On Mon, 4 Apr 2022 11:34:27 +0200 David Disseldorp <ddiss@xxxxxxx> wrote:
> initramfs cpio mtime preservation, as implemented in commit 889d51a10712
> ("initramfs: add option to preserve mtime from initramfs cpio images"),
> uses a linked list to defer directory mtime processing until after all
> other items in the cpio archive have been processed. This is done to
> ensure that parent directory mtimes aren't overwritten via subsequent
> child creation.
>
> The lkml link below indicates that the mtime retention use case was for
> embedded devices with applications running exclusively out of initramfs,
> where the 32-bit mtime value provided a rough file version identifier.
> Linux distributions which discard an extracted initramfs immediately
> after the root filesystem has been mounted may want to avoid the
> unnecessary overhead.
>
> This change adds a new INITRAMFS_PRESERVE_MTIME Kconfig option, which
> can be used to disable on-by-default mtime retention and in turn
> speed up initramfs extraction, particularly for cpio archives with large
> directory counts.
>
> Benchmarks with a one million directory cpio archive extracted 20 times
> demonstrated:
> mean extraction time (s) std dev
> INITRAMFS_PRESERVE_MTIME=y 3.808 0.006
> INITRAMFS_PRESERVE_MTIME unset 3.056 0.004
So about 35 nsec per directory?
By how much is this likely to reduce boot time in a real-world situation?
