207 lines
6.5 KiB
Plaintext
207 lines
6.5 KiB
Plaintext
Keeping data small
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When many applets are compiled into busybox, all rw data and
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bss for each applet are concatenated. Including those from libc,
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if static busybox is built. When busybox is started, _all_ this data
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is allocated, not just that one part for selected applet.
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What "allocated" exactly means, depends on arch.
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On NOMMU it's probably bites the most, actually using real
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RAM for rwdata and bss. On i386, bss is lazily allocated
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by COWed zero pages. Not sure about rwdata - also COW?
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In order to keep busybox NOMMU and small-mem systems friendly
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we should avoid large global data in our applets, and should
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minimize usage of libc functions which implicitly use
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such structures.
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Small experiment to measure "parasitic" bbox memory consumption:
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here we start 1000 "busybox sleep 10" in parallel.
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busybox binary is practically allyesconfig static one,
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built against uclibc. Run on x86-64 machine with 64-bit kernel:
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bash-3.2# nmeter '%t %c %m %p %[pn]'
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23:17:28 .......... 168M 0 147
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23:17:29 .......... 168M 0 147
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23:17:30 U......... 168M 1 147
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23:17:31 SU........ 181M 244 391
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23:17:32 SSSSUUU... 223M 757 1147
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23:17:33 UUU....... 223M 0 1147
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23:17:34 U......... 223M 1 1147
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23:17:35 .......... 223M 0 1147
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23:17:36 .......... 223M 0 1147
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23:17:37 S......... 223M 0 1147
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23:17:38 .......... 223M 1 1147
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23:17:39 .......... 223M 0 1147
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23:17:40 .......... 223M 0 1147
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23:17:41 .......... 210M 0 906
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23:17:42 .......... 168M 1 147
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23:17:43 .......... 168M 0 147
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This requires 55M of memory. Thus 1 trivial busybox applet
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takes 55k of memory on 64-bit x86 kernel.
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On 32-bit kernel we need ~26k per applet.
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(Data from NOMMU arches are sought. Provide 'size busybox' output too)
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Example 1
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One example how to reduce global data usage is in
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archival/libunarchive/decompress_unzip.c:
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/* This is somewhat complex-looking arrangement, but it allows
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* to place decompressor state either in bss or in
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* malloc'ed space simply by changing #defines below.
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* Sizes on i386:
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* text data bss dec hex
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* 5256 0 108 5364 14f4 - bss
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* 4915 0 0 4915 1333 - malloc
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*/
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#define STATE_IN_BSS 0
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#define STATE_IN_MALLOC 1
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(see the rest of the file to get the idea)
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This example completely eliminates globals in that module.
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Required memory is allocated in unpack_gz_stream() [its main module]
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and then passed down to all subroutines which need to access 'globals'
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as a parameter.
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Example 2
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In case you don't want to pass this additional parameter everywhere,
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take a look at archival/gzip.c. Here all global data is replaced by
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single global pointer (ptr_to_globals) to allocated storage.
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In order to not duplicate ptr_to_globals in every applet, you can
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reuse single common one. It is defined in libbb/messages.c
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as struct globals *const ptr_to_globals, but the struct globals is
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NOT defined in libbb.h. You first define your own struct:
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struct globals { int a; char buf[1000]; };
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and then declare that ptr_to_globals is a pointer to it:
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#define G (*ptr_to_globals)
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ptr_to_globals is declared as constant pointer.
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This helps gcc understand that it won't change, resulting in noticeably
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smaller code. In order to assign it, use PTR_TO_GLOBALS macro:
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PTR_TO_GLOBALS = xzalloc(sizeof(G));
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Typically it is done in <applet>_main().
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Now you can reference "globals" by G.a, G.buf and so on, in any function.
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bb_common_bufsiz1
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There is one big common buffer in bss - bb_common_bufsiz1. It is a much
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earlier mechanism to reduce bss usage. Each applet can use it for
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its needs. Library functions are prohibited from using it.
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'G.' trick can be done using bb_common_bufsiz1 instead of malloced buffer:
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#define G (*(struct globals*)&bb_common_bufsiz1)
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Be careful, though, and use it only if globals fit into bb_common_bufsiz1.
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Since bb_common_bufsiz1 is BUFSIZ + 1 bytes long and BUFSIZ can change
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from one libc to another, you have to add compile-time check for it:
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if (sizeof(struct globals) > sizeof(bb_common_bufsiz1))
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BUG_<applet>_globals_too_big();
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Drawbacks
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You have to initialize it by hand. xzalloc() can be helpful in clearing
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allocated storage to 0, but anything more must be done by hand.
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All global variables are prefixed by 'G.' now. If this makes code
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less readable, use #defines:
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#define dev_fd (G.dev_fd)
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#define sector (G.sector)
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Word of caution
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If applet doesn't use much of global data, converting it to use
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one of above methods is not worth the resulting code obfuscation.
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If you have less than ~300 bytes of global data - don't bother.
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gcc's data alignment problem
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The following attribute added in vi.c:
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static int tabstop;
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static struct termios term_orig __attribute__ ((aligned (4)));
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static struct termios term_vi __attribute__ ((aligned (4)));
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reduces bss size by 32 bytes, because gcc sometimes aligns structures to
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ridiculously large values. asm output diff for above example:
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tabstop:
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.zero 4
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.section .bss.term_orig,"aw",@nobits
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- .align 32
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+ .align 4
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.type term_orig, @object
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.size term_orig, 60
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term_orig:
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.zero 60
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.section .bss.term_vi,"aw",@nobits
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- .align 32
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+ .align 4
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.type term_vi, @object
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.size term_vi, 60
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gcc doesn't seem to have options for altering this behaviour.
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gcc 3.4.3 and 4.1.1 tested:
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char c = 1;
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// gcc aligns to 32 bytes if sizeof(struct) >= 32
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struct {
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int a,b,c,d;
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int i1,i2,i3;
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} s28 = { 1 }; // struct will be aligned to 4 bytes
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struct {
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int a,b,c,d;
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int i1,i2,i3,i4;
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} s32 = { 1 }; // struct will be aligned to 32 bytes
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// same for arrays
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char vc31[31] = { 1 }; // unaligned
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char vc32[32] = { 1 }; // aligned to 32 bytes
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-fpack-struct=1 reduces alignment of s28 to 1 (but probably
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will break layout of many libc structs) but s32 and vc32
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are still aligned to 32 bytes.
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I will try to cook up a patch to add a gcc option for disabling it.
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Meanwhile, this is where it can be disabled in gcc source:
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gcc/config/i386/i386.c
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int
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ix86_data_alignment (tree type, int align)
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{
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#if 0
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if (AGGREGATE_TYPE_P (type)
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&& TYPE_SIZE (type)
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&& TREE_CODE (TYPE_SIZE (type)) == INTEGER_CST
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&& (TREE_INT_CST_LOW (TYPE_SIZE (type)) >= 256
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|| TREE_INT_CST_HIGH (TYPE_SIZE (type))) && align < 256)
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return 256;
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#endif
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Result (non-static busybox built against glibc):
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# size /usr/srcdevel/bbox/fix/busybox.t0/busybox busybox
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text data bss dec hex filename
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634416 2736 23856 661008 a1610 busybox
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632580 2672 22944 658196 a0b14 busybox_noalign
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