On Tue, Nov 8, 2011 at 7:11 AM, David Fox wrote:

On Tue, Nov 8, 2011 at 6:31 AM, Daniel Fischer wrote:

...

On the haskell-cafe as well as the beginners mailing lists, there frequently (for some value of frequent) are posts where the author inquires about a badly performing programme, in the form of stack overflows, space leaks or slowness.

Often this is because they compiled their programme without optimisations, simply recompiling with -O or -O2 yields a decently performing programme.

So I wonder, should ghc compile with -O1 by default? What would be the downsides?

I think this is an excellent idea. Even better -O2.

FWIW gcc defaults to -O0.

On Tue, Nov 8, 2011 at 8:16 AM, Simon Marlow wrote:

On 08/11/2011 14:31, Daniel Fischer wrote:

...

On the haskell-cafe as well as the beginners mailing lists, there frequently (for some value of frequent) are posts where the author inquires about a badly performing programme, in the form of stack overflows, space leaks or slowness.

Often this is because they compiled their programme without optimisations, simply recompiling with -O or -O2 yields a decently performing programme.

So I wonder, should ghc compile with -O1 by default? What would be the downsides?

I understand the problem.  However, -O has a couple of serious downsides:

 1. it costs about 2x compile time and memory usage

 2. it forces a lot more recompilation to happen after changes,     due to inter-module optimisations.

most people know about 1, but I think 2 is probably less well-known. When in the edit-compile-debug cycle it really helps to have -O off, because your compiles will be so much quicker due to both factors 1 & 2.

So the default -O setting is a careful compromise, trying to hit a good compile-time/runtime tradeoff.  Perhaps we're more sensitive in Haskell because -O can easily give you an order of magnitude or more speedup, whereas in C you're likely to get a pretty consistent 30% or so.  The difference between -O and -O2 is another careful tradeoff.

Also bear in mind that using GHCi gives you another 2x speedup in compilation (approx), but 30x slowdown in runtime (varies wildly from program to program though).  And subsequent recompiles are much faster because GHCi has cached a lot of interfaces.

I suppose we should really run an up to date set of benchmarks on some real Haskell programs (i.e. not nofib) and reconsider how we set these defaults.  I really doubt that we'll want to turn on -O by default, though.

We should remember that we are only talking about which default leads to the best outcome when, due to inexperience, someone fails to set the option the way they want it.

On Tue, Nov 8, 2011 at 3:01 PM, Daniel Fischer wrote:

On Tuesday 08 November 2011, 17:16:27, Simon Marlow wrote:

...

most people know about 1, but I think 2 is probably less well-known. When in the edit-compile-debug cycle it really helps to have -O off, because your compiles will be so much quicker due to both factors 1 & 2.

Of course. So defaulting to -O1 would mean one has to specify -O0 in the .cabal or Makefile resp. on the command line during development, which certainly is an inconvenience.

AFAIK, Cabal already uses -O1 by default. Cheers, -- Felipe.

How much does using ghc without cabal imply a newer programmer? I don't use cabal when trying out small bits of code (maybe I should be using ghci), but am otherwise always using cabal. On Wed, Nov 9, 2011 at 3:18 AM, Duncan Coutts wrote:

...

On Tue, Nov 8, 2011 at 3:01 PM, Daniel Fischer wrote:

...

On Tuesday 08 November 2011, 17:16:27, Simon Marlow wrote:

...

most people know about 1, but I think 2 is probably less well-known. When in the edit-compile-debug cycle it really helps to have -O off, because your compiles will be so much quicker due to both factors 1 &

On 9 November 2011 00:17, Felipe Almeida Lessa wrote: 2.

...

...

Of course. So defaulting to -O1 would mean one has to specify -O0 in the .cabal or Makefile resp. on the command line during development, which certainly is an inconvenience.

AFAIK, Cabal already uses -O1 by default.

Indeed, and cabal check / hackage upload complain if you put -O{n} in your .cabal file.

The recommended method during development is to use:

$ cabal configure -O0

Duncan

_______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users

On 9 November 2011 00:16, Simon Marlow wrote:

On 08/11/2011 14:31, Daniel Fischer wrote:

...

On the haskell-cafe as well as the beginners mailing lists, there frequently (for some value of frequent) are posts where the author inquires about a badly performing programme, in the form of stack overflows, space leaks or slowness.

Often this is because they compiled their programme without optimisations, simply recompiling with -O or -O2 yields a decently performing programme.

So I wonder, should ghc compile with -O1 by default? What would be the downsides?

I understand the problem.  However, -O has a couple of serious downsides:

 1. it costs about 2x compile time and memory usage

 2. it forces a lot more recompilation to happen after changes,     due to inter-module optimisations.

most people know about 1, but I think 2 is probably less well-known. When in the edit-compile-debug cycle it really helps to have -O off, because your compiles will be so much quicker due to both factors 1 & 2.

So the default -O setting is a careful compromise, trying to hit a good compile-time/runtime tradeoff.  Perhaps we're more sensitive in Haskell because -O can easily give you an order of magnitude or more speedup, whereas in C you're likely to get a pretty consistent 30% or so.  The difference between -O and -O2 is another careful tradeoff.

Also bear in mind that using GHCi gives you another 2x speedup in compilation (approx), but 30x slowdown in runtime (varies wildly from program to program though).  And subsequent recompiles are much faster because GHCi has cached a lot of interfaces.

I suppose we should really run an up to date set of benchmarks on some real Haskell programs (i.e. not nofib) and reconsider how we set these defaults.  I really doubt that we'll want to turn on -O by default, though.

I don't think compile time is an issue for new users when building HelloWorld.hs and getting the hang of basic algorithms and data structures. Anyone could explicitly set -O0 if they are worried about compile times for a larger project. Having some optimizations on by default would help give new users a better impression of how code written in Haskell performs. Currently the first impression people have comes from seeing their code compiled into the slowest version GHC can produce :) Conrad.

On Tue, Nov 8, 2011 at 11:28 PM, wrote:

I don't agree that GHC's user interface should be optimized for newcomers to Haskell. GHC is an industrial-strength compiler with some very advanced features; the majority of its target audience is professional programmers. Let its interface reflect that fact.

As Simon explained, GHC's current defaults are a very nice point in the programming space for people who are actively building and changing their programs.

It's easy to build arguments for either side, but my experience as a professional developer is that new devs don't know what arguments they need for reasonable performance, often knowing even what various optimization flags do, but experienced developers do know the difference between -O0 and -O1, and frequently need -debug (not a default option) more than -O0. Seasoned GHC users can find that -O0 gives miserably slow compile times, and fall back to GHCi for edit/rebuild cycles... which still aren't terribly fast if you're using GHC's advanced features. I have a couple small modules that take 10 minutes each to compile on a current Core i7 at -O0, and -O2 really doesn't take much longer. GHCi is very slightly faster but I'll still head directly downstairs for a coffee as soon as either of these bad boys need rebuilding... and still make it back upstairs before they're done. And so I'd prefer the default to be -O1 or even -O2 and have people who really need it use -O0. GHC shouldn't be painful on purpose, industrial strength or not. -n