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| author | Denis Vlasenko <vda.linux@googlemail.com> | 2007-03-20 11:08:39 +0000 |
|---|---|---|
| committer | Denis Vlasenko <vda.linux@googlemail.com> | 2007-03-20 11:08:39 +0000 |
| commit | e84aeb5bcb1a8398fce53aad9c0072ad73a8b5c9 (patch) | |
| tree | 7681cc079c04adbd75e40183f3b65059413634a9 /docs/keep_data_small.txt | |
| parent | 88a2aa98e06cf6b0aa00a3cf88376a1d3497fb2c (diff) | |
| download | busybox-w32-e84aeb5bcb1a8398fce53aad9c0072ad73a8b5c9.tar.gz busybox-w32-e84aeb5bcb1a8398fce53aad9c0072ad73a8b5c9.tar.bz2 busybox-w32-e84aeb5bcb1a8398fce53aad9c0072ad73a8b5c9.zip | |
update docs
Diffstat (limited to 'docs/keep_data_small.txt')
| -rw-r--r-- | docs/keep_data_small.txt | 82 |
1 files changed, 51 insertions, 31 deletions
diff --git a/docs/keep_data_small.txt b/docs/keep_data_small.txt index fcd8df4a9..72de2d1ef 100644 --- a/docs/keep_data_small.txt +++ b/docs/keep_data_small.txt | |||
| @@ -2,44 +2,48 @@ | |||
| 2 | 2 | ||
| 3 | When many applets are compiled into busybox, all rw data and | 3 | When many applets are compiled into busybox, all rw data and |
| 4 | bss for each applet are concatenated. Including those from libc, | 4 | bss for each applet are concatenated. Including those from libc, |
| 5 | if static bbox is built. When bbox is started, _all_ this data | 5 | if static busybox is built. When busybox is started, _all_ this data |
| 6 | is allocated, not just that one part for selected applet. | 6 | is allocated, not just that one part for selected applet. |
| 7 | 7 | ||
| 8 | What "allocated" exactly means, depends on arch. | 8 | What "allocated" exactly means, depends on arch. |
| 9 | On nommu it's probably bites the most, actually using real | 9 | On NOMMU it's probably bites the most, actually using real |
| 10 | RAM for rwdata and bss. On i386, bss is lazily allocated | 10 | RAM for rwdata and bss. On i386, bss is lazily allocated |
| 11 | by COWed zero pages. Not sure about rwdata - also COW? | 11 | by COWed zero pages. Not sure about rwdata - also COW? |
| 12 | 12 | ||
| 13 | In order to keep bbox NOMMU and small-mem systems friendly | 13 | In order to keep busybox NOMMU and small-mem systems friendly |
| 14 | we should avoid large global data in our applets, and should | 14 | we should avoid large global data in our applets, and should |
| 15 | minimize usage of libc functions which implicitly use | 15 | minimize usage of libc functions which implicitly use |
| 16 | such structures in libc. | 16 | such structures. |
| 17 | 17 | ||
| 18 | Small experiment measures "parasitic" bbox memory consumption. | 18 | Small experiment to measure "parasitic" bbox memory consumption: |
| 19 | Here we start 1000 "busybox sleep 10" in parallel. | 19 | here we start 1000 "busybox sleep 10" in parallel. |
| 20 | bbox binary is practically allyesconfig static one, | 20 | busybox binary is practically allyesconfig static one, |
| 21 | built against uclibc: | 21 | built against uclibc. Run on x86-64 machine with 64-bit kernel: |
| 22 | 22 | ||
| 23 | bash-3.2# nmeter '%t %c %b %m %p %[pn]' | 23 | bash-3.2# nmeter '%t %c %m %p %[pn]' |
| 24 | 23:17:28 .......... 0 0 168M 0 147 | 24 | 23:17:28 .......... 168M 0 147 |
| 25 | 23:17:29 .......... 0 0 168M 0 147 | 25 | 23:17:29 .......... 168M 0 147 |
| 26 | 23:17:30 U......... 0 0 168M 1 147 | 26 | 23:17:30 U......... 168M 1 147 |
| 27 | 23:17:31 SU........ 0 188k 181M 244 391 | 27 | 23:17:31 SU........ 181M 244 391 |
| 28 | 23:17:32 SSSSUUU... 0 0 223M 757 1147 | 28 | 23:17:32 SSSSUUU... 223M 757 1147 |
| 29 | 23:17:33 UUU....... 0 0 223M 0 1147 | 29 | 23:17:33 UUU....... 223M 0 1147 |
| 30 | 23:17:34 U......... 0 0 223M 1 1147 | 30 | 23:17:34 U......... 223M 1 1147 |
| 31 | 23:17:35 .......... 0 0 223M 0 1147 | 31 | 23:17:35 .......... 223M 0 1147 |
| 32 | 23:17:36 .......... 0 0 223M 0 1147 | 32 | 23:17:36 .......... 223M 0 1147 |
| 33 | 23:17:37 S......... 0 0 223M 0 1147 | 33 | 23:17:37 S......... 223M 0 1147 |
| 34 | 23:17:38 .......... 0 0 223M 1 1147 | 34 | 23:17:38 .......... 223M 1 1147 |
| 35 | 23:17:39 .......... 0 0 223M 0 1147 | 35 | 23:17:39 .......... 223M 0 1147 |
| 36 | 23:17:40 .......... 0 0 223M 0 1147 | 36 | 23:17:40 .......... 223M 0 1147 |
| 37 | 23:17:41 .......... 0 0 210M 0 906 | 37 | 23:17:41 .......... 210M 0 906 |
| 38 | 23:17:42 .......... 0 0 168M 1 147 | 38 | 23:17:42 .......... 168M 1 147 |
| 39 | 23:17:43 .......... 0 0 168M 0 147 | 39 | 23:17:43 .......... 168M 0 147 |
| 40 | 40 | ||
| 41 | This requires 55M of memory. Thus 1 trivial busybox applet | 41 | This requires 55M of memory. Thus 1 trivial busybox applet |
| 42 | takes 55k of memory. | 42 | takes 55k of memory on 64-bit x86 kernel. |
| 43 | |||
| 44 | On 32-bit kernel we need ~26k per applet. | ||
| 45 | |||
| 46 | (Data from NOMMU arches are sought. Provide 'size busybox' output too) | ||
| 43 | 47 | ||
| 44 | 48 | ||
| 45 | Example 1 | 49 | Example 1 |
| @@ -104,8 +108,12 @@ its needs. Library functions are prohibited from using it. | |||
| 104 | 108 | ||
| 105 | #define G (*(struct globals*)&bb_common_bufsiz1) | 109 | #define G (*(struct globals*)&bb_common_bufsiz1) |
| 106 | 110 | ||
| 107 | Be careful, though, and use it only if | 111 | Be careful, though, and use it only if globals fit into bb_common_bufsiz1. |
| 108 | sizeof(struct globals) <= sizeof(bb_common_bufsiz1). | 112 | Since bb_common_bufsiz1 is BUFSIZ + 1 bytes long and BUFSIZ can change |
| 113 | from one libc to another, you have to add compile-time check for it: | ||
| 114 | |||
| 115 | if(sizeof(struct globals) > sizeof(bb_common_bufsiz1)) | ||
| 116 | BUG_<applet>_globals_too_big(); | ||
| 109 | 117 | ||
| 110 | 118 | ||
| 111 | Drawbacks | 119 | Drawbacks |
| @@ -135,7 +143,7 @@ static int tabstop; | |||
| 135 | static struct termios term_orig __attribute__ ((aligned (4))); | 143 | static struct termios term_orig __attribute__ ((aligned (4))); |
| 136 | static struct termios term_vi __attribute__ ((aligned (4))); | 144 | static struct termios term_vi __attribute__ ((aligned (4))); |
| 137 | 145 | ||
| 138 | reduced bss size by 32 bytes, because gcc sometimes aligns structures to | 146 | reduces bss size by 32 bytes, because gcc sometimes aligns structures to |
| 139 | ridiculously large values. asm output diff for above example: | 147 | ridiculously large values. asm output diff for above example: |
| 140 | 148 | ||
| 141 | tabstop: | 149 | tabstop: |
| @@ -154,3 +162,15 @@ ridiculously large values. asm output diff for above example: | |||
| 154 | .size term_vi, 60 | 162 | .size term_vi, 60 |
| 155 | 163 | ||
| 156 | gcc doesn't seem to have options for altering this behaviour. | 164 | gcc doesn't seem to have options for altering this behaviour. |
| 165 | |||
| 166 | gcc 3.4.3: | ||
| 167 | // gcc aligns to 32 bytes if sizeof(struct) >= 32 | ||
| 168 | struct st { | ||
| 169 | int c_iflag,c_oflag,c_cflag,c_lflag; | ||
| 170 | int i1,i2,i3; // struct will be aligned to 4 bytes | ||
| 171 | // int i1,i2,i3,i4; // struct will be aligned to 32 bytes | ||
| 172 | }; | ||
| 173 | struct st t = { 1 }; | ||
| 174 | // same for arrays | ||
| 175 | char vc31[31] = { 1 }; // unaligned | ||
| 176 | char vc32[32] = { 1 }; // aligned to 32 bytes | ||