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update docs

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This commit is contained in:
Denis Vlasenko 2007-03-20 11:08:39 +00:00
parent 88a2aa98e0
commit e84aeb5bcb
1 changed files with 49 additions and 29 deletions
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docs/keep_data_small.txt
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@ -2,44 +2,48 @@
When many applets are compiled into busybox, all rw data and When many applets are compiled into busybox, all rw data and
bss for each applet are concatenated. Including those from libc, bss for each applet are concatenated. Including those from libc,
if static bbox is built. When bbox is started, _all_ this data if static busybox is built. When busybox is started, _all_ this data
is allocated, not just that one part for selected applet. is allocated, not just that one part for selected applet.
What "allocated" exactly means, depends on arch. What "allocated" exactly means, depends on arch.
On nommu it's probably bites the most, actually using real On NOMMU it's probably bites the most, actually using real
RAM for rwdata and bss. On i386, bss is lazily allocated RAM for rwdata and bss. On i386, bss is lazily allocated
by COWed zero pages. Not sure about rwdata - also COW? by COWed zero pages. Not sure about rwdata - also COW?
In order to keep bbox NOMMU and small-mem systems friendly In order to keep busybox NOMMU and small-mem systems friendly
we should avoid large global data in our applets, and should we should avoid large global data in our applets, and should
minimize usage of libc functions which implicitly use minimize usage of libc functions which implicitly use
such structures in libc. such structures.
Small experiment measures "parasitic" bbox memory consumption. Small experiment to measure "parasitic" bbox memory consumption:
Here we start 1000 "busybox sleep 10" in parallel. here we start 1000 "busybox sleep 10" in parallel.
bbox binary is practically allyesconfig static one, busybox binary is practically allyesconfig static one,
built against uclibc: built against uclibc. Run on x86-64 machine with 64-bit kernel:
bash-3.2# nmeter '%t %c %b %m %p %[pn]' bash-3.2# nmeter '%t %c %m %p %[pn]'
23:17:28 .......... 0 0 168M 0 147 23:17:28 .......... 168M 0 147
23:17:29 .......... 0 0 168M 0 147 23:17:29 .......... 168M 0 147
23:17:30 U......... 0 0 168M 1 147 23:17:30 U......... 168M 1 147
23:17:31 SU........ 0 188k 181M 244 391 23:17:31 SU........ 181M 244 391
23:17:32 SSSSUUU... 0 0 223M 757 1147 23:17:32 SSSSUUU... 223M 757 1147
23:17:33 UUU....... 0 0 223M 0 1147 23:17:33 UUU....... 223M 0 1147
23:17:34 U......... 0 0 223M 1 1147 23:17:34 U......... 223M 1 1147
23:17:35 .......... 0 0 223M 0 1147 23:17:35 .......... 223M 0 1147
23:17:36 .......... 0 0 223M 0 1147 23:17:36 .......... 223M 0 1147
23:17:37 S......... 0 0 223M 0 1147 23:17:37 S......... 223M 0 1147
23:17:38 .......... 0 0 223M 1 1147 23:17:38 .......... 223M 1 1147
23:17:39 .......... 0 0 223M 0 1147 23:17:39 .......... 223M 0 1147
23:17:40 .......... 0 0 223M 0 1147 23:17:40 .......... 223M 0 1147
23:17:41 .......... 0 0 210M 0 906 23:17:41 .......... 210M 0 906
23:17:42 .......... 0 0 168M 1 147 23:17:42 .......... 168M 1 147
23:17:43 .......... 0 0 168M 0 147 23:17:43 .......... 168M 0 147
This requires 55M of memory. Thus 1 trivial busybox applet This requires 55M of memory. Thus 1 trivial busybox applet
takes 55k of memory. takes 55k of memory on 64-bit x86 kernel.
On 32-bit kernel we need ~26k per applet.
(Data from NOMMU arches are sought. Provide 'size busybox' output too)
Example 1 Example 1
@ -104,8 +108,12 @@ its needs. Library functions are prohibited from using it.
#define G (*(struct globals*)&bb_common_bufsiz1) #define G (*(struct globals*)&bb_common_bufsiz1)
Be careful, though, and use it only if Be careful, though, and use it only if globals fit into bb_common_bufsiz1.
sizeof(struct globals) <= sizeof(bb_common_bufsiz1). Since bb_common_bufsiz1 is BUFSIZ + 1 bytes long and BUFSIZ can change
from one libc to another, you have to add compile-time check for it:
if(sizeof(struct globals) > sizeof(bb_common_bufsiz1))
BUG_<applet>_globals_too_big();
Drawbacks Drawbacks
@ -135,7 +143,7 @@ static int tabstop;
static struct termios term_orig __attribute__ ((aligned (4))); static struct termios term_orig __attribute__ ((aligned (4)));
static struct termios term_vi __attribute__ ((aligned (4))); static struct termios term_vi __attribute__ ((aligned (4)));
reduced bss size by 32 bytes, because gcc sometimes aligns structures to reduces bss size by 32 bytes, because gcc sometimes aligns structures to
ridiculously large values. asm output diff for above example: ridiculously large values. asm output diff for above example:
tabstop: tabstop:
@ -154,3 +162,15 @@ ridiculously large values. asm output diff for above example:
.size term_vi, 60 .size term_vi, 60
gcc doesn't seem to have options for altering this behaviour. gcc doesn't seem to have options for altering this behaviour.
gcc 3.4.3:
// gcc aligns to 32 bytes if sizeof(struct) >= 32
struct st {
int c_iflag,c_oflag,c_cflag,c_lflag;
int i1,i2,i3; // struct will be aligned to 4 bytes
// int i1,i2,i3,i4; // struct will be aligned to 32 bytes
};
struct st t = { 1 };
// same for arrays
char vc31[31] = { 1 }; // unaligned
char vc32[32] = { 1 }; // aligned to 32 bytes