Re: [PATCH] lz4: Fix kernel decompression speed

From: Arvind Sankar
Date: Tue Aug 04 2020 - 11:17:19 EST


On Tue, Aug 04, 2020 at 10:32:36AM +0200, Pavel Machek wrote:
> Hi!
>
> > >> 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.
>
>
> Layered defense against performance-only problem, happening on
> emulation-only?
>
> IMO that's a bit of overkill.
>
> Best regards,
> Pavel
> --
> (english) http://www.livejournal.com/~pavelmachek
> (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

Why would it be emulation-only? QEMU is just being used for ease of
testing, but the performance impact should be similar on bare metal.

Thanks.