I don't know exactly when the 3.9 / 4.0 branches were cut, and what was done
between those two points, but in general we don't expect/want to see
performance regressions like that. I'm more familiar with the perf
characteristics on macos, Linux is different in some important regards, so I
can only speak in general terms here.
In your example, you're measuring three things, assuming you have debug
information for MY_PROGRAM. The first is "Do the initial read of the main
binary and its debug information". The second is "Find all symbol names
'main'". The third is "Scan a newly loaded solib's symbols" (assuming you
don't have debug information from solibs from /usr/lib etc). Technically
there's some additional stuff here -- launching the process, detecting solibs
as they're loaded, looking up the symbol context when we hit the breakpoint,
backtracing a frame or two, etc, but that stuff is rarely where you'll see perf
issues on a local debug session.
Which of these is likely to be important will depend on your MY_PROGRAM. If
you have a 'int main(){}', it's not going to be dwarf parsing. If your binary
only pulls in three solib's by the time it is running, it's not going to be new
module scanning. A popular place to spend startup time is in C++ name
demangling if you have a lot of solibs with C++ symbols.
On Darwin systems, we have a nonstandard accelerator table in our DWARF emitted
by clang that lldb reads. The "apple_types", "apple_names" etc tables. So
when we need to find a symbol named "main", for Modules that have a SymbolFile,
we can look in the accelerator table. If that SymbolFile has a 'main', the
accelerator table gives us a reference into the DWARF for the definition, and
we can consume the DWARF lazily. We should never need to do a full scan over
the DWARF, that's considered a failure.
(in fact, I'm working on a branch of the llvm.org sources from mid-October and
I suspect Darwin lldb is often consuming a LOT more dwarf than it should be
when I'm debugging, I need to figure out what is causing that, it's a big
problem.)
In general, I've been wanting to add a new "perf counters" infrastructure &
testsuite to lldb, but haven't had time. One thing I work on a lot is
debugging over a bluetooth connection; it turns out that BT is very slow, and
any extra packets we send between lldb and debugserver are very costly. The
communication is so fast over a local host, or over a usb cable, that it's easy
for regressions to sneak in without anyone noticing. So the original idea was
hey, we can have something that counts packets for distinct operations. Like,
this "next" command should take no more than 40 packets, that kind of thing.
And it could be expanded -- "b main should fully parse the DWARF for only 1
symbol", or "p *this should only look up 5 types", etc.
> On Apr 12, 2017, at 11:26 AM, Scott Smith via lldb-dev
> <lldb-dev@lists.llvm.org> wrote:
>
> I worked on some performance improvements for lldb 3.9, and was about to
> forward port them so I can submit them for inclusion, but I realized there
> has been a major performance drop from 3.9 to 4.0. I am using the official
> builds on an Ubuntu 16.04 machine with 16 cores / 32 hyperthreads.
>
> Running: time lldb-4.0 -b -o 'b main' -o 'run' MY_PROGRAM > /dev/null
>
> With 3.9, I get:
> real 0m31.782s
> user 0m50.024s
> sys 0m4.348s
>
> With 4.0, I get:
> real 0m51.652s
> user 1m19.780s
> sys 0m10.388s
>
> (with my changes + 3.9, I got real down to 4.8 seconds! But I'm not
> convinced you'll like all the changes.)
>
> Is this expected? I get roughly the same results when compiling llvm+lldb
> from source.
>
> I guess I can spend some time trying to bisect what happened. 5.0 looks to
> be another 8% slower.
>
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