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.
+
+Script:
+
+i=1000; while test $i != 0; do
+ echo -n .
+ busybox sleep 30 &
+ i=$((i - 1))
+done
+echo
+wait
+
+(Data from NOMMU arches are sought. Provide 'size busybox' output too)
Example 1
(see the rest of the file to get the idea)
This example completely eliminates globals in that module.
-Required memory is allocated in inflate_gunzip() [its main module]
+Required memory is allocated in unpack_gz_stream() [its main module]
and then passed down to all subroutines which need to access 'globals'
as a parameter.
ptr_to_globals is declared as constant pointer.
This helps gcc understand that it won't change, resulting in noticeably
-smaller code. In order to assign it, use PTR_TO_GLOBALS macro:
+smaller code. In order to assign it, use SET_PTR_TO_GLOBALS macro:
- PTR_TO_GLOBALS = xzalloc(sizeof(G));
+ SET_PTR_TO_GLOBALS(xzalloc(sizeof(G)));
Typically it is done in <applet>_main().
#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
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:
.size term_vi, 60
gcc doesn't seem to have options for altering this behaviour.
+
+gcc 3.4.3 and 4.1.1 tested:
+char c = 1;
+// gcc aligns to 32 bytes if sizeof(struct) >= 32
+struct {
+ int a,b,c,d;
+ int i1,i2,i3;
+} s28 = { 1 }; // struct will be aligned to 4 bytes
+struct {
+ int a,b,c,d;
+ int i1,i2,i3,i4;
+} s32 = { 1 }; // struct will be aligned to 32 bytes
+// same for arrays
+char vc31[31] = { 1 }; // unaligned
+char vc32[32] = { 1 }; // aligned to 32 bytes
+
+-fpack-struct=1 reduces alignment of s28 to 1 (but probably
+will break layout of many libc structs) but s32 and vc32
+are still aligned to 32 bytes.
+
+I will try to cook up a patch to add a gcc option for disabling it.
+Meanwhile, this is where it can be disabled in gcc source:
+
+gcc/config/i386/i386.c
+int
+ix86_data_alignment (tree type, int align)
+{
+#if 0
+ if (AGGREGATE_TYPE_P (type)
+ && TYPE_SIZE (type)
+ && TREE_CODE (TYPE_SIZE (type)) == INTEGER_CST
+ && (TREE_INT_CST_LOW (TYPE_SIZE (type)) >= 256
+ || TREE_INT_CST_HIGH (TYPE_SIZE (type))) && align < 256)
+ return 256;
+#endif
+
+Result (non-static busybox built against glibc):
+
+# size /usr/srcdevel/bbox/fix/busybox.t0/busybox busybox
+ text data bss dec hex filename
+ 634416 2736 23856 661008 a1610 busybox
+ 632580 2672 22944 658196 a0b14 busybox_noalign
projects / oweals / busybox.git / blobdiff