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Date:   Tue, 26 Sep 2023 22:49:26 +0200 (CEST)
From:   Nicolas Pitre <nico@fluxnic.net>
To:     Arnd Bergmann <arnd@arndb.de>
cc:     Zhangjin Wu <falcon@tinylab.org>, linux-kernel@vger.kernel.org,
        linux-mips@vger.kernel.org, linux-riscv@lists.infradead.org,
        Palmer Dabbelt <palmer@rivosinc.com>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        paulburton@kernel.org, "Paul E. McKenney" <paulmck@kernel.org>,
        Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
        Willy Tarreau <w@1wt.eu>,
        =?ISO-8859-15?Q?Thomas_Wei=DFschuh?= <linux@weissschuh.net>,
        Tim Bird <tim.bird@sony.com>,
        Geert Uytterhoeven <geert@linux-m68k.org>
Subject: Re: [PATCH v1 0/7] DCE/DSE: Add Dead Syscalls Elimination support,
 part1
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On Tue, 26 Sep 2023, Arnd Bergmann wrote:

> On Tue, Sep 26, 2023, at 09:14, Arnd Bergmann wrote:
> > On Tue, Sep 26, 2023, at 00:33, Zhangjin Wu wrote:
> >
> > It would be nice to include some size numbers here for at least
> > one practical use case. If you have a defconfig for a shipping
> > product with a small kernel, what is the 'size -B' output you
> > see comparing with and without DCE and, and with DCE+DSE?
> 
> To follow up on this myself, for a very rough baseline,
> I tried a riscv tinyconfig build with and without 
> CONFIG_LD_DEAD_CODE_DATA_ELIMINATION (this is currently
> not supported on arm, so I did not try it there), and
> then another build with simply *all* system calls stubbed
> out by hacking asm/syscall-wrapper.h:
> 
> $ size build/tmp/vmlinux-*
>    text	   data	    bss	     dec    hex	filename
>   754772  220016  71841	 1046629  ff865	vmlinux-tinyconfig
>   717500  223368  71841	 1012709  f73e5	vmlinux-tiny+nosyscalls
>   567310  176200  71473	  814983  c6f87	vmlinux-tiny+gc-sections
>   493278  170752  71433	  735463  b38e7	vmlinux-tiny+gc-sections+nosyscalls
> 10120058 3572756 493701	14186515 d87813	vmlinux-defconfig
>  9953934 3529004 491525	13974463 d53bbf	vmlinux-defconfig+gc
>  9709856 3500600 489221	13699677 d10a5d	vmlinux-defconfig+gc+nosyscalls
> 
> This would put us at an upper bound of 10% size savings (80kb) for
> tinyconfig, which is clearly significant. For defconfig, it's
> still 2.0% or 275kb size reduction when all syscalls are dropped.

I did something similar a while ago. Results included here:

https://lwn.net/Articles/746780/

In my case, stubbing out all syscalls produced a 7.8% reduction which 
was somewhat disappointing compared to other techniques. Of course it 
all depends on what is your actual goal.


Nicolas