ArrayArrays

Fri Aug 28 21:43:06 UTC 2015

Some form of MutableStruct# with a known number of words and a known number
of pointers is basically what Ryan Yates was suggesting above, but where
the word counts were stored in the objects themselves.

Given that it'd have a couple of words for those counts it'd likely want to
be something we build in addition to MutVar# rather than a replacement.

On the other hand, if we had to fix those numbers and build info tables
that knew them, and typechecker support, for instance, it'd get rather
invasive.

Also, a number of things that we can do with the 'sized' versions above,
like working with evil unsized c-style arrays directly inline at the end of
the structure cease to be possible, so it isn't even a pure win if we did
the engineering effort.

I think 90% of the needs I have are covered just by adding the one
primitive. The last 10% gets pretty invasive.

-Edward

On Fri, Aug 28, 2015 at 5:30 PM, Ryan Newton <rrnewton at gmail.com> wrote:

> I like the possibility of a general solution for mutable structs (like Ed
> said), and I'm trying to fully understand why it's hard.
>
> So, we can't unpack MutVar into constructors because of object identity
> problems. But what about directly supporting an extensible set of unlifted
> MutStruct# objects, generalizing (and even replacing) MutVar#? That may be
> too much work, but is it problematic otherwise?
>
> Needless to say, this is also critical if we ever want best in class
> lockfree mutable structures, just like their Stm and sequential
> counterparts.
>
> On Fri, Aug 28, 2015 at 4:43 AM Simon Peyton Jones <simonpj at microsoft.com>
> wrote:
>
>> At the very least I'll take this email and turn it into a short article.
>>
>> Yes, please do make it into a wiki page on the GHC Trac, and maybe make a
>> ticket for it.
>>
>>
>> Thanks
>>
>>
>>
>> Simon
>>
>>
>>
>> *From:* Edward Kmett [mailto:ekmett at gmail.com]
>> *Sent:* 27 August 2015 16:54
>> *To:* Simon Peyton Jones
>> *Cc:* Manuel M T Chakravarty; Simon Marlow; ghc-devs
>> *Subject:* Re: ArrayArrays
>>
>>
>>
>> An ArrayArray# is just an Array# with a modified invariant. It points
>> directly to other unlifted ArrayArray#'s or ByteArray#'s.
>>
>>
>>
>> While those live in #, they are garbage collected objects, so this all
>> lives on the heap.
>>
>>
>>
>> They were added to make some of the DPH stuff fast when it has to deal
>> with nested arrays.
>>
>>
>>
>> I'm currently abusing them as a placeholder for a better thing.
>>
>>
>>
>> The Problem
>>
>> -----------------
>>
>>
>>
>> Consider the scenario where you write a classic doubly-linked list in
>> Haskell.
>>
>>
>>
>> data DLL = DLL (IORef (Maybe DLL) (IORef (Maybe DLL)
>>
>>
>>
>> Chasing from one DLL to the next requires following 3 pointers on the
>> heap.
>>
>>
>>
>> DLL ~> IORef (Maybe DLL) ~> MutVar# RealWorld (Maybe DLL) ~> Maybe DLL ~>
>> DLL
>>
>>
>>
>> That is 3 levels of indirection.
>>
>>
>>
>> We can trim one by simply unpacking the IORef with -funbox-strict-fields
>> or UNPACK
>>
>>
>>
>> We can trim another by adding a 'Nil' constructor for DLL and worsening
>> our representation.
>>
>>
>>
>> data DLL = DLL !(IORef DLL) !(IORef DLL) | Nil
>>
>>
>>
>> but now we're still stuck with a level of indirection
>>
>>
>>
>> DLL ~> MutVar# RealWorld DLL ~> DLL
>>
>>
>>
>> This means that every operation we perform on this structure will be
>> about half of the speed of an implementation in most other languages
>> assuming we're memory bound on loading things into cache!
>>
>>
>>
>> Making Progress
>>
>> ----------------------
>>
>>
>>
>> I have been working on a number of data structures where the indirection
>> of going from something in * out to an object in # which contains the real
>> pointer to my target and coming back effectively doubles my runtime.
>>
>>
>>
>> We go out to the MutVar# because we are allowed to put the MutVar# onto
>> the mutable list when we dirty it. There is a well defined write-barrier.
>>
>>
>>
>> I could change out the representation to use
>>
>>
>>
>> data DLL = DLL (MutableArray# RealWorld DLL) | Nil
>>
>>
>>
>> I can just store two pointers in the MutableArray# every time, but this
>> doesn't help _much_ directly. It has reduced the amount of distinct
>> addresses in memory I touch on a walk of the DLL from 3 per object to 2.
>>
>>
>>
>> I still have to go out to the heap from my DLL and get to the array
>> object and then chase it to the next DLL and chase that to the next array.
>> I do get my two pointers together in memory though. I'm paying for a card
>> marking table as well, which I don't particularly need with just two
>> pointers, but we can shed that with the "SmallMutableArray#" machinery
>> added back in 7.10, which is just the old array code a a new data type,
>> which can speed things up a bit when you don't have very big arrays:
>>
>>
>>
>> data DLL = DLL (SmallMutableArray# RealWorld DLL) | Nil
>>
>>
>>
>> But what if I wanted my object itself to live in # and have two mutable
>> fields and be able to share the sme write barrier?
>>
>>
>>
>> An ArrayArray# points directly to other unlifted array types. What if we
>> have one # -> * wrapper on the outside to deal with the impedence mismatch
>> between the imperative world and Haskell, and then just let the
>> ArrayArray#'s hold other arrayarrays.
>>
>>
>>
>> data DLL = DLL (MutableArrayArray# RealWorld)
>>
>>
>>
>> now I need to make up a new Nil, which I can just make be a special
>> MutableArrayArray# I allocate on program startup. I can even abuse pattern
>> synonyms. Alternately I can exploit the internals further to make this
>> cheaper.
>>
>>
>>
>> Then I can use the readMutableArrayArray# and writeMutableArrayArray#
>> calls to directly access the preceding and next entry in the linked list.
>>
>>
>>
>> So now we have one DLL wrapper which just 'bootstraps me' into a strict
>> world, and everything there lives in #.
>>
>>
>>
>> next :: DLL -> IO DLL
>>
>> next (DLL m) = IO $ \s -> case readMutableArrayArray# s of
>>
>>    (# s', n #) -> (# s', DLL n #)
>>
>>
>>
>> It turns out GHC is quite happy to optimize all of that code to keep
>> things unboxed. The 'DLL' wrappers get removed pretty easily when they are
>> known strict and you chain operations of this sort!
>>
>>
>>
>> Cleaning it Up
>>
>> ------------------
>>
>>
>>
>> Now I have one outermost indirection pointing to an array that points
>> directly to other arrays.
>>
>>
>>
>> I'm stuck paying for a card marking table per object, but I can fix that
>> by duplicating the code for MutableArrayArray# and using a
>> SmallMutableArray#. I can hack up primops that let me store a mixture of
>> SmallMutableArray# fields and normal ones in the data structure.
>> Operationally, I can even do so by just unsafeCoercing the existing
>> SmallMutableArray# primitives to change the kind of one of the arguments it
>> takes.
>>
>>
>>
>> This is almost ideal, but not quite. I often have fields that would be
>> best left unboxed.
>>
>>
>>
>> data DLLInt = DLL !Int !(IORef DLL) !(IORef DLL) | Nil
>>
>>
>>
>> was able to unpack the Int, but we lost that. We can currently at best
>> point one of the entries of the SmallMutableArray# at a boxed or at a
>> MutableByteArray# for all of our misc. data and shove the int in question
>> in there.
>>
>>
>>
>> e.g. if I were to implement a hash-array-mapped-trie I need to store
>> masks and administrivia as I walk down the tree. Having to go off to the
>> side costs me the entire win from avoiding the first pointer chase.
>>
>>
>>
>> But, if like Ryan suggested, we had a heap object we could construct that
>> had n words with unsafe access and m pointers to other heap objects, one
>> that could put itself on the mutable list when any of those pointers
>> changed then I could shed this last factor of two in all circumstances.
>>
>>
>>
>> Prototype
>>
>> -------------
>>
>>
>>
>> Over the last few days I've put together a small prototype implementation
>> with a few non-trivial imperative data structures for things like Tarjan's
>> link-cut trees, the list labeling problem and order-maintenance.
>>
>>
>>
>> https://github.com/ekmett/structs
>>
>>
>>
>> Notable bits:
>>
>>
>>
>> Data.Struct.Internal.LinkCut
>> <https://github.com/ekmett/structs/blob/9ff2818f888aff4789b7a41077a674a10d15e6ee/src/Data/Struct/Internal/LinkCut.hs>
>> provides an implementation of link-cut trees in this style.
>>
>>
>>
>> Data.Struct.Internal
>> <https://github.com/ekmett/structs/blob/9ff2818f888aff4789b7a41077a674a10d15e6ee/src/Data/Struct/Internal.hs>
>> provides the rather horrifying guts that make it go fast.
>>
>>
>>
>> Once compiled with -O or -O2, if you look at the core, almost all the
>> references to the LinkCut or Object data constructor get optimized away,
>> and we're left with beautiful strict code directly mutating out underlying
>> representation.
>>
>>
>>
>> At the very least I'll take this email and turn it into a short article.
>>
>>
>>
>> -Edward
>>
>>
>>
>> On Thu, Aug 27, 2015 at 9:00 AM, Simon Peyton Jones <
>> simonpj at microsoft.com> wrote:
>>
>> Just to say that I have no idea what is going on in this thread.  What is
>> ArrayArray?  What is the issue in general?  Is there a ticket? Is there a
>> wiki page?
>>
>>
>>
>> If it’s important, an ab-initio wiki page + ticket would be a good thing.
>>
>>
>>
>> Simon
>>
>>
>>
>> *From:* ghc-devs [mailto:ghc-devs-bounces at haskell.org] *On Behalf Of *Edward
>> Kmett
>> *Sent:* 21 August 2015 05:25
>> *To:* Manuel M T Chakravarty
>> *Cc:* Simon Marlow; ghc-devs
>> *Subject:* Re: ArrayArrays
>>
>>
>>
>> When (ab)using them for this purpose, SmallArrayArray's would be very
>> handy as well.
>>
>>
>>
>> Consider right now if I have something like an order-maintenance
>> structure I have:
>>
>>
>>
>> data Upper s = Upper {-# UNPACK #-} !(MutableByteArray s) {-# UNPACK #-}
>> !(MutVar s (Upper s)) {-# UNPACK #-} !(MutVar s (Upper s))
>>
>>
>>
>> data Lower s = Lower {-# UNPACK #-} !(MutVar s (Upper s)) {-# UNPACK #-}
>> !(MutableByteArray s) {-# UNPACK #-} !(MutVar s (Lower s)) {-# UNPACK #-}
>> !(MutVar s (Lower s))
>>
>>
>>
>> The former contains, logically, a mutable integer and two pointers, one
>> for forward and one for backwards. The latter is basically the same thing
>> with a mutable reference up pointing at the structure above.
>>
>>
>>
>> On the heap this is an object that points to a structure for the
>> bytearray, and points to another structure for each mutvar which each point
>> to the other 'Upper' structure. So there is a level of indirection smeared
>> over everything.
>>
>>
>>
>> So this is a pair of doubly linked lists with an upward link from the
>> structure below to the structure above.
>>
>>
>>
>> Converted into ArrayArray#s I'd get
>>
>>
>>
>> data Upper s = Upper (MutableArrayArray# s)
>>
>>
>>
>> w/ the first slot being a pointer to a MutableByteArray#, and the next 2
>> slots pointing to the previous and next previous objects, represented just
>> as their MutableArrayArray#s. I can use sameMutableArrayArray# on these for
>> object identity, which lets me check for the ends of the lists by tying
>> things back on themselves.
>>
>>
>>
>> and below that
>>
>>
>>
>> data Lower s = Lower (MutableArrayArray# s)
>>
>>
>>
>> is similar, with an extra MutableArrayArray slot pointing up to an upper
>> structure.
>>
>>
>>
>> I can then write a handful of combinators for getting out the slots in
>> question, while it has gained a level of indirection between the wrapper to
>> put it in * and the MutableArrayArray# s in #, that one can be basically
>> erased by ghc.
>>
>>
>>
>> Unlike before I don't have several separate objects on the heap for each
>> thing. I only have 2 now. The MutableArrayArray# for the object itself, and
>> the MutableByteArray# that it references to carry around the mutable int.
>>
>>
>>
>> The only pain points are
>>
>>
>>
>> 1.) the aforementioned limitation that currently prevents me from
>> stuffing normal boxed data through a SmallArray or Array into an ArrayArray
>> leaving me in a little ghetto disconnected from the rest of Haskell,
>>
>>
>>
>> and
>>
>>
>>
>> 2.) the lack of SmallArrayArray's, which could let us avoid the card
>> marking overhead. These objects are all small, 3-4 pointers wide. Card
>> marking doesn't help.
>>
>>
>>
>> Alternately I could just try to do really evil things and convert the
>> whole mess to SmallArrays and then figure out how to unsafeCoerce my way to
>> glory, stuffing the #'d references to the other arrays directly into the
>> SmallArray as slots, removing the limitation  we see here by aping the
>> MutableArrayArray# s API, but that gets really really dangerous!
>>
>>
>>
>> I'm pretty much willing to sacrifice almost anything on the altar of
>> speed here, but I'd like to be able to let the GC move them and collect
>> them which rules out simpler Ptr and Addr based solutions.
>>
>>
>>
>> -Edward
>>
>>
>>
>> On Thu, Aug 20, 2015 at 9:01 PM, Manuel M T Chakravarty <
>> chak at cse.unsw.edu.au> wrote:
>>
>> That’s an interesting idea.
>>
>> Manuel
>>
>> > Edward Kmett <ekmett at gmail.com>:
>>
>> >
>> > Would it be possible to add unsafe primops to add Array# and
>> SmallArray# entries to an ArrayArray#? The fact that the ArrayArray#
>> entries are all directly unlifted avoiding a level of indirection for the
>> containing structure is amazing, but I can only currently use it if my leaf
>> level data can be 100% unboxed and distributed among ByteArray#s. It'd be
>> nice to be able to have the ability to put SmallArray# a stuff down at the
>> leaves to hold lifted contents.
>> >
>> > I accept fully that if I name the wrong type when I go to access one of
>> the fields it'll lie to me, but I suppose it'd do that if i tried to use
>> one of the members that held a nested ArrayArray# as a ByteArray# anyways,
>> so it isn't like there is a safety story preventing this.
>> >
>> > I've been hunting for ways to try to kill the indirection problems I
>> get with Haskell and mutable structures, and I could shoehorn a number of
>> them into ArrayArrays if this worked.
>> >
>> > Right now I'm stuck paying for 2 or 3 levels of unnecessary indirection
>> compared to c/java and this could reduce that pain to just 1 level of
>> unnecessary indirection.
>> >
>> > -Edward
>>
>> > _______________________________________________
>> > ghc-devs mailing list
>> > ghc-devs at haskell.org
>> > http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
>>
>>
>>
>>
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