[lldb-dev] Parallelizing loading of shared libraries

Scott Smith via lldb-dev lldb-dev at lists.llvm.org
Thu Apr 27 07:02:08 PDT 2017

So as it turns out, at least on my platform (Ubuntu 14.04), the symbols are
loaded regardless.  I changed my test so:
1. main() just returns right away
2. cmdline is: lldb -b -o run /path/to/my/binary

and it takes the same amount of time as setting a breakpoint.

On Wed, Apr 26, 2017 at 5:00 PM, Jim Ingham <jingham at apple.com> wrote:

> We started out with the philosophy that lldb wouldn't touch any more
> information in a shared library than we actually needed.  So when a library
> gets loaded we might need to read in and resolve its section list, but we
> won't read in any symbols if we don't need to look at them.  The idea was
> that if you did "load a binary, and run it" until the binary stops for some
> reason, we haven't done any unnecessary work.  Similarly, if all the
> breakpoints the user sets are scoped to a shared library then there's no
> need for us to read any symbols for any other shared libraries.  I think
> that is a good goal, it allows the debugger to be used in special purpose
> analysis tools w/o forcing it to pay costs that a more general purpose
> debug session might require.
> I think it would be hard to convert all the usages of modules to from "do
> something with a shared library" mode to "tell me you are interested in a
> shared library and give me a callback" so that the module reading could be
> parallelized on demand.  But at the very least we need to allow a mode
> where symbol reading is done lazily.
> The other concern is that lldb keeps the modules it reads in a global
> cache, shared by all debuggers & targets.  It is very possible that you
> could have two targets or two debuggers each with one target that are
> reading in shared libraries simultaneously, and adding them to the global
> cache.  In some of the uses that lldb has under Xcode this is actually very
> common.  So the task pool will have to be built up as things are added to
> the global shared module cache, not at the level of individual targets
> noticing the read-in of a shared library.
> Jim
> > On Apr 26, 2017, at 4:12 PM, Scott Smith via lldb-dev <
> lldb-dev at lists.llvm.org> wrote:
> >
> > After a dealing with a bunch of microoptimizations, I'm back to
> parallelizing loading of shared modules.  My naive approach was to just
> create a new thread per shared library.  I have a feeling some users may
> not like that; I think I read an email from someone who has thousands of
> shared libraries.  That's a lot of threads :-)
> >
> > The problem is loading a shared library can cause downstream
> parallelization through TaskPool.  I can't then also have the loading of a
> shared library itself go through TaskPool, as that could cause a deadlock -
> if all the worker threads are waiting on work that TaskPool needs to run on
> a worker thread.... then nothing will happen.
> >
> > Three possible solutions:
> >
> > 1. Remove the notion of a single global TaskPool, but instead have a
> static pool at each callsite that wants it.  That way multiple paths into
> the same code would share the same pool, but different places in the code
> would have their own pool.
> >
> > 2. Change the wait code for TaskRunner to note whether it is already on
> a TaskPool thread, and if so, spawn another one.  However, I don't think
> that fully solves the issue of having too many threads loading shared
> libraries, as there is no guarantee the new worker would work on the
> "deepest" work.  I suppose each task would be annotated with depth, and the
> work could be sorted in TaskPool though...
> >
> > 3. Leave a separate thread per shared library.
> >
> > Thoughts?
> >
> > _______________________________________________
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> > lldb-dev at lists.llvm.org
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