[llvm-dev] Question about LLVM LLJIT Compile time

[Lang Hames via llvm-dev]

Hi Chris,

I can think of a couple of things to check up front:

(1) Are you timing this with a release build or a debug build? ORC uses
asserts liberally, including in code that is run under the session lock,
and this may decrease parallelism in debug builds.

(1) Are you using a fixed sized thread pool with an appropriate limit?
Compiling too many things in parallel can have negatively impact
performance if it leads to memory exhaustion.

(2) Are you loading each module on a different LLVMContext? Modules sharing
an LLVMContext cannot be compiled concurrently, as contexts cannot be
shared between threads.

And some follow up questions: What platform are you running on? Are you
using LLJIT or LLLazyJIT? What kind of slow-down do you see relative to
single-threaded compilation?

Finally, some thoughts: The performance of concurrent compilation has not
received any attention at all yet, as I have been busy with other feature
work. I definitely want to get this working though. There are no stats or
timings collected at the moment, but I can think of a few that i think
would be useful and relatively easy to implement: (1) Track time spent
under the session lock by adding timers to runSessionLocked, (2) Track time
spent waiting on LLVMContexts in ThreadSafeContext, (3) Add a
runAs<FunctionType> utility with timers to time execution of JIT functions.

What are your thoughts? Are there any other tools you would like to see
added?

Cheers,
Lang.

On Wed, Jan 29, 2020 at 2:12 PM chris boese via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi,
>
> We are using the new LLJIT class in our compiler. We have not been
> successful using the parallel JIT feature. When we tried it previously on
> multiple modules, our compile-time increased significantly. I don't know if
> we are using it incorrectly, or that we miss out on optimizations we get
> when running on a single merged module, but it hasn't worked for us yet. We
> are pretty far behind HEAD atm, but will try it again soon.
>
> In the meantime, we are trying to find ways to gauge the compilation time
> of a module. We pass a single module to the LLJIT instance. Is there is any
> information we can get during the JIT construction to let us compare
> against other modules we run through JIT? We're trying to find hot spots or
> performance issues in our modules. Timers or statistical data would be
> helpful if they exist during the execution of the JIT engine.
>
> I imagine parallelizing the JIT will be our best bet for increasing
> performance, but we have not been able to use that yet.
>
> Any help/ideas would be appreciated.
>
> Thanks,
> Chris
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