Re: JIT compiling with LLVM v12

On 2018-08-22 06:20:21 +0000, Noah Misch wrote:
> 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.

Similar.

> 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.

I don't think it really shows that. The reason that JITing gets started
there is that the tables aren't analyzed and we end up with crazy ass
estimates about the cost of the queries. No useful setting of the cost
limits will protect against that... :(

Greetings,

Andres Freund