Well, a few weeks ago Bertram Felgenhauer came up with a version of IO that acts more like lazy ST. That could be just the thing. He placed it in the public domain/CC0 and told me I could put it up on Hackage if I want. I'll try to do that this week, but no promises. I could forward his email if you just want to try it out. On May 14, 2016 2:31 PM, "Harendra Kumar" <harendra.ku...@gmail.com> wrote:
> The difference seems to be entirely due to memory pressure. At list size > 1000 both pure version and IO version perform equally. But as the size of > the list increases the pure version scales linearly while the IO version > degrades exponentially. Here are the execution times per list element in ns > as the list size increases: > > Size of list Pure IO > 1000 8.7 8.3 > 10000 8.7 18 > 100000 8.8 63 > 1000000 9.3 786 > > This seems to be due to increased GC activity in the IO case. The GC stats > for list size 1 million are: > > IO case: %GC time 66.1% (61.1% elapsed) > Pure case: %GC time 2.6% (3.3% elapsed) > > Not sure if there is a way to write this code in IO monad which can reduce > this overhead. > > -harendra > > > On 14 May 2016 at 17:10, Harendra Kumar <harendra.ku...@gmail.com> wrote: > > > > You are right about the way IO code is generated because of the ordering > semantics. The IO version builds the list entirely in a recursive fashion > before returning it while the pure code builds it lazily. I wrote very > simple versions to keep things simpler: > > > > Pure version: > > > > f [] = [] > > f (x : xs) = x : f xs > > > > > > time 11.08 ms (10.86 ms .. 11.34 ms) > > Measured for a million elements in the list > > > > 104,041,264 bytes allocated in the heap > > 16,728 bytes copied during GC > > 35,992 bytes maximum residency (1 sample(s)) > > 21,352 bytes maximum slop > > 1 MB total memory in use (0 MB lost due to fragmentation) > > > > > > IO version: > > f [] = return [] > > f (x : xs) = do > > rest <- f xs > > return $ x : rest > > > > time 79.66 ms (75.49 ms .. 82.55 ms) > > > > 208,654,560 bytes allocated in the heap > > 121,045,336 bytes copied during GC > > 27,679,344 bytes maximum residency (8 sample(s)) > > 383,376 bytes maximum slop > > 66 MB total memory in use (0 MB lost due to fragmentation) > > > > Even though this is a small program not doing much and therefore > enhancing even small differences to a great degree, I am not sure if I can > completely explain the difference in slowness of the order of 7.5x by just > the recursive vs lazy building of the list. I am wondering if there is > anything that is worth further investigating and improving here. > > > > -harendra > > > > On 12 May 2016 at 05:41, Dan Doel <dan.d...@gmail.com> wrote: > > > > > > On Tue, May 10, 2016 at 4:45 AM, Harendra Kumar > > > <harendra.ku...@gmail.com> wrote: > > > > Thanks Dan, that helped. I did notice and suspect the update frame > and the > > > > unboxed tuple but given my limited knowledge about ghc/core/stg/cmm > I was > > > > not sure what is going on. In fact I thought that the intermediate > tuple > > > > should get optimized out since it is required only because of the > realworld > > > > token which is not real. But it might be difficult to see that at > this > > > > level? > > > > > > The token exists as far as the STG level is concerned, I think, > > > because that is the only thing ensuring that things happen in the > > > right order. And the closure must be built to have properly formed > > > STG. So optimizing away the closure building would have to happen at a > > > level lower than STG, and I guess there is no such optimization. I'm > > > not sure how easy it would be to do. > > > > > > > What you are saying may be true for the current implementation but > in theory > > > > can we eliminate the intermediate closure? > > > > > > I don't know. I'm a bit skeptical that building this one closure is > > > the only thing causing your code to be a factor of 5 slower. For > > > instance, another difference in the core is that the recursive call > > > corresponding to the result s2 happens before allocating the > > > additional closure. That is the case statement that unpacks the > > > unboxed tuple. And the whole loop happens this way, so it is actually > > > building a structure corresponding to the entire output list in memory > > > rather eagerly. > > > > > > By contrast, your pure function is able to act in a streaming fashion, > > > if consumed properly, where only enough of the result is built to keep > > > driving the rest of the program. It probably runs in constant space, > > > while your IO-based loop has a footprint linear in the size of the > > > input list (in addition to having slightly more overhead per character > > > because of the one extra thunk), because it is a more eager program. > > > > > > And having an asymptotically larger memory footprint is more likely to > > > explain a 5x slowdown than allocating one extra thunk for each list > > > concatenation, I think. (Of course, it could also be some other > > > factor, as well.) > > > > > > You should probably run with +RTS -s (compiling with -rtsopts), and > > > see if the IO version has a much larger maximum residency. > > > > > > Anyhow, this is fundamental to writing the algorithm using IO. It's an > > > algorithm that's a sequence of steps that happen in order, the number > > > of steps is proportional to the input list, and part of those steps is > > > putting stuff in memory for the results. > > > > > > > So why is that? Why can't we generate (++) inline similar to (:)? > How do we > > > > make this decision? Is there a theoretical reason for this or this > is just > > > > an implementation artifact? > > > > > > The difference between these two is that (++) is a function call, and > > > (:) is a constructor. Constructors are a special case, and don't need > > > to have all the provisions that functions in general do. The best way > > > to learn what the differences are is probably to read the paper about > > > the STG machine. > > > > > > Hope this is a more fruitful lead, but it may be that there's not a > > > lot that can be done, without doing some baroque things to your > > > algorithm, because of the necessity of its using IO. > > > > > > -- Dan > > _______________________________________________ > Glasgow-haskell-users mailing list > Glasgow-haskell-users@haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/glasgow-haskell-users > >
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