Fwd: Restricted sums in BoxedRep

Carter Schonwald carter.schonwald at gmail.com
Thu Oct 15 20:27:47 UTC 2020

Ryan: would an unboxed value Mvar, Eg to write StrictMvar (), avoid
creating extra gc pressure / traversal a?

On Thu, Oct 15, 2020 at 4:23 PM Ryan Yates <fryguybob at gmail.com> wrote:

> Ah yes.  In my research work I extended Mutable Constructor fields to
> allow a mutable array field to help with that problem.  This allowed me to
> have a Nil constructor in a sum type and a Node constructor with normal
> fields as well as an array of mutable fields.  Pointer tagging worked as
> expected so a case match on a dereference of a field would not need to
> follow the pointer and no intermediate objects were between Node objects.
> On Thu, Oct 15, 2020 at 4:08 PM David Feuer <david.feuer at gmail.com> wrote:
>> This might be lost in the noise for MVar and TVar, but for arrays, there
>> are tremendous advantages to cutting out the extra heap objects.
>> On Thu, Oct 15, 2020, 1:10 PM Ryan Yates <fryguybob at gmail.com> wrote:
>>> I haven't been following this discussion too closely, but in my research
>>> work I found that some of the benefits that I wanted in this direction were
>>> already there with pointer tagging.
>>> On Thu, Oct 15, 2020 at 12:59 PM David Feuer <david.feuer at gmail.com>
>>> wrote:
>>>> 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> wrote:
>>>>> 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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