[llvm-dev] MCJit Runtine Performance

[Hal Finkel via llvm-dev]

----- Original Message -----
> From: "Morten Brodersen via llvm-dev" <llvm-dev at lists.llvm.org>
> To: "llvm-dev" <llvm-dev at lists.llvm.org>
> Sent: Thursday, February 4, 2016 9:21:58 PM
> Subject: Re: [llvm-dev] MCJit Runtine Performance
> 
> 
> Hi Lang,
> 
> > That suggests an optimization quality issue, rather than
> > compile-time overhead
> 
> Yes that makes sense. The long running applications (6+ hours) JIT
> the rules once (taking a few seconds) and then run the generated
> machine code for hours. With no additional JIT'ing.
> 
> > if we can configure the CodeGen pipeline properly we can get the
> > performance back to the same level as the legacy JIT.
> 
> Sounds great. Happy to help with whatever is needed.
> 
> Speaking of which:
> 
> We generate low overhead profiling code as part of the generated IR.
> We use it for identifying performance bottlenecks in the higher
> level (before IR) optimizing stages.
> 
> So I think it would be possible for me to identify a function that
> runs much slower in 3.7.1. than in 3.5.2. And extract the IR.
> 
> Would that help?

It seems quite likely to help. Please do.

 -Hal

> 
> Cheers
> Morten
> 
> 
> On 05/02/16 13:46, Lang Hames wrote:
> 
> 
> 
> Hi Morten,
> 
> 
> > Here are the results (for a small but representational run):
> 
> 
> That suggests an optimization quality issue, rather than compile-time
> overhead. That's good news - I'd take it as a good sign that the MC
> and linking overhead aren't a big deal either, and if we can
> configure the CodeGen pipeline properly we can get the performance
> back to the same level as the legacy JIT.
> 
> 
> Cheers,
> Lang.
> 
> 
> 
> 
> On Thu, Feb 4, 2016 at 6:41 PM, Morten Brodersen via llvm-dev <
> llvm-dev at lists.llvm.org > wrote:
> 
> 
> 
> Hi Keno,
> 
> Thanks for the fast ISel suggestion.
> 
> Here are the results (for a small but representational run):
> 
> LLVM 3.5.2 (old JIT): 4m44s
> 
> LLVM 3.7.1 (MCJit) no fast ISel: 7m31s
> 
> LLVM 3.7.1 (MCJit) fast ISel: 7m39s
> 
> So not much of a difference unfortunately.
> 
> 
> 
> 
> On 05/02/16 11:05, Keno Fischer wrote:
> 
> 
> 
> 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.
> 
> 
> 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