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

[Robbie Harwood <rharwood@redhat.com>]

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Mathieu Desnoyers <mathieu.desnoyers@efficios.com> writes:

> 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) 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 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.
>
> I notice that the same quadratic sorting is done for other supported
> OSes, so I suspect similar gains can be obtained there, but I limit the
> scope of this patch to Linux because this is the platform on which I can
> test.
>
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>

Reviewed-by: Robbie Harwood <rharwood@redhat.com>

Be well,
--Robbie

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