559f7d8e9
Rob Landley
2006-04-17
Subject
Elf dynamic linker stuff from Rich Felker.
SOURCE BUG
This commit was genuinely broken at the time (not related to our build system): scripts/config/mkconfigs.c (2005-08-01 - 2006-10-05 arasi) uretilen bbconfigopts.h icine .config satirlarini kacissiz, ham newline'larla gömüyordu - gecerli C degildi, busybox'un kendi bb_mkdep araci bile parse edemiyordu (7d219aab7 ile duzeltildi, mkconfigs shell scripte cevrildi)
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docs/busybox.net/programming.html+35 -0
| @@ -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 | |