Re: [PATCH] lz4: Fix kernel decompression speed

[Arvind Sankar]

On Mon, Aug 03, 2020 at 10:55:01PM +0000, Nick Terrell wrote:
> 
> 
> > 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.

Thanks. I tried using RDTSC to get some timings under QEMU, and I get
similar speedup as you have for LZ4, and around 15-20% or so for ZSTD
(on 64-bit) -- I see that ZSTD_copy8 is already using __builtin_memcpy,
but there must be more that can be optimized? There's a couple 1/2-byte
sized copies in huf_decompress.c.

For the rest of the algos there seems to be no difference, likely
because some casual grepping doesn't show any constant-size copies in
the other algorithms.