Обсуждение: Default JIT setting in V12

Hi,

On 2019-09-04 09:56:28 -0400, Jeff Janes wrote:
> I think it is intuitive, and with empirical evidence, that we do not want
> to JIT compile at all unless we are going to optimize the compiled code.

There's pretty clear counter-evidence however as well :(

I think it's probably more sensible to use some cheap minimal
optimization for the "unoptimized" mode - because there's some
non-linear cost algorithms with full optimizations enabled.

How does your example look with something like:

diff --git i/src/backend/jit/llvm/llvmjit.c w/src/backend/jit/llvm/llvmjit.c
index 82c4afb7011..85ddae2ea2b 100644
--- i/src/backend/jit/llvm/llvmjit.c
+++ w/src/backend/jit/llvm/llvmjit.c
@@ -428,7 +428,7 @@ llvm_optimize_module(LLVMJitContext *context, LLVMModuleRef module)
     if (context->base.flags & PGJIT_OPT3)
         compile_optlevel = 3;
     else
-        compile_optlevel = 0;
+        compile_optlevel = 1;
 
     /*
      * Have to create a new pass manager builder every pass through, as the

which I think - but I'd have to check - doesn't include any of the
non-linear cost optimizations.


> Is there a rationale for, or other examples to show, that it makes sense
> for the default value of jit_optimize_above_cost to be 5 fold higher than
> the default setting of jit_above_cost?

Yes. IIRC even tpc-h or something shows that with small scale one does
get noticable - but not crazy - speedups with unoptimized code, but that
it's a loss with optimized code.

Greetings,

Andres Freund

Hi,

On 2019-09-04 07:51:16 -0700, Andres Freund wrote:
> On 2019-09-04 09:56:28 -0400, Jeff Janes wrote:
> > I think it is intuitive, and with empirical evidence, that we do not want
> > to JIT compile at all unless we are going to optimize the compiled code.
> 
> There's pretty clear counter-evidence however as well :(
> 
> I think it's probably more sensible to use some cheap minimal
> optimization for the "unoptimized" mode - because there's some
> non-linear cost algorithms with full optimizations enabled.
> 
> How does your example look with something like:
> 
> diff --git i/src/backend/jit/llvm/llvmjit.c w/src/backend/jit/llvm/llvmjit.c
> index 82c4afb7011..85ddae2ea2b 100644
> --- i/src/backend/jit/llvm/llvmjit.c
> +++ w/src/backend/jit/llvm/llvmjit.c
> @@ -428,7 +428,7 @@ llvm_optimize_module(LLVMJitContext *context, LLVMModuleRef module)
>      if (context->base.flags & PGJIT_OPT3)
>          compile_optlevel = 3;
>      else
> -        compile_optlevel = 0;
> +        compile_optlevel = 1;
>  
>      /*
>       * Have to create a new pass manager builder every pass through, as the
> 
> which I think - but I'd have to check - doesn't include any of the
> non-linear cost optimizations.

Or better, something slightly more complete, like the attached (which
affects both code-gen time optimizations (which are more like peephole
ones), and both function/global ones that are cheap).

Greetings,

Andres Freund