[lldb-dev] LLDB performance drop from 3.9 to 4.0

Jason Molenda via lldb-dev lldb-dev at lists.llvm.org
Wed Apr 12 12:26:05 PDT 2017

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 at 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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