Re: JIT compiling with LLVM v12

On Wed, Mar 28, 2018 at 02:27:51PM -0700, Andres Freund wrote:
> For now LLVM is enabled by default when compiled --with-llvm. I'm mildly
> inclined to leave it like that until shortly before the release, and
> then disable it by default (i.e. change the default of jit=off). But I
> think we can make that decision based on experience during the testing
> window. I'm opening an open items entry for that.

I'll vote for jit=on and letting any bugs shake out earlier, but it's not a
strong preference.

I see jit slows the regression tests considerably:

# x86_64, non-assert, w/o llvm
$ for n in 1 2 3; do env time make -C src/bin/pg_upgrade check; done 2>&1 | grep elapsed
7.64user 4.24system 0:36.40elapsed 32%CPU (0avgtext+0avgdata 36712maxresident)k
8.09user 4.50system 0:37.71elapsed 33%CPU (0avgtext+0avgdata 36712maxresident)k
7.53user 4.18system 0:36.54elapsed 32%CPU (0avgtext+0avgdata 36712maxresident)k

# x86_64, non-assert, w/ llvm trunk
$ for n in 1 2 3; do env time make -C src/bin/pg_upgrade check; done 2>&1 | grep elapsed
9.58user 5.79system 0:49.61elapsed 30%CPU (0avgtext+0avgdata 36712maxresident)k
9.47user 5.92system 0:47.84elapsed 32%CPU (0avgtext+0avgdata 36712maxresident)k
9.09user 5.51system 0:47.94elapsed 30%CPU (0avgtext+0avgdata 36712maxresident)k

# mips32el, assert, w/o llvm (buildfarm member topminnow) [1]
28min install-check-*
35min check-pg_upgrade

# mips32el, assert, w/ llvm 6.0.1 [1]
63min install-check-*
166min check-pg_upgrade

Regardless of the choice of jit={on|off} default, these numbers tell me that
some or all of jit_*_cost defaults are too low.

[1] The mips32el runs used "nice -+20" and ran on a shared machine. I include
them to show the trend, but exact figures may be non-reproducible.