This patchset changes how BTF encoders collect and store function
information in a table searcheable by name, where their
btf_encoder_function_state is maintained.
Previously a separate array of elf_function structs was built and
stored by each btf_encoder, and when the encoders were merged before
dumping BTF their function tables would be traversed and merged too.
With these changes, the table used for searching for elf_function
becomes shared between encoder threads. However, additional
information in the table and in each individual elf_function has to be
stored to accomodate the sharing.
The memory footprint is now slightly higher, although it degrades
faster when number of threads is increased without this patchset.
/usr/bin/time -v with -j8:
With patchset: Maximum resident set size (kbytes): 1402792 (~2% higher)
Without: Maximum resident set size (kbytes): 1376700
/usr/bin/time -v with -j16:
With patchset: Maximum resident set size (kbytes): 1456044
Without: Maximum resident set size (kbytes): 1450748
The idea to make function table shared between encoders has been
mentioned previously [1].
I've been looking into ways of improving parallel reproducible_build
time, ideally making it as fast as non-reproducible. In an off-list
discussion Eduard Zingerman suggested creating a btf_encoder for each
input compilation unit. This would allow to enforce the order of BTF
merge after btf_encoders are done.
With each encoder maintaining it's own table this idea proved
infeasible [2]. And so making it shared became a requirement.
I implemented a prototype that improved the speed of parallel
reproducible_build by using a similar shared table and implementing
Eduard's idea [3]. On master (84ab453) reproducible_build was ~50%
slower than non-reproducible, and with my experimental changes it was
20% slower.
This patchset stems from that work and discussions. The longer term
goal is to make BTF encoding reproducible by default, and fast.
[1] https://lore.kernel.org/dwarves/20240909155031.1596249-1-alan.maguire@xxxxxxxxxx/
[2] https://lore.kernel.org/dwarves/aQtEzThpIhRiwQpkgeZepPGr5Mt_zuGfPbxQxZgS6SSoPYoqM1afjDqEmIZkdH3YzvhWmWwqCS_8ZvFTcSZHvpkAeBpLRTAZEmrOhq0svfo=@pm.me/
[3] https://github.com/theihor/dwarves/pull/1/files
Ihor Solodrai (6):
dwarf_loader: introduce pre/post cus__load_module hooks to conf_load
btf_encoder: split btf_encoder__collect_symbols()
btf_encoder: introduce elf_functions struct type
btf_encoder: collect elf_functions in pre_cus__load_module
btf_encoder: introduce elf_functions_entry
btf_encoder: switch to shared elf_functions table
btf_encoder.c | 341 ++++++++++++++++++++++++++++++++++++-------------
btf_encoder.h | 5 +-
dwarf_loader.c | 21 ++-
dwarves.h | 18 ++-
pahole.c | 28 ++--
5 files changed, 303 insertions(+), 110 deletions(-)
--
2.34.1
