Re: WIP: Faster Expression Processing and Tuple Deforming (including JIT)

Hi,

On 2016-12-06 13:56:28 -0500, Tom Lane wrote:
> Andres Freund <andres(at)anarazel(dot)de> writes:
> > I'm posting a quite massive series of WIP patches here, to get some
> > feedback.
>
> I guess the $64 question that has to be addressed here is whether we're
> prepared to accept LLVM as a run-time dependency. There are some reasons
> why we might not be:

Indeed. It'd only be a soft dependency obviously.

> * The sheer mass of the dependency. What's the installed footprint of
> LLVM, versus a Postgres server? How hard is it to install from source?

Worked for me first try, but I'm perhaps not the best person to judge.
It does take a while to compile though (~20min on my laptop).

> * How will we answer people who say they can't accept having a compiler
> installed on their production boxes for security reasons?

I think they'll just not enable --with-llvm in that case, and get
inferior performance. Note that installing llvm does not imply
installing a full blown C compiler (although I think that's largely
moot, you can get pretty much the same things done with just compiling
LLVM IR).

> * Are there any currently-interesting platforms that LLVM doesn't work
> for? (I'm worried about RISC-V as much as legacy systems.)

LLVM itself I don't think is a problem, it seems to target a wide range
of platforms. The platforms that don't support JIT compiling might be a
bit larger, since that involves more than just generating code.

> I concur with your feeling that hand-rolled JIT is right out. But
> I'm not sure that whatever performance gain we might get in this
> direction is worth the costs.

Well, I'm not impartial, but I don't think we do our users a service by
leaving significant speedups untackled, and after spending a *LOT* of
time on this, I don't see much other choice than JITing. Note that
nearly everything performance sensitive is moving towards doing JITing
in some form or another.

Greetings,

Andres Freund