linux-kernel - Re: [PATCH] lz4: Fix kernel decompression speed

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Message-ID: <20200804083236.zjkmfer37z5rn3r4@duo.ucw.cz>
Date:   Tue, 4 Aug 2020 10:32:36 +0200
From:   Pavel Machek <pavel@...x.de>
To:     Nick Terrell <terrelln@...com>
Cc:     Arvind Sankar <nivedita@...m.mit.edu>,
        Nick Terrell <nickrterrell@...il.com>,
        Ingo Molnar <mingo@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        X86 ML <x86@...nel.org>, Kernel Team <Kernel-team@...com>,
        Yann Collet <yann.collet.73@...il.com>,
        Gao Xiang <gaoxiang25@...wei.com>,
        Sven Schmidt <4sschmid@...ormatik.uni-hamburg.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] lz4: Fix kernel decompression speed

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

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