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From:   Arvind Sankar <nivedita@alum.mit.edu>
Date:   Tue, 4 Aug 2020 11:16:54 -0400
To:     Pavel Machek <pavel@denx.de>
Cc:     Nick Terrell <terrelln@fb.com>,
        Arvind Sankar <nivedita@alum.mit.edu>,
        Nick Terrell <nickrterrell@gmail.com>,
        Ingo Molnar <mingo@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        X86 ML <x86@kernel.org>, Kernel Team <Kernel-team@fb.com>,
        Yann Collet <yann.collet.73@gmail.com>,
        Gao Xiang <gaoxiang25@huawei.com>,
        Sven Schmidt <4sschmid@informatik.uni-hamburg.de>,
        Andrew Morton <akpm@linux-foundation.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH] lz4: Fix kernel decompression speed
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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.