On 15. 11. 19 13:24, Aleksandra Fedorova wrote:
Hi
On Fri, Nov 15, 2019 at 10:22 AM Victor Stinner <vstinner@xxxxxxxxxx> wrote:
Hi Jan,
With the helper of Florian Weimer and Charalampos Stratakis, we also agreed to test this flag in priority. I understood that it disables the LD_PRELOAD feature: it's no longer possible to override symbols in libpython with LD_PRELOAD. Thanks to that, the compiler can avoid PLT indirection for function calls and can inline more function functions in libpython. I'm talking about a function call from libpython to libpython: something which is very common in python. Basically, almost all function calls are calls from libpython to libpython.
I'm impressed. Thanks to -fno-semantic-interposition, I get the same speedup on a dynamically linked Python (libpython) compared to statically linked Python!
Yesterday, I tried on a vanilla Python compiled manually:
./configure --enable-optimizations --with-lto --enable-shared
CFLAGS="-fno-semantic-interposition" LDFLAGS="-fno-semantic-interposition"
I saw the same speed up than avoiding --enable-shared. Today I validated this result using the RPM generated by Charalampos's PR:
https://src.fedoraproject.org/rpms/python38/pull-request/53
In short, https://fedoraproject.org/wiki/Changes/PythonStaticSpeedup is useless: there is no need to modify Python to statically link it to libpython. We can keep the dynamically library libpython and keep Python dynamically linked to it. We only need to pass -fno-semantic-interposition to compiler and linker flags when building Python!
I'm not sure if we need a Fedora change just for a compiler flag. Again, the only drawback is that we will no longer be able to override a symbol using LD_PRELOAD. Honestly, I never did that. I don't see any use case for that. But I used LD_PRELOAD on the libc multiple times to mock the system clock for example.
Please do file a Change. It works not only as a coordination tracker,
but also as a tool to inform everyone about changes happening in
Fedora.
Already working on it.
The discussion on the topic has been very interesting, as well as the
outcome. I think it would be nice to see the summary with the
estimated impact and highlight it via Release Notes.
Yes indeed.
--
Miro Hrončok
--
Phone: +420777974800
IRC: mhroncok
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