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authorGravatar Denis Vlasenko <vda.linux@googlemail.com>2007-03-20 11:08:39 +0000
committerGravatar Denis Vlasenko <vda.linux@googlemail.com>2007-03-20 11:08:39 +0000
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tree7681cc079c04adbd75e40183f3b65059413634a9 /docs/keep_data_small.txt
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-rw-r--r--docs/keep_data_small.txt82
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 @@
When many applets are compiled into busybox, all rw data and
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.
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
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
minimize usage of libc functions which implicitly use
-such structures in libc.
-
-Small experiment measures "parasitic" bbox memory consumption.
-Here we start 1000 "busybox sleep 10" in parallel.
-bbox binary is practically allyesconfig static one,
-built against uclibc:
-
-bash-3.2# nmeter '%t %c %b %m %p %[pn]'
-23:17:28 .......... 0 0 168M 0 147
-23:17:29 .......... 0 0 168M 0 147
-23:17:30 U......... 0 0 168M 1 147
-23:17:31 SU........ 0 188k 181M 244 391
-23:17:32 SSSSUUU... 0 0 223M 757 1147
-23:17:33 UUU....... 0 0 223M 0 1147
-23:17:34 U......... 0 0 223M 1 1147
-23:17:35 .......... 0 0 223M 0 1147
-23:17:36 .......... 0 0 223M 0 1147
-23:17:37 S......... 0 0 223M 0 1147
-23:17:38 .......... 0 0 223M 1 1147
-23:17:39 .......... 0 0 223M 0 1147
-23:17:40 .......... 0 0 223M 0 1147
-23:17:41 .......... 0 0 210M 0 906
-23:17:42 .......... 0 0 168M 1 147
-23:17:43 .......... 0 0 168M 0 147
+such structures.
+
+Small experiment to measure "parasitic" bbox memory consumption:
+here we start 1000 "busybox sleep 10" in parallel.
+busybox binary is practically allyesconfig static one,
+built against uclibc. Run on x86-64 machine with 64-bit kernel:
+
+bash-3.2# nmeter '%t %c %m %p %[pn]'
+23:17:28 .......... 168M 0 147
+23:17:29 .......... 168M 0 147
+23:17:30 U......... 168M 1 147
+23:17:31 SU........ 181M 244 391
+23:17:32 SSSSUUU... 223M 757 1147
+23:17:33 UUU....... 223M 0 1147
+23:17:34 U......... 223M 1 1147
+23:17:35 .......... 223M 0 1147
+23:17:36 .......... 223M 0 1147
+23:17:37 S......... 223M 0 1147
+23:17:38 .......... 223M 1 1147
+23:17:39 .......... 223M 0 1147
+23:17:40 .......... 223M 0 1147
+23:17:41 .......... 210M 0 906
+23:17:42 .......... 168M 1 147
+23:17:43 .......... 168M 0 147
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
@@ -104,8 +108,12 @@ its needs. Library functions are prohibited from using it.
#define G (*(struct globals*)&bb_common_bufsiz1)
-Be careful, though, and use it only if
-sizeof(struct globals) <= sizeof(bb_common_bufsiz1).
+Be careful, though, and use it only if globals fit into 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
@@ -135,7 +143,7 @@ static int tabstop;
static struct termios term_orig __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:
tabstop:
@@ -154,3 +162,15 @@ ridiculously large values. asm output diff for above example:
.size term_vi, 60
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