128 lines
4.2 KiB
Plaintext
128 lines
4.2 KiB
Plaintext
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
|
|
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 bbox 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
|
|
|
|
This requires 55M of memory. Thus 1 trivial busybox applet
|
|
takes 55k of memory.
|
|
|
|
|
|
Example 1
|
|
|
|
One example how to reduce global data usage is in
|
|
archival/libunarchive/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 inflate_gunzip() [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 PTR_TO_GLOBALS macro:
|
|
|
|
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
|
|
sizeof(struct globals) <= sizeof(bb_common_bufsiz1).
|
|
|
|
|
|
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.
|