[llvm-dev] MCJit Runtine Performance

[Hal Finkel via llvm-dev]

----- Original Message -----
> From: "Keno Fischer via llvm-dev" <llvm-dev at lists.llvm.org>
> To: "Morten Brodersen" <Morten.Brodersen at constrainttec.com>
> Cc: "llvm-dev" <llvm-dev at lists.llvm.org>
> Sent: Thursday, February 4, 2016 6:05:29 PM
> Subject: Re: [llvm-dev] MCJit Runtine Performance
> 
> 
> 
> Yes, unfortunately, this is very much known. Over in the julia
> project, we've recently gone through this and taken the hit (after
> doing some work to fix the very extreme corner cases that we were
> hitting). We're not entirely sure why the slowdown is this
> noticable, but at least in our case, profiling didn't reveal any
> remaining low hanging fruits that are responsible. One thing you can
> potentially try if you haven't yet is to enable fast ISel and see if
> that brings you closer to the old runtimes.

And maybe the register allocator? Are you using the greedy one or the linear one? Are there any other MI-level optimizations running?

 -Hal

> 
> 
> On Thu, Feb 4, 2016 at 7:00 PM, Morten Brodersen via llvm-dev <
> llvm-dev at lists.llvm.org > wrote:
> 
> 
> Hi All,
> 
> We recently upgraded a number of applications from LLVM 3.5.2 (old
> JIT) to LLVM 3.7.1 (MCJit).
> 
> We made the minimum changes needed for the switch (no changes to the
> IR generated or the IR optimizations applied).
> 
> The resulting code pass all tests (8000+).
> 
> However the runtime performance dropped significantly: 30% to 40% for
> all applications.
> 
> The applications I am talking about optimize airline rosters and
> pairings. LLVM is used for compiling high level business rules to
> efficient machine code.
> 
> A typical optimization run takes 6 to 8 hours. So a 30% to 40%
> reduction in speed has real impact (=> we can't upgrade from 3.5.2).
> 
> We have triple checked and reviewed the changes we made from old JIT
> to MCJIt. We also tried different ways to optimize the IR.
> 
> However all results indicate that the performance drop happens in the
> (black box) IR to machine code stage.
> 
> So my question is if the runtime performance reduction is
> known/expected for MCJit vs. old JIT? Or if we might be doing
> something wrong?
> 
> If you need more information, in order to understand the issue,
> please tell us so that we can provide you with more details.
> 
> Thanks
> Morten
> 
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-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory