[llvm-dev] LLVM multithreading support

[Nicolai Hähnle via llvm-dev]

On Sun, Apr 12, 2020 at 10:17 PM Chris Lattner via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> On Apr 12, 2020, at 11:27 AM, Mehdi AMINI <joker.eph at gmail.com> wrote:
>
> Hey Chris,
>
>
> On Sat, Apr 11, 2020 at 5:15 PM Chris Lattner via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>>
>> Hi all,
>>
>> I was looking at the profile for a tool I’m working on, and noticed that it is spending 10% of its time doing locking related stuff.  The structure of the tool is that it reading in a ton of stuff (e.g. one moderate example I’m working with is 40M of input) into MLIR, then uses its multithreaded pass manager to do transformations.
>>
>> As it happens, the structure of this is that the parsing pass is single threaded, because it is parsing through a linear file (the parser is simple and fast, so this is bound by IR construction).  This means that none of the locking during IR construction is useful.
>
>
> I'm curious which are the places that show up on the profile? Do you have a few stacktraces to share?
>
>
> In my case, it is all MLIR attribute/type uniquification stuff which is guarded by a RWMutex.

If this can become a performance problem, is there a way to tackle the
problem head-on to reduce that cost even in scenarios that really are
multithreaded? E.g., an inlined initial atomic lock that falls back to
the "real" lock implementation on failure?

As long as it's only a small number of hotspots (and attribute/type
uniquing seem like plausible candidates), it'd seem justified to do
such things.

Cheers,
Nicolai

>
>> Historically, LLVM had a design where you could dynamically enable and disable multithreading support in a tool, which would be perfect for this use case, but it got removed by this patch: (xref https://reviews.llvm.org/D4216).  The rationale in the patch doesn’t make sense to me - this mode had nothing to do with the old LLVM global lock, this had to do with whether llvm::llvm_is_multithreaded() returned true or false … which all the locking stuff is guarded on.
>
>
> It seems that at the time the assumption was that this flag was there to alleviate the cost of the global lock only and removing the lock removed the motivation for the feature? Looks like you proved this wrong :)
>
> +Zach, David, and Reid to make sure they don't miss this.
>
>
> Yeah, it was about not paying the cost for synchronization when it wasn’t worthwhile.
>
>> Would it make sense to re-enable this, or am I missing something?
>
>
> Finding a way to re-enable it seems interesting. I wonder how much it'll interact with the places inside the compiler that are threaded now, maybe it isn't much more than tracking and auditing the uses of LLVM_ENABLE_THREADS (like lib/Support/ThreadPool.cpp for example). Have you already looked into it?
>
>
> It is super-easy to reenable, because the entire codebase is still calling llvm::llvm_is_multithreaded().  We just need to add the global back, along with the methods to set and clear the global, and change llvm::llvm_is_multithreaded() to something like:
>
> bool llvm::llvm_is_multithreaded() {
> #if LLVM_ENABLE_THREADS != 0
> return someGlobal;
> #else
> return false;
> #endif
> }
>
> -Chris
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