On Mon, Aug 03, 2020 at 10:55:01PM +0000, Nick Terrell wrote: > > > > On Aug 3, 2020, at 2:57 PM, Arvind Sankar <nived...@alum.mit.edu> wrote: > > > > On Mon, Aug 03, 2020 at 12:40:22PM -0700, Nick Terrell wrote: > >> From: Nick Terrell <terre...@fb.com> > >> > >> 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.
