Fwd: Restricted sums in BoxedRep

[Carter Schonwald]

Indeed, I mean things that aren’t pointery, and could be represented by a
tvar paired with a mutable byte array or mvar with mutable byte array, but
which we’d want considered as a single heap object from the rts/gc
perspective.

On Thu, Oct 15, 2020 at 11:58 AM David Feuer <david.feuer at gmail.com> wrote:

> Sorry, unlifted, not unboxed...
>
> On Thu, Oct 15, 2020, 11:57 AM David Feuer <david.feuer at gmail.com> wrote:
>
>> Putting unboxed things in TVar, MVar, etc., is part of Andrew Martin's
>> accepted BoxedRep proposal.
>>
>> On Thu, Oct 15, 2020, 11:44 AM Carter Schonwald <
>> carter.schonwald at gmail.com> wrote:
>>
>>> A related idea that came up recently and is perhaps simpler ties into
>>> this via the lens of having unboxed Mvars/tvars (even if it’s restricted to
>>> just things we can embed in a word64#)
>>>
>>> This came up in
>>> https://gitlab.haskell.org/ghc/ghc/-/issues/18798#note_307410, where
>>> viktor had millions of independent mvars holding what’s essentially a
>>> strict unit ()!
>>>
>>> The motivation in this later scenario is that in high concurrency
>>> settings, the less trivial stuff the gc needs to trace under updates, the
>>> better ghc scales.
>>>
>>> This may not be a use case david has in mind, but certainly seems
>>> related.
>>>
>>> Phrased more succinctly: gc perf dominates large heap / many core
>>> computation in Haskell via sensitivity to allocation volume / mutation
>>> volume (to ensure generational hypothesis stays valid), and providing tools
>>> to incrementally reduce the pressure with local changes would be good.
>>>
>>> So I’d propose / suggest that a baby step towards what david asks would
>>> be for us to work out some manner of unboxed tvar/mvar ref machinery that
>>> supports unboxed values.
>>>
>>>
>>>
>>> On Thu, Oct 15, 2020 at 5:32 AM Andreas Klebinger <
>>> klebinger.andreas at gmx.at> wrote:
>>>
>>>> From a implementors perspective my main questions would be:
>>>>
>>>> * How big is the benefit in practice? How many use cases are there?
>>>> * How bad are the costs? (Runtime overhead, rts complexity, ...)
>>>>
>>>> The details of how this would be exposed to a user would are important.
>>>> But if the costs are too high for the drawbacks then it becomes a moot
>>>> point.
>>>>
>>>>
>>>> David Feuer schrieb am 14.10.2020 um 22:21:
>>>>
>>>> Forwarded from Andrew Martin below. I think we want more than just
>>>> Maybe (more than one null), but the nesting I described is certainly more
>>>> convenience than necessity.
>>>>
>>>> ---------- Forwarded message ---------
>>>> From: Andrew Martin <andrew.thaddeus at gmail.com>
>>>> Date: Wed, Oct 14, 2020, 4:14 PM
>>>> Subject: Re: Restricted sums in BoxedRep
>>>> To: David Feuer <david.feuer at gmail.com>
>>>>
>>>>
>>>> You'll have to forward this to the ghc-devs list to share it with
>>>> others since I'm not currently subscribed to it, but I've had this same
>>>> thought before. It is discussed at
>>>> https://github.com/andrewthad/impure-containers/issues/12. Here's the
>>>> relevant excerpt:
>>>>
>>>>> Relatedly, I was thinking the other day that after finishing
>>>>> implementing
>>>>> https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0203-pointer-rep.rst,
>>>>> I should really look at seeing if it's possible to add this
>>>>> maybe-of-a-lifted value trick straight to GHC. I think that with:
>>>>>
>>>>> data RuntimpRep
>>>>>   = BoxedRep Levity
>>>>>   | MaybeBoxedRep Levity
>>>>>   | IntRep
>>>>>   | ...
>>>>>
>>>>> data BuiltinMaybe :: forall (v :: Levity). TYPE v -> TYPE ('MaybeBoxedRep v)
>>>>>
>>>>> This doesn't have the nesting issues because the kind system prevents
>>>>> nesting. But anyway, back to the original question. I would recommend not
>>>>> using Maybe.Unsafe and using unpacked-maybe instead. The latter is
>>>>> definitely safe, and it only costs an extra machine word of space in each
>>>>> data constructor it gets used in, and it doesn't introduce more
>>>>> indirections.
>>>>>
>>>>
>>>> On Tue, Oct 13, 2020 at 5:47 PM David Feuer <david.feuer at gmail.com>
>>>> wrote:
>>>>
>>>>> Null pointers are widely known to be a lousy language feature in
>>>>> general, but there are certain situations where they're *really* useful for
>>>>> compact representation. For example, we define
>>>>>
>>>>>     newtype TMVar a = TMVar (TVar (Maybe a))
>>>>>
>>>>> We don't, however, actually use the fact that (Maybe a) is lifted. So
>>>>> we could represent this much more efficiently using something like
>>>>>
>>>>>     newtype TMVar a = TMVar (TVar a)
>>>>>
>>>>> where Nothing is represented by a distinguished "null" pointer.
>>>>>
>>>>> While it's possible to implement this sort of thing in user code (with
>>>>> lots of fuss and care), it's not very nice at all. What I'd really like to
>>>>> be able to do is represent certain kinds of sums like this natively.
>>>>>
>>>>> Now that we're getting BoxedRep, I think we can probably make it
>>>>> happen. The trick is to add a special Levity constructor representing sums
>>>>> of particular shapes. Specifically, we can represent a type like this if it
>>>>> is a possibly-nested sum which, when flattened into a single sum, consists
>>>>> of some number of nullary tuples and at most one Lifted or Unlifted type.
>>>>> Then we can have (inline) primops to convert between the BoxedRep and the
>>>>> sum-of-sums representations.
>>>>>
>>>>> Anyone have thoughts on details for what the Levity constructor
>>>>> arguments might look like?
>>>>>
>>>>
>>>>
>>>> --
>>>> -Andrew Thaddeus Martin
>>>>
>>>>
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