Re: JIT compiling with LLVM v9.0

On 2018-01-31 14:45:46 -0500, Robert Haas wrote:
> On Wed, Jan 31, 2018 at 1:34 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:
> >> The first one is a problem that's not going to go away. If the
> >> problem of JIT being enabled "magically" is something we're concerned
> >> about, we need to figure out a good solution, not just disable the
> >> feature by default.
> >
> > That's a fair argument, and I don't really have a good answer to it. We
> > could have a jit = off/try/on, and use that to signal things? I.e. it
> > can be set to try (possibly default in version + 1), and things will
> > work if it's not installed, but if set to on it'll refuse to work if not
> > enabled. Similar to how huge pages work now.
>
> We could do that, but I'd be more inclined just to let JIT be
> magically enabled. In general, if a user could do 'yum install ip4r'
> (for example) and have that Just Work without any further database
> configuration, I think a lot of people would consider that to be a
> huge improvement. Unfortunately we can't really do that for various
> reasons, the biggest of which is that there's no way for installing an
> OS package to modify the internal state of a database that may not
> even be running at the time. But as a general principle, I think
> having to configure both the OS and the DB is an anti-feature, and
> that if installing an extra package is sufficient to get the
> new-and-improved behavior, users will like it.

I'm not seing a contradiction between what you describe as desired, and
what I describe? If it defaulted to try, that'd just do what you want,
no? I do think it's important to configure the system so it'll error if
JITing is not available.

> Bonus points if it doesn't require a server restart.

I think server restart might be doable (although it'll increase memory
usage because the shlib needs to be loaded in each backend rather than
postmaster), but once a session is running I'm fairly sure we do not
want to retry. Re-checking whether a shlib is available on the
filesystem every query does not sound like a good idea...

Greetings,

Andres Freund