[lldb-dev] Parallelizing loading of shared libraries

Pavel Labath via lldb-dev lldb-dev at lists.llvm.org
Tue May 2 03:04:54 PDT 2017

On 1 May 2017 at 22:58, Scott Smith <scott.smith at purestorage.com> wrote:
> On Mon, May 1, 2017 at 2:42 PM, Pavel Labath <labath at google.com> wrote:
>> Besides, hardcoding the nesting logic into "add" is kinda wrong.
>> Adding a task is not the problematic operation, waiting for the result
>> of one is. Granted, generally these happen on the same thread, but
>> they don't have to be -- you can write a continuation-style
>> computation, where you do a bit of work, and then enqueue a task to do
>> the rest. This would create an infinite pool depth here.
> True, but that doesn't seem to be the style of code here.  If it were you
> wouldn't need multiple pools, since you'd just wait for the callback that
> your work was done.
>> Btw, are we sure it's not possible to solve this with just one thread
>> pool. What would happen if we changed the implementation of "wait" so
>> that if the target task is not scheduled yet, we just go ahead an
>> compute it on our thread? I haven't thought through all the details,
>> but is sounds like this could actually give better performance in some
>> scenarios...
> My initial reaction was "that wouldn't work, what if you ran another posix
> dl load?"  But then I suppose *it* would run more work, and eventually you'd
> run a leaf task and finish something.
> You'd have to make sure your work could be run regardless of what mutexes
> the caller already had (since you may be running work for another
> subsystem), but that's probably not too onerous, esp given how many
> recursive mutexes lldb uses..

Is it any worse that if the thread got stuck in the "wait" call? Even
with a dead-lock-free thread pool the task at hand still would not be
able to make progress, as the waiter  would hold the mutex even while
blocked (and recursiveness will not save you here).

> I think that's all the more reason we *should* work on getting something into LLVM first.  Anything we already have in LLDB, or any modifications we make will likely not be pushed up to LLVM, especially since LLVM already has a ThreadPool, so any changes you make to LLDB's thread pool will likely have to be re-written when trying to get it to LLVM.  And since, as you said, more projects depend on LLVM than LLDB, there's a good chance that the baseline you'd be starting from when making improvements is more easily adaptable to what you want to do.  LLDB has a long history of being shy of making changes in LLVM where appropriate, and myself and others have started pushing back on that more and more, because it accumulates long term technical debt.
> In my experience, "let's just get it into LLDB first and then work on getting it up to LLVM later" ends up being "well, it's in LLDB now, so since my immediate problem is solved I may or may not have time to revisit this in the future"  (even if the original intent is sincere).
> If there is some resistance getting changes into LLVM, feel free to add me as a reviewer, and I can find the right people to move it along.  I'd still like to at least hear a strong argument for why the existing implementation in LLVM is unacceptable for what we need.  I'm ok with "non optimal".  Unless it's "unsuitable", we should start there and make incremental improvements.

I think we could solve our current problem by just having two global
instances of llvm::ThreadPool. The only issue I have with that is that
I will then have 100 threads around constantly, which will make
debugging lldb harder (although even that can be viewed as an
incentive to make debugging threaded programs easier :) ).

The reason I am not overly supportive of doing the design in llvm is
that I think we are trying to come up with a solution that will work
around issues with the lldb design, and I am not sure if that's the
right motivation. I am not against that either, though...

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