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[PATCH v9 0/6] elf: Use mmap to map in section contents

H.J. Lu hjl.tools@gmail.com
Wed Mar 13 16:08:09 GMT 2024 
Changes in v9:

1. Use MAP_FAILED for mmap failure.

Changes in v8:

1. Rebase against master branch.
2. Add _bfd_elf_link_mmap_section_contents and
_bfd_elf_link_munmap_section_contents.

Changes in v7:

1. Don't add the --keep-memory linker option.

Changes in v6:

1. Add the --keep-memory linker option and always cache symbol and
relocation tables for --keep-memory.
2. Always keep symbol table and relocation info for eh_frame to speedup
the Rust binary build by ~300x:

https://sourceware.org/bugzilla/show_bug.cgi?id=31466

Changes in v5:

1. Drop 2 patches which have been merged onto master branch.
2. Rename _bfd_elf_mmap_section to _bfd_elf_mmap_section_contents.
3. Rename _bfd_mmap_readonly_tracked, _bfd_mmap_readonly_untracked,
_bfd_munmap_readonly_untracked, _bfd_mmap_read_untracked to
_bfd_mmap_readonly_persistent, _bfd_mmap_readonly_temporary,
_bfd_munmap_readonly_temporary and _bfd_mmap_read_temporary.
4. Drop the setup_group change.
5. Fix a typo.
6. Update comments.

Changes in v4:

1. Change don't cache symbol nor relocation tables with mmap to opt-in.

Changes in v3:

1. Fix non-mmap build.
2. Change the argument name of bfd_mmap_local from flags to prot since
its values are PROT_XXX.

Changes in v2:

1. Don't hard-code BFD_JUMP_TABLE_COPY in bfd so that elf-bfd.h can be
included in libbfd.c.
2. Change the --with-mmap default to true.
3. Check USE_MMAP instead of HAVE_MMAP.
4. Remove the asize parameter to _bfd_mmap_readonly_tracked.
5. Add contents_addr and contents_size to bfd_elf_section_data.
6. Rename _bfd_link_keep_memory to _bfd_elf_link_keep_memory.

---
We can use mmap to map in ELF section contents, instead of copying them
into memory by hand.  We don't need to cache symbol nor relocation tables
if they are mapped in.  Data to link the 3.5GB clang executable in LLVM
17 debug build on Linux/x86-64 with 32GB RAM is:

		stdio		mmap		improvement
user		86.73		87.02		-0.3%
system		9.55		9.21		3.6%
total		100.40		97.66		0.7%
maximum set(GB)	17.34		13.14		24%
page faults	4047667		3042877		25%

and data to link the 275M cc1plus executable in GCC 14 stage 1 build is:

user		5.41		5.44		-0.5%
system		0.80		0.76		5%
total		6.25		6.26		-0.2%
maximum set(MB)	1323		968		27%
page faults	323451		236371		27%

Data shows that these won't improve the single copy linker performance.
But they improve the overall system performance when linker is used by
reducing linker memory usage and page faults.  They allow more parallel
linker jobs on LLVM debug build.

Here is a quote from Noah Goldstein: "on a large project they are an
extremely large speedup".

H.J. Lu (6):
  elf: Use mmap to map in read-only sections
  elf: Add _bfd_elf_m[un]map_section_contents
  elf: Use mmap to map in symbol and relocation tables
  elf: Don't cache symbol nor relocation tables with mmap
  elf: Always keep symbol table and relocation info for eh_frame
  elf: Add _bfd_elf_link_m[un]map_section_contents

 bfd/bfd-in2.h      |  24 ++++-
 bfd/bfd.c          |  17 ++++
 bfd/bfdwin.c       |   8 +-
 bfd/cache.c        |   7 +-
 bfd/compress.c     |   2 +-
 bfd/elf-bfd.h      |  33 ++++++
 bfd/elf-eh-frame.c |   4 +-
 bfd/elf-sframe.c   |   4 +-
 bfd/elf.c          | 236 +++++++++++++++++++++++++++++++++++--------
 bfd/elf32-i386.c   |   8 +-
 bfd/elf64-x86-64.c |  12 +--
 bfd/elfcode.h      |   7 +-
 bfd/elflink.c      | 188 +++++++++++++++++++++++++----------
 bfd/elfxx-target.h |   6 +-
 bfd/elfxx-x86.c    |   8 +-
 bfd/elfxx-x86.h    |   1 +
 bfd/libbfd-in.h    |  33 +++++-
 bfd/libbfd.c       | 243 ++++++++++++++++++++++++++++++++++++++++++++-
 bfd/libbfd.h       |  33 +++++-
 bfd/linker.c       |  35 -------
 bfd/lynx-core.c    |   2 +-
 bfd/opncls.c       |  21 ++++
 bfd/section.c      |   9 +-
 bfd/sysdep.h       |   4 +
 24 files changed, 770 insertions(+), 175 deletions(-)

-- 
2.44.0
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