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Elf dynamic linker stuff from Rich Felker.

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Rob Landley 2006-04-17 21:47:03 +00:00
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@ -376,6 +376,41 @@ relocations with the environment variable "LD_DEBUG". Try
"LD_DEBUG=help /bin/true" for a list of commands. Learning to interpret
"LD_DEBUG=statistics cat /proc/self/statm" could be interesting.</p>
<p>For more on this topic, here's Rich Felker:</p>
<blockquote>
<p>Dynamic linking (without fixed load addresses) fundamentally requires
at least one dirty page per dso that uses symbols. Making calls (but
never taking the address explicitly) to functions within the same dso
does not require a dirty page by itself, but will with ELF unless you
use -Bsymbolic or hidden symbols when linking.</p>
<p>ELF uses significant additional stack space for the kernel to pass all
the ELF data structures to the newly created process image. These are
located above the argument list and environment. This normally adds 1
dirty page to the process size.</p>
<p>The ELF dynamic linker has its own data segment, adding one or more
dirty pages. I believe it also performs relocations on itself.</p>
<p>The ELF dynamic linker makes significant dynamic allocations to manage
the global symbol table and the loaded dso's. This data is never
freed. It will be needed again if libdl is used, so unconditionally
freeing it is not possible, but normal programs do not use libdl. Of
course with glibc all programs use libdl (due to nsswitch) so the
issue was never addressed.</p>
<p>ELF also has the issue that segments are not page-aligned on disk.
This saves up to 4k on disk, but at the expense of using an additional
dirty page in most cases, due to a large portion of the first data
page being filled with a duplicate copy of the last text page.</p>
<p>The above is just a partial list of the tiny memory penalties of ELF
dynamic linking, which eventually add up to quite a bit. The smallest
I've been able to get a process down to is 8 dirty pages, and the
above factors seem to mostly account for it (but some were difficult
to measure).</p>
</blockquote>
<h2><a name="who">Who are the BusyBox developers?</a></h2>
<p>The following login accounts currently exist on busybox.net. (I.E. these