X-Git-Url: https://git.librecmc.org/?a=blobdiff_plain;f=docs%2Fkeep_data_small.txt;h=21d732674c64f3a456b7ca7d992234b053fe0465;hb=c2a06db69de7562024524a89a7b0f0f7e61c5999;hp=ec13b4d3f86163083892834684a8989977bbae7a;hpb=972288e62fa0798f59caf209ab58c911d289a8b2;p=oweals%2Fbusybox.git diff --git a/docs/keep_data_small.txt b/docs/keep_data_small.txt index ec13b4d3f..21d732674 100644 --- a/docs/keep_data_small.txt +++ b/docs/keep_data_small.txt @@ -1,50 +1,65 @@ - Keeping data small + Keeping data small 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? -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 +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. + +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 userspace memory (nmeter doesn't account for kernel-side -allocations). Definitely can be improved. +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) -Thus we should avoid large global data in our applets, -and should minimize usage of libc functions which implicitly use -such structures in libc. - Example 1 + Example 1 One example how to reduce global data usage is in -archival/libunarchive/decompress_unzip.c: +archival/libarchive/decompress_unzip.c: /* This is somewhat complex-looking arrangement, but it allows * to place decompressor state either in bss or in @@ -57,12 +72,15 @@ archival/libunarchive/decompress_unzip.c: #define STATE_IN_BSS 0 #define STATE_IN_MALLOC 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. - Example 2 + + Example 2 In case you don't want to pass this additional parameter everywhere, take a look at archival/gzip.c. Here all global data is replaced by @@ -70,7 +88,7 @@ single global pointer (ptr_to_globals) to allocated storage. In order to not duplicate ptr_to_globals in every applet, you can reuse single common one. It is defined in libbb/messages.c -as struct globals *ptr_to_globals, but the struct globals is +as struct globals *const ptr_to_globals, but the struct globals is NOT defined in libbb.h. You first define your own struct: struct globals { int a; char buf[1000]; }; @@ -79,13 +97,160 @@ and then declare that ptr_to_globals is a pointer to it: #define G (*ptr_to_globals) -Linker magic ensures that these two merge into single pointer object. -Now initialize it in _main(): +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 SET_PTR_TO_GLOBALS macro: + + SET_PTR_TO_GLOBALS(xzalloc(sizeof(G))); + +Typically it is done in _main(). + +Now you can reference "globals" by G.a, G.buf and so on, in any function. + + + bb_common_bufsiz1 + +There is one big common buffer in bss - bb_common_bufsiz1. It is a much +earlier mechanism to reduce bss usage. Each applet can use it for +its needs. Library functions are prohibited from using it. + +'G.' trick can be done using bb_common_bufsiz1 instead of malloced buffer: + +#define G (*(struct globals*)&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__globals_too_big(); + + + Drawbacks + +You have to initialize it by hand. xzalloc() can be helpful in clearing +allocated storage to 0, but anything more must be done by hand. + +All global variables are prefixed by 'G.' now. If this makes code +less readable, use #defines: + +#define dev_fd (G.dev_fd) +#define sector (G.sector) + + + Word of caution + +If applet doesn't use much of global data, converting it to use +one of above methods is not worth the resulting code obfuscation. +If you have less than ~300 bytes of global data - don't bother. + + + Finding non-shared duplicated strings + +strings busybox | sort | uniq -c | sort -nr + + + gcc's data alignment problem + +The following attribute added in vi.c: + +static int tabstop; +static struct termios term_orig __attribute__ ((aligned (4))); +static struct termios term_vi __attribute__ ((aligned (4))); + +reduces bss size by 32 bytes, because gcc sometimes aligns structures to +ridiculously large values. asm output diff for above example: + + tabstop: + .zero 4 + .section .bss.term_orig,"aw",@nobits +- .align 32 ++ .align 4 + .type term_orig, @object + .size term_orig, 60 + term_orig: + .zero 60 + .section .bss.term_vi,"aw",@nobits +- .align 32 ++ .align 4 + .type term_vi, @object + .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 + + + + Keeping code small + +Set CONFIG_EXTRA_CFLAGS="-fno-inline-functions-called-once", +produce "make bloatcheck", see the biggest auto-inlined functions. +Now, set CONFIG_EXTRA_CFLAGS back to "", but add NOINLINE +to some of these functions. In 1.16.x timeframe, the results were +(annotated "make bloatcheck" output): - ptr_to_globals = xzalloc(sizeof(G)); +function old new delta +expand_vars_to_list - 1712 +1712 win +lzo1x_optimize - 1429 +1429 win +arith_apply - 1326 +1326 win +read_interfaces - 1163 +1163 loss, leave w/o NOINLINE +logdir_open - 1148 +1148 win +check_deps - 1148 +1148 loss +rewrite - 1039 +1039 win +run_pipe 358 1396 +1038 win +write_status_file - 1029 +1029 almost the same, leave w/o NOINLINE +dump_identity - 987 +987 win +mainQSort3 - 921 +921 win +parse_one_line - 916 +916 loss +summarize - 897 +897 almost the same +do_shm - 884 +884 win +cpio_o - 863 +863 win +subCommand - 841 +841 loss +receive - 834 +834 loss -and you can reference "globals" by G.a, G.buf and so on, in any function. +855 bytes saved in total. -The drawback is that now you have to initialize it by hand. xzalloc() -can be helpful in clearing allocated storage to 0, but anything more -must be done by hand. +scripts/mkdiff_obj_bloat may be useful to automate this process: run +"scripts/mkdiff_obj_bloat NORMALLY_BUILT_TREE FORCED_NOINLINE_TREE" +and select modules which shrank.