to expect arguments, and call "version_sort -r",
>   rather than copying it as a "version_reverse_sort".
> - Specify that O(n*log(n)) merge sort is specific to GNU coreutils' sort.
> ---
>  util/grub-mkconfig_lib.in |  8 ++++----
>  util/grub.d/10_linux.in   | 12 ++++++++----
>  2 files changed, 12 insertions(+), 8 deletions(-)
>
> diff --git a/util/grub-mkconfig_lib.in b/util/grub-mkconfig_lib.in
> index 301d1ac22..fc14afdb3 100644
> --- a/util/grub-mkconfig_lib.in
> +++ b/util/grub-mkconfig_lib.in
> @@ -204,16 +204,16 @@ version_sort ()
>  {
>    case $version_sort_sort_has_v in
>      yes)
> -      LC_ALL=C sort -V;;
> +      LC_ALL=C sort -V $@;;
>      no)
> -      LC_ALL=C sort -n;;
> +      LC_ALL=C sort -n $@;;
>      *)
>        if sort -V </dev/null > /dev/null 2>&1; then
>          version_sort_sort_has_v=yes
> -     LC_ALL=C sort -V
> +     LC_ALL=C sort -V $@
>        else
>          version_sort_sort_has_v=no
> -        LC_ALL=C sort -n
> +        LC_ALL=C sort -n $@
>        fi;;
>     esac
>  }
> diff --git a/util/grub.d/10_linux.in b/util/grub.d/10_linux.in
> index ca068038e..001a97ce3 100644
> --- a/util/grub.d/10_linux.in
> +++ b/util/grub.d/10_linux.in
> @@ -195,9 +195,15 @@ title_correction_code=
>  # yet, so it's empty. In a submenu it will be equal to '\t' (one tab).
>  submenu_indentation=""
>
> +# Perform a reverse version sort on the entire list.
> +# Temporarily replace the '.old' suffix by ' 1' and append ' 2' for all
> +# other files to order the '.old' files after their non-old counterpart
> +# in reverse-sorted order.
> +
> +reverse_sorted_list=$(echo $list | tr ' ' '\n' | sed -e 's/\.old$/ 1/' -e '/ 
> 1$/! s/$/ 2/' | version_sort -r | sed -e 's/ 1$/.old/' -e 's/ 2$//')

Nit, I think you can use one "-e" argument for sed, e.g. sed -e 's/\.old$/ 1/; 
/ 1$/! s/$/ 2/'.

Otherwise patches LGTM.

Please hold on with rebase. I am going to push one more patch before
your patch series which may potentially conflict with your changes.
I will drop you a line when you can do it.

Daniel