Messages in this thread | | | Subject | Re: Very slow clang kernel config .. | From | Tom Stellard <> | Date | Fri, 30 Apr 2021 20:32:29 -0700 |
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On 4/30/21 6:48 PM, 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. >
BUILD_SHARED_LIBS is the only one that is discouraged and we don't use that in Fedora any more. We just use LLVM_LINK_LLVM_DYLIB and the clang equivalent.
> 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? >
Yes, it's intentional. Dynamic linking libraries from other packages is the Fedora policy[1], and clang and llvm are separate packages (in Fedora).
- Tom
[1] https://docs.fedoraproject.org/en-US/packaging-guidelines/#_statically_linking_executables
>> 0.83% [kernel.vmlinux] rmqueue_bulk >> 0.77% conf yylex >> 0.75% libc-2.33.so __gconv_transform_utf8_internal >> 0.74% libc-2.33.so _int_malloc >> 0.69% libc-2.33.so __strlen_avx2 >> 0.62% [kernel.vmlinux] pagecache_get_page >> 0.58% [kernel.vmlinux] page_remove_rmap >> 0.56% [kernel.vmlinux] __handle_mm_fault >> 0.54% [kernel.vmlinux] filemap_map_pages >> 0.54% libc-2.33.so __strcmp_avx2 >> 0.54% [kernel.vmlinux] __free_one_page >> 0.52% [kernel.vmlinux] release_pages
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