Hi Austin,
first of all, thanks a lot for taking the time to report your results!
On 23/09/2008, at 11:48, Austin Seipp wrote:
* The vectorise pass boosts compilation times *a lot*. I don't think
this is exactly unwarrented since it seems like a pretty complicated
transformation, but while making the primitive version using just
the unlifted interface the compilation takes about 1.5 seconds, for
the vectorised version it's on the order of 15 seconds. For
something as trivial as this dot-product thing, that's a bit
of a compilation time, though.
The problem here is not the vectoriser but rather the subsequent
optimisations. The vectoriser itself is (or should be - I haven't
really timed it, to be honest) quite fast. It generates very complex
code, however, which GHC takes a lot of time to optimise. We'll
improve the output of the vectoriser eventually, but not before it is
complete. For the moment, there is no solution for this, I'm afraid.
* It's pretty much impossible to use ghc-core to examine the output
core of the vectorised version - I let it run and before anything
started showing up in `less` it was already using on the order of
100mb of memory. If I just add -ddump-simpl to the command line, the
reason is obvious: the core generated is absolutely huge.
Yes. Again, this is something we'll try to improve eventually.
* For the benchmark included, the vectorised ver. spends about 98% of
its time from what I can see in the GC before it dies from stack
overflow. I haven't tried something like +RTS -A1G -RTS yet, though.
IIUC, the code is
dotp :: [:Int:] -> [:Int:] -> Int
dotp v w = I.sumP [: (I.*) x y | x <- v, y <- w :]
The way the vectoriser works at the moment, it will repeat the array w
(lengthP v) times, i.e., create an array of length (lengthP v *
lengthP w). This is quite unfortunate and needs to be fused away but
isn't at the moment. The only advice I can give is to stay away from
array comprehensions for now. They work but are extremely slow. This
definition should work fine:
dotp v w = I.sumP (zipWithP (I.*) v w)
* The vectoriser is really, really touchy. For example, the below code
sample works (from DotPVect.hs):
import Data.Array.Parallel.Prelude.Int as I
dotp :: [:Int:] -> [:Int:] -> Int
dotp v w = I.sumP [: (I.*) x y | x <- v, y <- w :]
This however, does not work:
dotp :: [:Int:] -> [:Int:] -> Int
dotp v w = I.sumP [: (Prelude.*) x y | x <- v, y <- w :]
This is because the vectorised code needs to call the vectorised
version of (*). Internally, the vectoriser has a hardwired mapping
from top-level functions to their vectorised versions. That is, it
knows that it should replace calls to
(Data.Array.Parallel.Prelude.Int.*) by calls to
Data.Array.Parallel.Prelude.Base.Int.plusV. There is no vectorised
version of (Prelude.*), however, and there won't be one until we can
vectorise the Prelude. In fact, the vectoriser doesn't even support
classes at the moment. So the rule of thumb is: unless it's in
Data.Array.Parallel.Prelude or you wrote and vectorised it yourself,
it will choke the vectoriser.
I also ran into a few other errs relating to the vectoriser dying - if
I can find some I'll reply to this with some results.
Please do! And please keep using DPH and reporting your results, that
is really useful to us!
FWIW, we'll include some DPH documentation in 6.10 but it still has to
be written...
Roman
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