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

Paul Menzel
2018-04-24T09:08:42Z

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Dear Adam,

Thank you very much to join the discussion.

On 04/24/18 04:08, Adam Borowski wrote:
> On Mon, Apr 23, 2018 at 07:02:30PM +0200, Pavel Machek 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.
>
> I took a somewhat different approach: I recorded the output from grub+kernel
> to ttyrec over serial line, and rigged ttyrec2ansi to output timestamp
> difference from the last checkpoint every time an '\e' or '\n' is seen.
> '\e' is important, as there's no other marking for when grub stops the
> interactive phase and starts the actual boot.

Nice work. It’d be great, if you shared these patches, so others and I
can reproduce it.

(Also, `scripts/extract-vmlinux` needs to be updated for LZ4.)

> Turns out that, reading from SSD, grub is way way slower than it should take
> normally. The machine is old (AMD Phenom II X6 1055T), SSD is Crucial
> CT240M500SSD1.

What firmware does the device (mainboard) have? Legacy BIOS or UEFI (or
even coreboot ;-)). It’s my understanding, that GRUB does not have a
native driver with the legacy BIOS and UEFI, and just uses the BIOS
calls or the UEFI equivalent.

> I also have the zstd patch applied, which adds another data point.
>
> The two "Loading XXX ..." lines come from grub, those timestamped within []
> brackets from the kernel, 〈〉are ttyrec timestamps, ⤸ is wrapped lines.
>
>
> zstd:
>
> Loading Linux 4.17.0-rc2-debug-00025-gd426b0ba363d ...〈0.739823〉
> ^MLoading initial ramdisk ...〈0.402010〉
> ^M[ 0.000000] Linux version 4.17.0-rc2-debug-00025-gd426b0ba363d ⤸
> (kilobyte@umbar) (gcc version 7.3.0 (Debian 7.3.0-16)) #1 SMP Mon Apr 23⤸
> 10:25:58 CEST 2018^M〈0.785922〉
> [ 0.000000] Command line: BOOT_IMAGE=/sys/boot/vmlinuz-4.17.0-rc2-debug-00025-gd426b0ba363d⤸
> root=UUID=b7c38da9-ae84-4083-a1f8-6d7b4fc33961 ro rootflags=subvol=sys syscall.x32=y⤸
> console=tty0 console=ttyS0,115200n8 no_console_suspend^M〈0.020199〉
>
> gzip:
>
> Loading Linux 4.17.0-rc2-debug-gz-00025-gd426b0ba363d ...〈0.724988〉
> ^MLoading initial ramdisk ...〈0.357951〉
> ^M[ 0.000000] Linux version 4.17.0-rc2-debug-gz-00025-gd426b0ba363d ⤸
> (kilobyte@umbar) (gcc version 7.3.0 (Debian 7.3.0-16)) #1 SMP Mon Apr 23 ⤸
> 23:15:07 CEST 2018^M〈0.777977〉
> [ 0.000000] Command line: BOOT_IMAGE=/sys/boot/vmlinuz-4.17.0-rc2-debug-gz-00025-gd426b0ba363d⤸
> root=UUID=b7c38da9-ae84-4083-a1f8-6d7b4fc33961 ro rootflags=subvol=sys syscall.x32=y⤸
> console=tty0 console=ttyS0,115200n8 no_console_suspend^M〈0.020117〉
>
> lz4:
>
> Loading Linux 4.17.0-rc2-debug-none-00025-gd426b0ba363d ...〈0.799969〉
> ^MLoading initial ramdisk ...〈0.424959〉
> ^M[ 0.000000] Linux version 4.17.0-rc2-debug-lz4-00025-gd426b0ba363d ⤸
> (kilobyte@umbar) (gcc version 7.3.0 (Debian 7.3.0-16)) #1 SMP Tue Apr 24 ⤸
> 00:34:59 CEST 2018^M〈0.732925〉
> [ 0.000000] Command line: BOOT_IMAGE=/sys/boot/vmlinuz-4.17.0-rc2-debug-lz4-00025-gd426b0ba363d⤸
> root=UUID=b7c38da9-ae84-4083-a1f8-6d7b4fc33961 ro rootflags=subvol=sys syscall.x32=y⤸
> console=tty0 console=ttyS0,115200n8 no_console_suspend^M〈0.021019〉
>
> zstd again:
>
> Loading Linux 4.17.0-rc2-debug-00025-gd426b0ba363d ...〈0.728852〉
> ^MLoading initial ramdisk ...〈0.399968〉
> ^M[ 0.000000] Linux version 4.17.0-rc2-debug-00025-gd426b0ba363d ⤸
> (kilobyte@umbar) (gcc version 7.3.0 (Debian 7.3.0-16)) #1 SMP Mon Apr 23 ⤸
> 10:25:58 CEST 2018^M〈0.786964〉
> [ 0.000000] Command line: BOOT_IMAGE=/sys/boot/vmlinuz-4.17.0-rc2-debug-00025-gd426b0ba363d⤸
> root=UUID=b7c38da9-ae84-4083-a1f8-6d7b4fc33961 ro rootflags=subvol=sys syscall.x32=y⤸
> console=tty0 console=ttyS0,115200n8 no_console_suspend^M〈0.020071〉
>
> lz4 rigged for no compression:
>
> Loading Linux 4.17.0-rc2-debug-none-00025-gd426b0ba363d-dirty ...〈0.479834〉
> ^MLoading initial ramdisk ...〈2.246985〉

Just to be sure. The 2.2 seconds are from loading the 27 MB Linux kernel
image, right?

> ^M[ 0.000000] Linux version 4.17.0-rc2-debug-none-00025-gd426b0ba363d-dirty ⤸
> (kilobyte@umbar) (gcc version 7.3.0 (Debian 7.3.0-16)) #5 SMP Tue Apr 24 ⤸
> 02:57:18 CEST 2018^M〈0.711949〉
> [ 0.000000] Command line: BOOT_IMAGE=/sys/boot/vmlinuz-4.17.0-rc2-debug-none-00025-gd426b0ba363d-dirty⤸
> root=UUID=b7c38da9-ae84-4083-a1f8-6d7b4fc33961 ro rootflags=subvol=sys syscall.x32=y⤸
> console=tty0 console=ttyS0,115200n8 no_console_suspend^M〈0.021902〉
>
> Sizes of relevant files:
>
> 14826134 initrd.img-4.17.0-rc2-debug-00025-gd426b0ba363d (zstd)
> 14826352 initrd.img-4.17.0-rc2-debug-gz-00025-gd426b0ba363d
> 14826909 initrd.img-4.17.0-rc2-debug-lz4-00025-gd426b0ba363d
> 14826761 initrd.img-4.17.0-rc2-debug-none-00025-gd426b0ba363d-dirty
> 6567408 vmlinuz-4.17.0-rc2-debug-00025-gd426b0ba363d (zstd)
> 7230960 vmlinuz-4.17.0-rc2-debug-gz-00025-gd426b0ba363d
> 8775152 vmlinuz-4.17.0-rc2-debug-lz4-00025-gd426b0ba363d
> 27821552 vmlinuz-4.17.0-rc2-debug-none-00025-gd426b0ba363d-dirty
> (I did not alter initrd compression, which is zstd in all cases).

Does the size of the uncompressed image match the size in
`arch/x86/boot/compressed/vmlinux.bin`?

>>> So yes, looks like uncompressed kernel image may be good idea.
>
> Seems like the time to actually read this far bigger file from the disk
> using grub's inefficient way, takes longer than the gains from faster
> decompression. You can eliminate the decompression step altogether by
> avoiding copying, but it still looks like it's not a win.
>
> I've seen u-boot taking ~60 seconds to read from a SD card, too.

I could test on my coreboot systems, if GRUB is faster with the native
AHCI driver.

> Another surprise is that zstd is a notch _slower_ than gzip (in userspace
> it's drastically faster for the same compression ratio), but reduced disk
> space is still nice. It's worth investigating why it's not as fast as it
> should be.

Maybe that should be done in a separate thread. I’ll split it out.

>> 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.
>
> I can't seem to find any. IIRC xz format can store uncompressed blocks but
> the tool doesn't appear to expose this as an option.
>
>> 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?
>
> Perhaps my method is totally wrong, but differences in decompression speed
> are surprisingly small, dwarfed by whatever else the kernel does between
> messages.
>
> I did not test xz, nor ran tests more than once, but it's 4am so these are
> things to do tomorrow.

It’d be interesting to find out, what is happening in the first 700 ms,
before the first Linux message is transmitted over serial line. It could
be, that the serial line affects the time stamps for example, or takes
so long to set up the serial console.

Kind regards,

Paul

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