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| -rw-r--r-- | docs/busybox.net/programming.html | 35 |
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diff --git a/docs/busybox.net/programming.html b/docs/busybox.net/programming.html index f5433f5..61777af 100644 --- a/docs/busybox.net/programming.html +++ b/docs/busybox.net/programming.html @@ -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 |