Fwd: Restricted sums in BoxedRep

David Feuer david.feuer at gmail.com
Thu Oct 15 16:58:05 UTC 2020

Yes, that's something quite different. We'd need a whole different heap
object type for such MVars and TVars. My approach covers the case where the
unboxed thing can only take on a few values, for some value of "a few"
which, depending on implementation, may or may not be very small. If the
nulls point to actual heap objects in pre-allocated pinned memory (say),
then up to 64 or so might be reasonable. If they point to "invalid" address
space, then the numbers could go up a good bit.

On Thu, Oct 15, 2020, 12:50 PM Carter Schonwald <carter.schonwald at gmail.com>

> 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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