Re: [PATCH RFC] x86: Add CONFIG_KERNEL_UNCOMPRESSED support

[Tor Vic <torvic9@mailbox.org>]

On 1/22/25 14:54, Jann Horn wrote:
> On Wed, Jan 22, 2025 at 2:31 PM Ard Biesheuvel <ardb@kernel.org> wrote:
>> Hi Jann,
>>
>> On Tue, 21 Jan 2025 at 23:16, Jann Horn <jannh@google.com> wrote:
>>>
>>> Support storing the kernel uncompressed for developers who want to quickly
>>> iterate with one-off kernel builds.
>>> Store it in the usual format with a 4-byte length suffix and keep this new
>>> codepath as close as possible to the normal path where decompression
>>> happens.
>>>
>>> The other compression methods offered by the kernel take some time;
>>> even LZ4 (which the kernel uses at compression level 9) takes ~2.8
>>> seconds to compress a 110M large vmlinux.bin on my machine.
>>>
>>> An alternate approach to this would be to offer customization of the LZ4
>>> compression level through a kconfig variable; and yet another approach
>>> would be to abuse the existing gzip decompression logic by storing the
>>> kernel as "non-compressed" DEFLATE blocks, so that the decompression code
>>> will essentially end up just doing a bunch of memcpy() calls.
>>>
>>
>> This all seems pretty complicated, and adding yet another
>> (pseudo-)compression method is not great in terms of maintenance
>> burden, especially because there are other consumers of the compressed
>> images (both for bzImage and EFI zboot)
>>
>> Did you try running gzip with -1 instead of -9? On my build machine,
>> this reduces the compression time of a defconfig bzImage build from
>> 4.3 seconds to 0.9 seconds.
> 
> I tried lz4 with -1; that is very fast (240ms wall clock time on my
> machine, and just 120ms user time):
> 
> $ ls -lh arch/x86/boot/compressed/vmlinux.bin
> -rwxr-x--- 1 [...] 110M Jan 22 00:01 arch/x86/boot/compressed/vmlinux.bin
> $ cat arch/x86/boot/compressed/vmlinux.bin | time lz4 -l -9 - - | wc -c
> 2.86user 0.04system 0:02.96elapsed 97%CPU (0avgtext+0avgdata 15756maxresident)k
> 0inputs+0outputs (0major+220minor)pagefaults 0swaps
> 46309676
> $ cat arch/x86/boot/compressed/vmlinux.bin | time lz4 -l -1 - - | wc -c
> 0.12user 0.06system 0:00.24elapsed 75%CPU (0avgtext+0avgdata 15524maxresident)k
> 0inputs+0outputs (0major+94minor)pagefaults 0swaps
> 56029608
> 
> But I wasn't sure how to wire that up in a nice way. I guess the
> nicest option would be to create a separate kconfig variable for the
> compression level to use for any cmd_lz4/cmd_lz4_with_size invocations
> in the build process; and then maybe only make this option visible if
> LZ4 is selected as kernel compression method?
> 
> Another option would be to create a new option in the "Kernel
> compression mode" choice menu with a name like "LZ4 (fast)", turn
> CONFIG_KERNEL_LZ4 into an internal flag that is selected by both LZ4
> variants shown in the choice menu, and duplicate some of the make
> rules, but that seems overly complicated.
> 

Hello,

In my opinion 'lz4 -9' doesn't make much sense.
It's terribly slow and the compression ratio is also not exactly good.

Instead, zstd seems to be a much better choice. Not quite as ultra fast 
as lz4 levels 1 to 3, but much better compression.

As an example, I compressed a kernel source tarball (zstd is 
multithreaded with 4 threads here, which are not fully used with 
small-ish files like vmlinux):

   - zstd -3: from 1.32 GB to 199 MB in 5.23 seconds
   - zstd -6:              to 173 MB in 10.77 seconds
   - zstd -10:             to 165 MB in 24.52 seconds
   - lz4 -1:               to 373 MB in 1.60 seconds
   - lz4 -3:               to 278 MB in 6.45 seconds
   - lz4 -9:               to 266 MB in 22.03 seconds

And a vmlinux from my installed kernel 6.13:

   - zstd -3: from 51.8 MB to 16.7 MB in 0.60 seconds
   - zstd -6:              to 15.8 MB in 1.39 seconds
   - zstd -10:             to 15.3 MB in 3.54 seconds
   - lz4 -1:               to 23.7 MB in 0.08 seconds
   - lz4 -3:               to 20.2 MB in 0.51 seconds
   - lz4 -9:               to 19.7 MB in 1.23 seconds

For my kernel, I use a Kconfig option to choose a zstd compression 
level. I could submit it if there is interest.

Tor