Re: [PATCH 0/4] Micro-optimize DWARF partial symbol reading

Simon Marchi
2020-05-20T21:08:07Z

On 2020-05-20 1:40 p.m., Tom Tromey wrote:
> A personal goal of mine is to improve the startup time of gdb. In the
> long run, I think the answer lies partly with threading, and perhaps
> with a more radical rewrite of the DWARF psymbol reader. However,
> those are difficult goals; and in the short term, I found that just
> profiling the reader and making small improvements can make a
> difference.
>
> This series improves the performance of the DWARF partial symbol
> reader about 10% (more in one case) on some real-world executables.
> See the first patch for details (I chose to put the details there so
> they would end up in the eventual git log).
>
> Regression tested on x86-64 Fedora 30.
>
> Let me know what you think.
>
> Tom

I tried the series as a whole, with these two files, libxul.so, which reads this debug info file:

$ l /usr/lib/debug/.build-id/06/bc3dd11d2331977ff78ce8e18c59216a8b9a61.debug
-rwxrwxr-x 1 root root 1.5G May 8 12:21 /usr/lib/debug/.build-id/06/bc3dd11d2331977ff78ce8e18c59216a8b9a61.debug

and libwebkit2gtk-4.0.so.37.28.5, which reads this debug info file:

$ l /usr/lib/debug/.build-id/77/5b4022ee4a85d12697b8791001b40570c25f98.debug
-rwxrwxr-x 1 root root 1.4G Aug 15 2018 /usr/lib/debug/.build-id/77/5b4022ee4a85d12697b8791001b40570c25f98.debug

So both are about the same size. This is without the patchset applied

$ for i in 1 2 3 4 5; do time ./gdb -nx --data-directory=data-directory /usr/lib/firefox/libxul.so -batch; done
./gdb -nx --data-directory=data-directory /usr/lib/firefox/libxul.so -batch 97.10s user 1.81s system 102% cpu 1:36.94 total
./gdb -nx --data-directory=data-directory /usr/lib/firefox/libxul.so -batch 97.61s user 1.96s system 102% cpu 1:37.55 total
./gdb -nx --data-directory=data-directory /usr/lib/firefox/libxul.so -batch 99.33s user 1.90s system 101% cpu 1:39.34 total
./gdb -nx --data-directory=data-directory /usr/lib/firefox/libxul.so -batch 96.87s user 1.95s system 101% cpu 1:36.92 total
./gdb -nx --data-directory=data-directory /usr/lib/firefox/libxul.so -batch 97.19s user 1.94s system 102% cpu 1:37.10 total

$ for i in 1 2 3 4 5; do time ./gdb -nx --data-directory=data-directory /usr/lib/x86_64-linux-gnu/libwebkit2gtk-4.0.so.37.28.5 -batch; done
./gdb -nx --data-directory=data-directory -batch 96.66s user 1.27s system 101% cpu 1:36.76 total
./gdb -nx --data-directory=data-directory -batch 95.63s user 1.45s system 101% cpu 1:35.92 total
./gdb -nx --data-directory=data-directory -batch 92.45s user 1.24s system 101% cpu 1:32.62 total
./gdb -nx --data-directory=data-directory -batch 96.55s user 1.45s system 101% cpu 1:36.85 total
./gdb -nx --data-directory=data-directory -batch 92.75s user 1.34s system 101% cpu 1:32.93 total

And this is with the patchset applied:

$ for i in 1 2 3 4 5; do time ./gdb -nx --data-directory=data-directory /usr/lib/firefox/libxul.so -batch; done
./gdb -nx --data-directory=data-directory /usr/lib/firefox/libxul.so -batch 58.08s user 1.71s system 103% cpu 57.780 total
./gdb -nx --data-directory=data-directory /usr/lib/firefox/libxul.so -batch 57.89s user 1.75s system 103% cpu 57.618 total
./gdb -nx --data-directory=data-directory /usr/lib/firefox/libxul.so -batch 57.85s user 1.67s system 103% cpu 57.492 total
./gdb -nx --data-directory=data-directory /usr/lib/firefox/libxul.so -batch 58.03s user 1.85s system 103% cpu 57.883 total
./gdb -nx --data-directory=data-directory /usr/lib/firefox/libxul.so -batch 58.16s user 1.73s system 103% cpu 57.833 total

$ for i in 1 2 3 4 5; do time ./gdb -nx --data-directory=data-directory /usr/lib/x86_64-linux-gnu/libwebkit2gtk-4.0.so.37.28.5 -batch; do
ne
./gdb -nx --data-directory=data-directory -batch 57.81s user 1.17s system 102% cpu 57.788 total
./gdb -nx --data-directory=data-directory -batch 57.60s user 1.27s system 101% cpu 57.728 total
./gdb -nx --data-directory=data-directory -batch 57.75s user 1.18s system 101% cpu 57.847 total
./gdb -nx --data-directory=data-directory -batch 57.33s user 1.19s system 102% cpu 57.318 total
./gdb -nx --data-directory=data-directory -batch 57.95s user 1.17s system 101% cpu 57.967 total

It's still a bit too long for an interactive user to wait, but it's quite an improvement!

Simon