Re: [PATCH] lz4: Fix kernel decompression speed

[Nick Terrell]

> On Aug 3, 2020, at 2:57 PM, Arvind Sankar <nivedita@xxxxxxxxxxxx> wrote:
> 
> On Mon, Aug 03, 2020 at 12:40:22PM -0700, Nick Terrell wrote:
>> From: Nick Terrell <terrelln@xxxxxx>
>> 
>> This patch replaces all memcpy() calls with LZ4_memcpy() which calls
>> __builtin_memcpy() so the compiler can inline it.
>> 
>> LZ4 relies heavily on memcpy() with a constant size being inlined. In
>> x86 and i386 pre-boot environments memcpy() cannot be inlined because
>> memcpy() doesn't get defined as __builtin_memcpy().
>> 
>> An equivalent patch has been applied upstream so that the next import
>> won't lose this change [1].
>> 
>> I've measured the kernel decompression speed using QEMU before and after
>> this patch for the x86_64 and i386 architectures. The speed-up is about
>> 10x as shown below.
>> 
>> Code	Arch	Kernel Size	Time	Speed
>> v5.8	x86_64	11504832 B	148 ms	 79 MB/s
>> patch	x86_64	11503872 B	 13 ms	885 MB/s
>> v5.8	i386	 9621216 B	 91 ms	106 MB/s
>> patch	i386	 9620224 B	 10 ms	962 MB/s
>> 
>> I also measured the time to decompress the initramfs on x86_64, i386,
>> and arm. All three show the same decompression speed before and after,
>> as expected.
>> 
>> [1] https://github.com/lz4/lz4/pull/890
>> 
> 
> Hi Nick, would you be able to test the below patch's performance to
> verify it gives the same speedup? It removes the #undef in misc.c which
> causes the decompressors to not use the builtin version. It should be
> equivalent to yours except for applying it to all the decompressors.
> 
> Thanks.

I will measure it. I would expect it to provide the same speed up. It would be great to fix
the problem for x86/i386 in general.

But, I believe that this is also a problem for ARM, though I have a hard time measuring
because I can’t get pre-boot print statements in QEMU. I will attempt to take a look at the
assembly, because I’m fairly certain that memcpy() isn’t inlined in master.

Even if we fix all the architectures, I would still like to merge the LZ4 patch. It seems like it
is pretty easy to merge a patch that is a boot speed regression, because people aren’t
actively measuring it. So I prefer a layered defense.

Additionally, this is following upstream. At some point LZ4 will be imported from upstream
and get an equivalent of this patch https://github.com/lz4/lz4/pull/890.

Best,
Nick