linux-kernel - Re: How to disable Linux kernel self-extraction (KERNEL_GZIP, KERNEL

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Date:   Mon, 23 Apr 2018 19:02:30 +0200
From:   Pavel Machek <pavel@....cz>
To:     Paul Menzel <pmenzel+linux-kernel@...gen.mpg.de>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: How to disable Linux kernel self-extraction (KERNEL_GZIP, KERNEL_BZIP2, …)?

Hi!

> > >>I try to decrease boot time, and my system has an SSD and enough space, so
> > >>loading 18 instead of 12 MB doesn’t make a difference, but the
> > >>self-extraction is noticeable. So, I like to disable it.
> > >
> > >How long does GZIP extraction take on your hardware?
> > 
> > It’s hard to measure – at least I didn’t find a way to do so –, but counting
> > from the last GRUB message to the first message of Linux (with `quiet`
> > removed from the the command line), it takes roughly *two* seconds.
> > 
> > ```
> > $ ls -l /boot/vmlinuz-4.15.0-3-686-pae
> > -rw-r--r-- 1 root root  3987200 Apr 19 12:13 /boot/vmlinuz-4.15.0-3-686-pae
> > $ ls -lh /boot/vmlinuz-4.15.0-3-686-pae
> > -rw-r--r-- 1 root root 3,9M Apr 19 12:13 /boot/vmlinuz-4.15.0-3-686-pae
> > $ time scripts/extract-vmlinux vmlinuz-4.15.0-3-686-pae > bla.txt
> > 
> > real    0m1,204s
> > user    0m1,041s
> > sys     0m0,245s
> > ```
> 
> Interesting; looks like I have faster machine (thinkpad X220).
> 
> pavel@duo:/data/l/linux$ time scripts/extract-vmlinux /tmp/vmlinux.bin
> > /tmp/delme
> 0.21user 0.15system 0.66 (0m0.660s) elapsed 56.36%CPU
> pavel@duo:/data/l/linux$ ls -al /tmp/delme
> -rw-r--r-- 1 pavel pavel 18275356 Apr 22 22:44 /tmp/delme
> pavel@duo:/data/l/linux$ gzip /tmp/delme
> pavel@duo:/data/l/linux$ time gzip -d /tmp/delme.gz
> 0.21user 0.02system 0.23 (0m0.232s) elapsed 100.03%CPU
> pavel@duo:/data/l/linux$ gzip -1 /tmp/delme
> pavel@duo:/data/l/linux$ time gzip -d /tmp/delme.gz
> 0.22user 0.03system 0.25 (0m0.257s) elapsed 100.11%CPU
> pavel@duo:/data/l/linux$ time gzip -d /tmp/delme.gz
> 
> ..and it also looks like extract-kernel is significantly slower than
> gzip -9, because it is not using all the CPU. Strange.
> 
> Interesting. I somehow assumed gzip -d would be faster than that on
> modern machines.
> 
> So yes, looks like uncompressed kernel image may be good idea.

Actually... Compressors usually have a mode when they store the data
uncompressed. So you should be able to prepare .gz image which is not
really compressed inside, and thus really fast to uncompress.

Or maybe even better -- there should be some compression algorithms
that are fast enough to uncompress that there should be no
slowdown. Maybe use one of those?
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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