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Date: Sat, 24 Oct 2015 00:00:56 +0200
From: Denys Vlasenko <vda.linux@...glemail.com>
To: Rich Felker <dalias@...c.org>
Cc: musl <musl@...ts.openwall.com>
Subject: Re: [PATCH] configure: add gcc flags for better link-time optimization
On Fri, Oct 23, 2015 at 4:48 PM, Rich Felker <dalias@...c.org> wrote:
>> Minimizing the number of data pages is more important
>> than text pages. A text page is shared among all processes linked
>> to this libc.so; data page is allocated in every process
>> (as soon as even one byte in this page is written to.
>> With only 4 pages in total like in this example, I'm pretty sure
>> all of them get dirtied by libc init, use of stdio or malloc).
>>
>> Make libc (.data + .bss) fit into one page less and you get about
>> as many pages saved as you have processes running.
>
> FYI all the data/bss in libc except a few large objects _easily_ fits
> in a single page.
What's importand is how many pages are dirtied.
Here's a test with Aboriginal's x86_64 static busybox:
# sleep 9999 | sh -c 'echo $$; exec ./busybox dd bs=1'
16290
# pmap 16290
16290: ./busybox dd bs 1
0000000000400000 320K r-x--
/home/srcdevel/aboriginal/a.0/build/root-filesystem-x86_64/usr/bin/busybox
^^^^ text + rodata
000000000064f000 4K rw---
/home/srcdevel/aboriginal/a.0/build/root-filesystem-x86_64/usr/bin/busybox
^^^^ data + start of bss
0000000000650000 8K rw--- [ anon ]
^^^^ the rest of bss
0000000001655000 4K rw--- [ anon ]
^^^^ brk
00007fff57196000 132K rw--- [ stack ]
00007fff571df000 8K r---- [ anon ]
00007fff571e1000 8K r-x-- [ anon ]
ffffffffff600000 4K r-x-- [ anon ]
total 488K
Thus, for this binary, three RW pages mapped immediately for .data and .bss,
for any applet.
Are all these pages touched?
# cat /proc/16290/smaps
00400000-00450000 r-xp 00000000 08:02 1810890
/home/srcdevel/aboriginal/a.0/build/root-filesystem-x86_64/usr/bin/busybox
Size: 320 kB
...
0064f000-00650000 rw-p 0004f000 08:02 1810890
/home/srcdevel/aboriginal/a.0/build/root-filesystem-x86_64/usr/bin/busybox
Size: 4 kB
Rss: 4 kB
Pss: 4 kB
Shared_Clean: 0 kB
Shared_Dirty: 0 kB
Private_Clean: 0 kB
Private_Dirty: 4 kB
Referenced: 4 kB
...
00650000-00652000 rw-p 00000000 00:00 0
Size: 8 kB
Rss: 4 kB
Pss: 4 kB
Shared_Clean: 0 kB
Shared_Dirty: 0 kB
Private_Clean: 0 kB
Private_Dirty: 4 kB
Referenced: 4 kB
Anonymous: 4 kB
No. Only two pages are mapped, not three.
This is pretty impressive. However, this is a small busybox config:
only 31 applet.
I have a complete (~320 applets) 32-bit static busybox config built
against uclibc, and it has only 2 pages .data+.bss
Will test & see how close to that can musl get.
I'll continue sending patches which allow to carry over some
data size reductions from uclibc to musl.
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