hush/docs/keep_data_small.txt
Denys Vlasenko 8a6a2f9c9c update seamless uncompression code
This change makes "tar tf hello_world.txz" work without
adding special-casing for ".txz" extension. It also removes
ever-growing magic checking code in rpm2cpio and get_header_tar -
we reuse one which lives in setup_unzip_on_fd.

function                                             old     new   delta
unpack_gz_stream                                       7     566    +559
check_signature16                                      -      70     +70
setup_unzip_on_fd                                     99     142     +43
handle_SIGCHLD                                         -      41     +41
unpack_bz2_stream                                    342     376     +34
unzip_main                                          2352    2385     +33
bbunpack                                             503     533     +30
open_transformer                                      74     102     +28
unpack_Z_stream                                     1278    1304     +26
unpack_gunzip                                        101     123     +22
init_transformer_aux_data                              -      18     +18
unpack_xz_stream                                    2388    2402     +14
open_zipped                                          131     141     +10
rpm_main                                            1358    1363      +5
get_header_tar_lzma                                   52      57      +5
get_header_tar_bz2                                    52      57      +5
unpack_lzma_stream                                  2698    2702      +4
hash_find                                            234     233      -1
get_header_tar                                      1759    1733     -26
get_header_tar_gz                                     92      57     -35
unpack_uncompress                                     51      12     -39
rpm2cpio_main                                        201     147     -54
unpack_unxz                                           67      12     -55
unpack_bz2_stream_prime                               55       -     -55
get_header_tar_Z                                      86       -     -86
unpack_gz_stream_with_info                           539       -    -539
------------------------------------------------------------------------------
(add/remove: 3/3 grow/shrink: 14/6 up/down: 947/-890)          Total: 57 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
2012-03-06 16:27:48 +01:00

257 lines
8.0 KiB
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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 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
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 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 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
One example how to reduce global data usage is in
archival/libarchive/decompress_unzip.c:
/* This is somewhat complex-looking arrangement, but it allows
* to place decompressor state either in bss or in
* malloc'ed space simply by changing #defines below.
* Sizes on i386:
* text data bss dec hex
* 5256 0 108 5364 14f4 - bss
* 4915 0 0 4915 1333 - malloc
*/
#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 unpack_gz_stream() [its main module]
and then passed down to all subroutines which need to access 'globals'
as a parameter.
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
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 *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]; };
and then declare that ptr_to_globals is a pointer to it:
#define G (*ptr_to_globals)
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 <applet>_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_<applet>_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):
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
855 bytes saved in total.
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.