[SR-2137] Poor startup and backtrace performance when debugging a core file on Ubuntu 16.04

[trfiala]

Previous ID	SR-2137
Radar	None
Original Reporter	@trfiala
Type	Bug

Environment

Ubuntu 16.04 x86_64, swift-lldb/master-next, 24-cores, 20 GB RAM

Additional Detail from JIRA

Votes	0
Component/s	LLDB for Swift
Labels	Bug, Linux
Assignee	None
Priority	Medium

md5: b74240e224800fc51af3c88b361079d0

Issue Description:

When debugging SR-2136 using a debug build of LLDB with release versions of LLVM/clang/Swift, I experienced the following timings:

startup - first time to '(lldb)' prompt: 80 seconds.
first backtrace - 100 seconds.

This was debugging a ~26 MB (that is MB, not GB) core file of the system Python with the lldb module loaded, in the manner in which LLDB tests run. The command line looked like this:

$ /path/to/lldb /usr/bin/python -c /path/to/core

The debug build of LLDB had all symbols exported from liblldb so that it could be debugged.

We'll want to tease out which of these are at play in the painfully slow perf:
(1) Linux corefiles,
(2) Linux general symbol-lookup,
(3) Linux debug LLDB build in general,

[trfiala]

What's all the more surprising with this one is that we, in general, attempt to do as little work as possible on startup and be as lazy as possible on all heavy-duty work. Thus, regardless of potential slow code paths on Linux, I would have expected the first time to '(lldb)' prompt to have been zippy.

[belkadan]

This isn't really the appropriate place to report general bugs about LLDB; that would be https://llvm.org/bugs.

[trfiala]

I filed it here because this is the version of LLDB where I saw it. As we do quite an extended set of symbolic lookup that only enters the LLDB codebase in GitHub, I want to first ensure the slowdown didn't come from here.

If it proves to be the case that this is the perf in LLVM.org, then I will of course move this into LLVM.org bugzilla.