Re: How to disable Linux kernel self-extraction (KERNEL_GZIP, KERNEL_BZIP2, …)?

From: Paul Menzel <pmenzel+linux-kernel@molgen.mpg.de>
To: Pavel Machek <pavel@ucw.cz>
Cc: linux-kernel@vger.kernel.org, x86@kernel.org,
	Ingo Molnar <mingo@kernel.org>
Subject: Re: How to disable Linux kernel self-extraction (KERNEL_GZIP, KERNEL_BZIP2, …)?
Date: Sun, 22 Apr 2018 21:55:51 +0200	[thread overview]
Message-ID: <b22efb58-b63e-7130-f2d7-a9d28225ef9b@molgen.mpg.de> (raw)
In-Reply-To: <20180422102033.GB26243@amd>

Dear Pavel,

Am 22.04.2018 um 12:20 schrieb Pavel Machek:
> On Fri 2018-04-20 16:36:00, Paul Menzel wrote:

>> 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
```

As another data point, my self-built (*bigger*) image with XZ used for 
compression.

```
$ ls -lh /boot/vmlinuz-4.16.0+
-rw-r--r-- 1 root root 6,1M Apr 14 10:48 /boot/vmlinuz-4.16.0+
$ time /usr/src/linux-headers-4.17.0-rc1+/scripts/extract-vmlinux 
vmlinuz-4.16.0+ > bla.txt

real    0m2,190s
user    0m1,792s
sys     0m0,500s
```

So, it’s really noticeable if the rest of the system starts in less then 
five seconds to a login prompt, or you are trying to achieve to display 
the LUKS passphrase dialog more or less instantly.

Kind regards,

Paul