[lldb-dev] Lack of parallelism

Scott Smith via lldb-dev lldb-dev at lists.llvm.org
Tue May 2 12:31:17 PDT 2017

As it turns out, it was lock contention in the memory allocator.  Using
tcmalloc brought it from 8+ seconds down to 4.2.

I think this didn't show up in mutrace because glibc's malloc doesn't use
pthread mutexes.

Greg, that joke about adding tcmalloc wholesale is looking less funny and
more serious....  Or maybe it's enough to make it a cmake link option (use
if present or use if requested).

On Tue, May 2, 2017 at 8:42 AM, Jim Ingham <jingham at apple.com> wrote:

> I'm not sure about Linux, on OS X lldb will mmap the debug information
> rather that using straight reads.  But that should just be once per loaded
> module.
> Jim
> > On May 2, 2017, at 8:09 AM, Scott Smith via lldb-dev <
> lldb-dev at lists.llvm.org> wrote:
> >
> > I've been trying to improve the parallelism of lldb but have run into an
> odd roadblock.  I have the code at the point where it creates 40 worker
> threads, and it stays that way because it has enough work to do.  However,
> running 'top -d 1' shows that for the time in question, cpu load never gets
> above 4-8 cpus (even though I have 40).
> >
> > 1. I tried mutrace, which measures mutex contention (I had to call
> unsetenv("LD_PRELOAD") in main() so it wouldn't propagate to the process
> being tested).  It indicated some minor contention, but not enough to be
> the problem.  Regardless, I converted everything I could to lockfree
> structures (TaskPool and ConstString) and it didn't help.
> >
> > 2. I tried strace, but I don't think strace can figure out how to trace
> lldb.  It says it waits on a single futex for 8 seconds, and then is done.
> >
> > I'm about to try lttng to trace all syscalls, but I was wondering if
> anyone else had any ideas?  At one point I wondered if it was mmap kernel
> semaphore contention, but that shouldn't affect faulting individual pages,
> and I assume lldb doesn't call mmap all the time.
> >
> > I'm getting a bit frustrated because lldb should be taking 1-2 seconds
> to start up (it has ~45s of user+system work to do), but instead is taking
> 8-10, and I've been stuck there for a while.
> >
> > _______________________________________________
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> > lldb-dev at lists.llvm.org
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