Re: [PATCH v7 3/6] initramfs: add INITRAMFS_PRESERVE_MTIME Kconfig option

[Andrew Morton <akpm@linux-foundation.org>]

On Mon,  4 Apr 2022 11:34:27 +0200 David Disseldorp <ddiss@suse.de> 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?