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Notes about pic, static linking, and debugging dynamic linking.

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Rob Landley 2006-04-10 17:54:23 +00:00
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docs/busybox.net/programming.html
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@ -17,6 +17,7 @@
<li><a href="#tips_encrypted_passwords">Encrypted Passwords</a></li>
<li><a href="#tips_vfork">Fork and vfork</a></li>
<li><a href="#tips_short_read">Short reads and writes</a></li>
<li><a href="#tips_memory">Memory used by relocatable code, PIC, and static linking.</a></li>
</ul>
<li><a href="#who">Who are the BusyBox developers?</a></li>
</ul>
@ -346,6 +347,35 @@ data comes in that can be merged into the same packet. (In case you were
wondering why action games that use TCP/IP set TCP_NODELAY to lower the latency
on their their sockets, now you know.)</p>
<h2><a name="tips_memory">Memory used by relocatable code, PIC, and static linking.</a></h2>
<p>The downside of standard dynamic linking is that it results in self-modifying
code. Although each executable's pages are mmaped() into a process's address
space from the executable file and are thus naturally shared between processes
out of the page cache, the library loader (ld-linux.so.2 or ld-uClibc.so.0)
writes to these pages to supply addresses for relocatable symbols. This
dirties the pages, triggering copy-on-write allocation of new memory for each
processes's dirtied pages.</p>
<p>One solution to this is Position Independent Code (PIC), a way of linking
a file so all the relocations are grouped together. This dirties fewer
pages (often just a single page) for each process's relocations. The down
side is this results in larger executables, which take up more space on disk
(and a correspondingly larger space in memory). But when many copies of the
same program are running, PIC dynamic linking trades a larger disk footprint
for a smaller memory footprint, by sharing more pages.</p>
<p>A third solution is static linking. A statically linked program has no
relocations, and thus the entire executable is shared between all running
instances. This tends to have a significantly larger disk footprint, but
on a system with only one or two executables, shared libraries aren't much
of a win anyway.</p>
<p>You can tell the glibc linker to display debugging information about its
relocations with the environment variable "LD_DEBUG". Try
"LD_DEBUG=help /bin/true" for a list of commands. Learning to interperet
"LD_DEBUG=statistics cat /proc/self/statm" could be interesting.</p>
<h2><a name="who">Who are the BusyBox developers?</a></h2>
<p>The following login accounts currently exist on busybox.net. (I.E. these
@ -375,7 +405,6 @@ solar :Ned Ludd
timr :Tim Riker
tobiasa :Tobias Anderberg
vapier :Mike Frysinger
vodz :Vladimir N. Oleynik
</pre>
<p>The following accounts used to exist on busybox.net, but don't anymore so
@ -395,6 +424,7 @@ miles
proski
rjune
tausq
vodz :Vladimir N. Oleynik
</pre>