Horton

Re: JIT compiling with LLVM v11

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Peter Eisentraut <peter.eisentraut@2ndquadrant.com>
Cc: Pg Hackers <pgsql-hackers@postgresql.org>
Date: 2018-03-06T20:16:01Z
Lists: pgsql-hackers 
Hi,

On 2018-03-06 10:29:47 -0500, Peter Eisentraut wrote:
> I think taking the total cost as the triggering threshold is probably
> good enough for a start.  The cost modeling can be refined over time.

Cool.


> We should document that both jit_optimize_above_cost and
> jit_inline_above_cost require jit_above_cost to be set, or otherwise
> nothing happens.

Yea, that's a good plan. We could also change it so it would, but I
don't think there's much point?


> One problem I see is that if someone sets things like
> enable_seqscan=off, the artificial cost increase created by those
> settings would quite likely bump the query over the jit threshold, which
> would alter the query performance characteristics in a way that the user
> would not have intended.  I don't have an idea how to address this right
> now.

I'm not too worried about that scenario. If, for a cheap plan, the
planner ends up with a seqscan despite it being disabled, you're pretty
close to randomly choosing plans already, as the pruning doesn't work
well anymore (as the %1 percent fuzz factor in
compare_path_costs_fuzzily() swamps the actual plan costs).


> I ran some performance assessments:
>
> merge base (0b1d1a038babff4aadf0862c28e7b667f1b12a30)
>
> make installcheck  3.14s user 3.34s system 17% cpu 37.954 total
>
> jit branch default settings
>
> make installcheck  3.17s user 3.30s system 13% cpu 46.596 total
>
> jit_above_cost=0
>
> make installcheck  3.30s user 3.53s system 5% cpu 1:59.89 total
>
> jit_optimize_above_cost=0 (and jit_above_cost=0)
>
> make installcheck  3.44s user 3.76s system 1% cpu 8:12.42 total
>
> jit_inline_above_cost=0 (and jit_above_cost=0)
>
> make installcheck  3.32s user 3.62s system 2% cpu 5:35.58 total
>
> One can see the CPU savings quite nicely.

I'm not quite sure what you mean by that.


> One obvious problem is that with the default settings, the test suite
> run gets about 15% slower.  (These figures are reproducible over several
> runs.)  Is there some debugging stuff turned on that would explain this?
>  Or would just loading the jit module in each session cause this?

I suspect it's loading the module.  There's two pretty easy avenues to
improve this:

1) Attempt to load the JIT provider in postmaster, thereby avoiding a
   lot of redundant dynamic linker work if already installed. That's
   ~5-10 lines or such.  I basically refrained from that because it's
   convenient to not have to restart the server during development (one
   can just reconnect and get a newer jit plugin).

2) Don't load the JIT provider until fully needed. Right now
   jit_compile_expr() will load the jit provider even if not really
   needed. We should probably move the first two return blocks in
   llvm_compile_expr() into jit_compile_expr(), to avoid that.


> From the other results, we can see that one clearly needs quite a big
> database to see a solid benefit from this.

Right, until we've got caching this'll only be beneficial for ~1s+
analytics queries. Unfortunately caching requires some larger planner &
executor surgery, so I don't want to go there at the same time (I'm
already insane enough).


> Do you have any information gathered about this so far?  Any scripts
> to create test databases and test queries?

Yes. I've used tpc-h. Not because it's the greatest, but because it's
semi conveniently available and a lot of others have experience with it
already.  Do you mean whether I've run a couple benchmarks? If so, yes.
I'll schedule some more later - am on battery power rn.

Greetings,

Andres Freund

Commits

  1. Improve JIT docs.

  2. Add documentation for the JIT feature.

  3. Add EXPLAIN support for JIT.

  4. Add inlining support to LLVM JIT provider.

  5. JIT tuple deforming in LLVM JIT provider.

  6. Add FIELDNO_* macro designating offset into structs required for JIT.

  7. Add expression compilation support to LLVM JIT provider.

  8. Expand list of synchronized types and functions in LLVM JIT provider.

  9. Add helpers for emitting LLVM IR.

  10. Basic planner and executor integration for JIT.

  11. Debugging and profiling support for LLVM JIT provider.

  12. Support for optimizing and emitting code in LLVM JIT provider.

  13. Add file containing extensions of the LLVM C API.

  14. Basic JIT provider and error handling infrastructure.

  15. Add configure infrastructure (--with-llvm) to enable LLVM support.

  16. Add C++ support to configure.

  17. Add PGAC_PROG_VARCC_VARFLAGS_OPT autoconf macro.

  18. Fix VM buffer pin management in heap_lock_updated_tuple_rec().

  19. Allow tupleslots to have a fixed tupledesc, use in executor nodes.

  20. Expression evaluation based aggregate transition invocation.

  21. Perform slot validity checks in a separate pass over expression.

  22. Rely on executor utils to build targetlist for DML RETURNING.

  23. Refer to OS X as "macOS", except for the port name which is still "darwin".