On 11/5/19, Ben Cotton wrote:
https://fedoraproject.org/wiki/Changes/PythonStaticSpeedup
== Summary ==
Python 3 traditionally in Fedora was built with a shared library
libpython3.?.so and the final binary was dynamically linked against
that shared library. This change is about creating the static library
and linking the final python3 binary against it, as it provides
significant performance improvement, up to 27% depending on the
workload. The static library will not be shipped. The shared library
will continue to exist in a separate subpackage. In essence, python3
will no longer depend on libpython.
<<snip>>
There are alternatives that provide gradations in the tradeoffs.
0) Include _Py_UnixMain in libpython3.POINTVER.so, also set ElfXX_Ehdr.e_entry
and include enough -startfiles so that execve(libpython3.POINTVER.so, ...)
will act as if execve(python, ...). Compare execve("/lib64/libc.so.6", ...)
which prints the credits for glibc. Then python3 and libpython3.POINTVER.so
can be hardlinked or symlinked. This removes one DT_NEEDED from the
startup of python3.
1) Do not flag python3 and libpython3.POINTVER.so with DT_BIND_NOW or DF_BIND_NOW.
This removes the need to perform relocation processing for every slot in the PLT
at process startup.
2) Use -Wl,-Bsymbolic during the build (static bind) of libpython3.POINTVER.so.
This removes all intra-library symbolic relocations (hence PLT slots) at the cost
of also removing the ability to override (interpose) them.
3) Compile and build libpython3.POINTVER.so as ET_EXEC (without -fPIC, without
-shared, without -fPIE), static bind with
-Wl,-Ttext-segment=$(< /proc/sys/vm/mmap-min-addr)
to put the library below the pages of any default ET_EXEC, static bind with
--export-dynamic (or --dynamic-list=) to make visible all Python primitives,
and enhance the dynamic linker ld-linux to dlopen(ET_EXEC, ...) as if ET_DYN
but OR-in MAP_FIXED when mmap() of PT_LOAD. The dynamic linker can be
tricked today by changing ElfXX_Ehdr.e_type from ET_EXEC to ET_DYN,
as long as the linux kernel honors the hint of mmap(non_zero, ...)
without MAP_FIXED.
Today's /lib64/libpython3.7m.so.1.0 occupies about 3.4 MB of pages, which fits
between default mmap-min-addr of 64K and default -Ttext-segment of 4M.
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