From e84aeb5bcb1a8398fce53aad9c0072ad73a8b5c9 Mon Sep 17 00:00:00 2001
From: Denis Vlasenko
Date: Tue, 20 Mar 2007 11:08:39 +0000
Subject: update docs

---
 docs/keep_data_small.txt | 82 ++++++++++++++++++++++++++++++------------------
 1 file changed, 51 insertions(+), 31 deletions(-)

(limited to 'docs')

diff --git a/docs/keep_data_small.txt b/docs/keep_data_small.txt
index fcd8df4..72de2d1 100644
--- a/docs/keep_data_small.txt
+++ b/docs/keep_data_small.txt
@@ -2,44 +2,48 @@
 
 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.
+
+(Data from NOMMU arches are sought. Provide 'size busybox' output too)
 
 
 		Example 1
@@ -104,8 +108,12 @@ its needs. Library functions are prohibited from using it.
 
 #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
@@ -135,7 +143,7 @@ static int tabstop;
 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:
@@ -154,3 +162,15 @@ ridiculously large values. asm output diff for above example:
         .size   term_vi, 60
 
 gcc doesn't seem to have options for altering this behaviour.
+
+gcc 3.4.3:
+// gcc aligns to 32 bytes if sizeof(struct) >= 32
+struct st {
+    int c_iflag,c_oflag,c_cflag,c_lflag;
+    int i1,i2,i3;    // struct will be aligned to 4 bytes
+//  int i1,i2,i3,i4; // struct will be aligned to 32 bytes
+};
+struct st t = { 1 };
+// same for arrays
+char vc31[31] = { 1 }; // unaligned
+char vc32[32] = { 1 }; // aligned to 32 bytes
-- 
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