Re: Performance degradation when factoring out common code

While trying to come up with a minimal example I discovered one more puzzling thing. runghc is fastest, ghc is slower, ghc with optimization is slowest. This is completely reverse of the expected order. ghc -O1 (-O2 is similar): time 15.23 ms (14.72 ms .. 15.73 ms) ghc -O0: time 3.612 ms (3.548 ms .. 3.728 ms) runghc: time 2.250 ms (2.156 ms .. 2.348 ms) I am grokking it further. Any pointers will be helpful. I understand that -O2 can sometimes be slower e.g. aggressive inlining can sometimes be counterproductive. But 4x variation is a lot and this is the case with -O1 as well which should be relatively safer than -O2 in general. Worst of all runghc is significantly faster than ghc. What's going on? -harendra On 8 September 2017 at 18:49, Harendra Kumar wrote:

I will try creating a minimal example and open a ticket for the inlining problem, the one I am sure about.

-harendra

On 8 September 2017 at 18:35, Simon Peyton Jones wrote:

...

*I know that this is not an easy request*, but can either of you produce a small example that demonstrates your problem? If so, please open a ticket.

I don’t like hearing about people having to use trial and error with INLINE or SPECIALISE pragmas. But I can’t even begin to solve the problem unless I can reproduce it.

Simon

*From:* ghc-devs [mailto:ghc-devs-bounces@haskell.org] *On Behalf Of *Harendra Kumar *Sent:* 08 September 2017 13:50 *To:* Mikolaj Konarski *Cc:* ghc-devs@haskell.org *Subject:* Re: Performance degradation when factoring out common code

I should also point out that I saw performance improvements by manually factoring out and propagating some common expressions to outer loops in performance sensitive paths. Now I have made this a habit to do this manually. Not sure if something like this has also been fixed with that ticket or some other ticket.

-harendra

On 8 September 2017 at 17:34, Harendra Kumar wrote:

Thanks Mikolaj! I have seen some surprising behavior quite a few times recently and I was wondering whether GHC should do better. In one case I had to use SPECIALIZE very aggressively, in another version of the same code it worked well without that. I have been doing a lot of trial and error with the INLINE/NOINLINE pragmas to figure out what the right combination is. Sometimes it just feels like black magic, because I cannot find a rationale to explain the behavior. I am not sure if there are any more such problems lurking in, perhaps this is an area where some improvement looks possible.

-harendra

On 8 September 2017 at 17:10, Mikolaj Konarski < mikolaj.konarski@gmail.com> wrote:

Hello,

I've had a similar problem that's been fixed in 8.2.1:

https://ghc.haskell.org/trac/ghc/ticket/12603

You can also use some extreme global flags, such as

ghc-options: -fexpose-all-unfoldings -fspecialise-aggressively

to get most the GHC subtlety and shyness out of the way when experimenting.

Good luck Mikolaj

On Fri, Sep 8, 2017 at 11:21 AM, Harendra Kumar wrote:

...

Hi,

I have this code snippet for the bind implementation of a Monad:

AsyncT m >>= f = AsyncT $ \_ stp yld -> let run x = (runAsyncT x) Nothing stp yld yield a _ Nothing = run $ f a yield a _ (Just r) = run $ f a <> (r >>= f) in m Nothing stp yield

I want to have multiple versions of this implementation parameterized by a function, like this:

bindWith k (AsyncT m) f = AsyncT $ \_ stp yld -> let run x = (runAsyncT x) Nothing stp yld yield a _ Nothing = run $ f a yield a _ (Just r) = run $ f a `k` (bindWith k r f) in m Nothing stp yield

And then the bind function becomes:

(>>=) = bindWith (<>)

But this leads to a performance degradation of more than 10%. inlining does not help, I tried INLINE pragma as well as the "inline" GHC builtin. I thought this should be a more or less straightforward replacement making the second version equivalent to the first one. But apparently there is something going on here that makes it perform worse.

I did not look at the core, stg or asm yet. Hoping someone can quickly comment on it. Any ideas why is it so? Can this be worked around somehow?

Thanks, Harendra

...

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