Re: New LLVM JIT Features

Hi Andres,

Thanks for your thoughts , please see my comments inline.

On 2019-04-03 10:44:06 +0530, preejackie wrote:

Hi Andres,

Thanks for the reply! Please see my comments inline.

On 03/04/19 3:20 AM, Andres Freund wrote:

Hi,

On 2019-04-02 00:51:51 +0530, preejackie wrote:

As LLVM ORC supports compiling in multiple backend threads, it would be
effective if we compile the functions speculatively before they are called
by the executing function. So when we request JIT to compile a function, JIT
will immediately returns the function address for raw executable bits. This
will greatly reduce the JIT latencies in modern multi-core machines.

I personally think this should be approached somewhat differently -
putting patchpoints into code reduces the efficiency of the generated
code, so I don't think that's the right approach. What I think we should

 What do you mean by patch points here? To my knowledge, LLVM symbols have
arbitrary stub associated which resolve to function address at function
address.

I was assuming that you'd want to improve latency by not compiling all
the functions at the start of the executor (like we currently do), but
have sub-functions compiled in the background. That'd require
patchpoints to be able to initially redirect to a function to wait for
compilation, which then can be changed to directly jump to the function.
Because we already just compile all the functions reachable at the start
of execution in one go, so it's not a one-by-one function affair.

  Compiling the whole module will increase your start-up time of the application right? Is there any techniques applied in Pgsql to handle this ? Sometimes, you will compile functions that you don't need immediately or even it will not called in run time. This is the trade-off between different JIT implementations.  Also adding patch points in the generated code will degrade performance only when we didn't compile the function ahead-of-time, theoretically this will patch points miss will go down when we increase the number of compiler threads. And practically every computer have at least 4 cores nowadays.

do is to, if we decide it's worthwhile at plan time, generate the LLVM
IR time at the beginning of execution, but continue to use interpreted
execution initially. The generated IR would then be handed over to a
background [process|thread|whatnot] for optimization of code
generation. Then, when finished, I'd switch over from interpreted to JIT
compiled execution.  That approach will, in my view, yield better
latency behaviour because we can actually evaluate quals etc for which
we've not yet finished code generation.

And also I'm working on designing a ORC in-place dynamic profiling support, by
this JIT will automatically able to identify the hot functions, and compile
it in higher optimization level to achieve good performance.

I think that's a nice concept, but at the moment the generated code is
so bad that it's much more likely to get big benefits by improving the
generated IR, compared to giving more hints to the optimizer.

By improving the generated IR, you mean by turning pgsql queries into LLVM
IR? If it is the case, this design doesn't handles that, it works only when
the given program representation is in LLVM IR.

My point is that we generate IR that's hard for LLVM to optimize. And
that fixing that is going to give you way bigger wins than profile
guided optimization.

  I hope this is problem of Pgsql, but I'm proposing this project for LLVM Community.  

Greetings,

Andres Freund

-- 
Have a great day!
PreeJackie