38 lines
1.9 KiB
Markdown
38 lines
1.9 KiB
Markdown
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# Linkers part 10
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## Parallel Linking
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It is possible to parallelize the linking process somewhat. This can help hide
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I/O latency and can take better advantage of modern multi-core systems. My
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intention with gold is to use these ideas to speed up the linking process.
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The first area which can be parallelized is reading the symbols and relocation
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entries of all the input files. The symbols must be processed in order;
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otherwise, it will be difficult for the linker to resolve multiple definitions
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correctly. In particular all the symbols which are used before an archive must
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be fully processed before the archive is processed, or the linker won’t know
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which members of the archive to include in the link (I guess I haven’t talked
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about archives yet). However, despite these ordering requirements, it can be
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beneficial to do the actual I/O in parallel.
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After all the symbols and relocations have been read, the linker must complete
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the layout of all the input contents. Most of this can not be done in parallel,
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as setting the location of one type of contents requires knowing the size of
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all the preceding types of contents. While doing the layout, the linker can
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determine the final location in the output file of all the data which needs to
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be written out.
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After layout is complete, the process of reading the contents, applying
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relocations, and writing the contents to the output file can be fully
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parallelized. Each input file can be processed separately.
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Since the final size of the output file is known after the layout phase, it is
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possible to use `mmap` for the output file. When not doing relaxation, it is
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then possible to read the input contents directly into place in the output
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file, and to relocation them in place. This reduces the number of system calls
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required, and ideally will permit the operating system to do optimal disk I/O
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for the output file.
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Just a short entry tonight. More tomorrow.
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