Re: JIT compiling with LLVM v9.0

On Tue, Jan 30, 2018 at 02:08:30PM -0800, Andres Freund wrote:
> Hi,
>
> On 2018-01-30 22:57:06 +0100, David Fetter wrote:
> > On Tue, Jan 30, 2018 at 01:46:37PM -0500, Robert Haas wrote:
> > > On Mon, Jan 29, 2018 at 1:40 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:
> > > > It's an optional dependency, and it doesn't increase build
> > > > time that much... If we were to move the llvm interfacing code
> > > > to a .so, there'd not even be a packaging issue, you can just
> > > > package that .so separately and get errors if somebody tries
> > > > to enable LLVM without that .so being installed.
> > >
> > > I suspect that would be really valuable. If 'yum install
> > > postgresql-server' (or your favorite equivalent) sucks down all
> > > of LLVM,
> >
> > As I understand it, LLVM is organized in such a way as not to
> > require this. Andres, am I understanding correctly that what
> > you're using doesn't require much of LLVM at runtime?
>
> I'm not sure what you exactly mean. Yes, you need the llvm library
> at runtime. Perhaps you're thinking of clang or llvm binarieries?
> The latter we *not* need.

I was, and glad I understood correctly.

> What's required is something like:
> $ apt show libllvm5.0
> Package: libllvm5.0
> Version: 1:5.0.1-2
> Priority: optional
> Section: libs
> Source: llvm-toolchain-5.0
> Maintainer: LLVM Packaging Team <pkg-llvm-team(at)lists(dot)alioth(dot)debian(dot)org>
> Installed-Size: 56.9 MB
> Depends: libc6 (>= 2.15), libedit2 (>= 2.11-20080614), libffi6 (>= 3.0.4), libgcc1 (>= 1:3.4), libstdc++6 (>= 6), libtinfo5 (>= 6), zlib1g (>= 1:1.2.0)
> Breaks: libllvm3.9v4
> Replaces: libllvm3.9v4
> Homepage: http://www.llvm.org/
> Tag: role::shared-lib
> Download-Size: 13.7 MB
> APT-Manual-Installed: no
> APT-Sources: http://debian.osuosl.org/debian unstable/main amd64 Packages
> Description: Modular compiler and toolchain technologies, runtime library
> LLVM is a collection of libraries and tools that make it easy to build
> compilers, optimizers, just-in-time code generators, and many other
> compiler-related programs.
> .
> This package contains the LLVM runtime library.
>
> So ~14MB to download, ~57MB on disk. We only need a subset of
> libllvm5.0, and LLVM allows to build such a subset. But obviously
> distributions aren't going to target their LLVM just for postgres.

True, although if they're using an LLVM only for PostgreSQL and care
about 57MB of disk, they're probably also ready to do that work.

> > > Unfortunately, that has the pretty significant downside that a
> > > lot of people who actually want the postgresql-server-jit
> > > package will not realize that they need to install it, which
> > > sucks.
> >
> > It does indeed.
>
> With things like apt recommends and such I don't think this is a
> huge problem. It'll be installed by default unless somebody is on a
> space constrained system and doesn't want that...

Don't most of the wins for JITing come in the OLAP space anyway? I'm
having trouble picturing a severely space-constrained OLAP system, but
of course it's a possible scenario.

Best,
David.
--
David Fetter <david(at)fetter(dot)org> http://fetter.org/
Phone: +1 415 235 3778

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