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SubjectRe: Very slow clang kernel config ..
On Fri, Apr 30, 2021 at 06:48:11PM -0700, Nick Desaulniers wrote:
> On Fri, Apr 30, 2021 at 6:22 PM Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
> >
> > On Fri, Apr 30, 2021 at 5:25 PM Nick Desaulniers
> > <ndesaulniers@google.com> wrote:
> > >
> > > Ah, no, sorry, these are the runtime link editor/loader. So probably
> > > spending quite some time resolving symbols in large binaries.
> >
> > Yeah. Appended is the profile I see when I profile that "make
> > oldconfig", so about 45% of all time seems to be spent in just symbol
> > lookup and relocation.
> >
> > And a fair amount of time just creating and tearing down that huge
> > executable (with a lot of copy-on-write overhead too), with the kernel
> > side of that being another 15%. The cost of that is likely also fairly
> > directly linked to all the dynamic linking costs, which brings in all
> > that data.
> >
> > Just to compare, btw, this is the symbol lookup overhead for the gcc case:
> >
> > 1.43% ld-2.33.so do_lookup_x
> > 0.96% ld-2.33.so _dl_relocate_object
> > 0.69% ld-2.33.so _dl_lookup_symbol_x
> >
> > so it really does seem to be something very odd going on with the clang binary.
> >
> > Maybe the Fedora binary is built some odd way, but it's likely just
> > the default clang build.
> >
> > Linus
> >
> > ----
> > 23.59% ld-2.33.so _dl_lookup_symbol_x
> > 11.41% ld-2.33.so _dl_relocate_object
> > 9.95% ld-2.33.so do_lookup_x
> > 4.00% [kernel.vmlinux] copy_page
> > 3.98% [kernel.vmlinux] next_uptodate_page
> > 3.05% [kernel.vmlinux] zap_pte_range
> > 1.81% [kernel.vmlinux] clear_page_rep
> > 1.68% [kernel.vmlinux] asm_exc_page_fault
> > 1.33% ld-2.33.so strcmp
> > 1.33% ld-2.33.so check_match
>
> 47.61% spent in symbol table lookup. Nice. (Not counting probably a
> fair amount of the libc calls below).
>
> > 0.92% libLLVM-12.so llvm::StringMapImpl::LookupBucketFor
>
> ^ wait a minute; notice how in your profile the `Shared Object` is
> attributed to `libLLVM-12.so` while mine is `clang-13`? Clang can be
> built as either having libllvm statically linked or dynamically; see
> the cmake variables
> LLVM_BUILD_LLVM_DYLIB:BOOL
> LLVM_LINK_LLVM_DYLIB:BOOL
> BUILD_SHARED_LIBS:BOOL
> https://llvm.org/docs/CMake.html
>
> I think those are frowned upon; useful for cutting down on developers
> iteration speed due to not having to relink llvm when developing
> clang. But shipping that in production? I just checked and it doesn't
> look like we do that for AOSP's build of LLVM.
>
> Tom, is one of the above intentionally set for clang builds on Fedora?
> I'm guessing it's intentional that there are packages for
> libLLVM-12.so and libclang-cpp.so.12, perhaps they have other
> dependents?

Have you tried building clang/llvm with -Bsymbolic-functions?

Mike

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