Using Z3 with parallelization from SBV - haskell

Question

I'd like to use Z3 via SBV using multiple cores. Based on this answer I should be able to do that just by passing parallel.enable=true to the z3 executable on the command line. Since I am using SBV, I need to go through SBV's interface to various SMTLib solvers, so here's what I tried:
foo = runSMTWith z3par $ do
...
where
z3par = z3
{ SBV.solver = (SBV.solver z3)
{ SBV.options = \cfg -> SBV.options (SBV.solver z3) cfg ++ ["parallel.enable=true"]
}
}
However, I am not seeing any signs of Z3 running with parallelization enabled:
CPU usage doesn't go above one core
No speedup compared to running without this flag
How do I enable Z3 parallelization, when going via SBV?

Answer 1

What you're doing is essentially how it is done from SBV. You might want to increase verbosity of z3 and output the diagnostics to a file to inspect later. Something like:
import Data.SBV
import Data.SBV.Control
foo :: IO (Word64, Word64)
foo = runSMTWith z3{solver = par} $ do
x <- sWord64 "x"
y <- sWord64 "y"
setOption $ DiagnosticOutputChannel "diagnostic_output"
constrain $ x * y .== 13
constrain $ x .> 1
constrain $ y .> 1
query $ do ensureSat
(,) <$> getValue x <*> getValue y
where par = (solver z3) {options = \cfg -> options (solver z3) cfg ++ extras}
extras = [ "parallel.enable=true"
, "-v:3"
]
Here, we're not only setting z3's parallel-mode, but we're also telling it to increase verbosity and put all the diagnostics in a file. (Side note: There are many other settings in the parallel section of z3 config, you can see what they are by issuing z3 -pd in your command line and looking at the output. You can set any other parameters from there by adding it to the extras variable above.)
When I run the above, I get:
*Main> foo
(6379316565415788539,3774100875216427415)
But I also get a file named diagnostic_output created in the current directory, which contains the following lines, amongst others:
(tactic.parallel :progress 0% :open 1)
(tactic.parallel :split-cube 0)
(parallel.tactic simplify-1)
(tactic.parallel :progress 100.00% :status sat :open 0)
So z3 is indeed in the parallel mode and things are happening. Of course, what exactly it does is more or less a black-box, and it's impossible to interpret the above output without inspecting z3 internals. (I don't think the meaning of these stats nor the strategies for the parallel solver are that well documented. If you find a good documentation on the details, please do report!)
Update
As of this commit, you can now simply say:
runSMTWith z3{extraArgs = ["parallel.enable=true"]} $ do ...
simplifying the programming a bit further.
Solver agnostic concurrency directly from SBV
Note that SBV also has combinators for running things concurrently directly from Haskell. See the functions:
satConcurrentWithAny
satConcurrentWithAll
proveConcurrentWithAny
proveConcurrentWithAll
These functions are solver agnostic, you can use them with any solver of your choosing. Of course, they require you to restructure your problem and do a manual decomposition to take advantage of the multiple-cores in your computer and stitch the solutions together yourself. But they also give you full control over how you want to structure your expensive search.