Best way to get to CAS/fetch-and-add on unboxed vectors?
Ryan Newton
rrnewton at gmail.com
Thu Nov 21 19:05:49 UTC 2013
I'm not sure why this would be a ticket, because it's not really a bug or
an issue with GHC. But I did go ahead and write up the state of affairs in
a Haskell wiki page here:
http://www.haskell.org/haskellwiki/AtomicMemoryOps
There's just a fundamental impedance mismatch between having a lazy/pure
world where pointer equality is meaningless, and operations based on
pointer-equality like CAS. I think we've papered over that as best we
could with the "Ticketed" interface describe in that wiki, but I don't
think there's anything else I could have asked of GHC to make this any
easier.
But as you know there are several improvements that can be made to make
what we've got better. Are you blocked on anything wrt to adding direct
LLVM support for the primops in question?
On Wed, Nov 20, 2013 at 4:38 PM, Carter Schonwald <
carter.schonwald at gmail.com> wrote:
> I though Vector instances had their constructors exported so that you can
> get at the underlying Primitive vectors, and and Primitive vectors have a
> bytearray underneath!
> I don't understand why Vector/MVector needs to be changed here to support
> that info.
> For the Atomic operations, it seems like you just really want to only
> support the operations on vector types directly expressable with Primitive
> vectors. Perhaps i'm not understanding though.
>
As far as I can see if the user gives you a "Data.Vector.Unboxed.MVector s
Int8", what you're getting is the result of this type function:
newtype instance MVector s Int8 = MV_Int8 (P.MVector s Int8)
.. from Unboxed.Base.hs. The only thing that's provided for "MVector s a"
types generally are instances of the Vector/MVector classes, which don't
say how to get the pointer(s).
BUT... you make a good point about the constructors (e.g. MV_Int8) being
exposed! It seems like we CAN make our own type family that mirrors the
MVector one, and which leverages *each* of those "newtype instance"
equations, matching on an MV_Int8 and grabbing the underlying Primitive
vector... Nice!
On Wed, Nov 20, 2013 at 10:57 AM, Ryan Newton <rrnewton at gmail.com> wrote:
>
>> We do expose CAS for boxed data, and we depend on that for various
>> concurrent data structures (e.g. Michael-scott queues).
>>
>> But it's a messy business:
>>
>> <explanation> You have to fight hard against the GHC optimizer to prevent
>> it from performing optimizations (e.g. unbox and rebox an Int) that
>> completely destroy pointer equality (in all runs of the program). The way
>> that we've codified our solution to this is to use abstract "Ticket"s
>> (based on the Any kind) that GHC is not supposed to be able to see through.
>> These are used to abstractly represent old reads when you come back to do
>> a new CAS operation:
>>
>>
>> http://hackage.haskell.org/package/atomic-primops-0.4/docs/Data-Atomics.html#g:2
>>
>> Even then, there was a bunch of careful NOINLINE's and other business
>> necessary to achieve something workable, and it's highly GHC specific. If
>> we were to expose CAS on boxed "Data.Vector", I think it would only make
>> sense as the ticketed interface above (raw CAS is useless, and will just
>> condemn others to the same problems we had).
>> </explanation>
>>
>> As a result of all this the CAS for Data.Vector would have a different
>> signature (unless we wanted to force Storable/Unbox to use Tickets), and
>> thus couldn't go in the "MVector" type class along with all the other basic
>> operations that are uniform across representations.
>>
>> That said, it's easy to provide CAS from a separate "vector-atomics"
>> package, because Data.Vector exposes the MVector constructor that lets you
>> get at the underlying MutableArray. So we certainly can provide the
>> functionality somewhere, somehow.
>>
>> -Ryan
>>
>>
>>
>> On Wed, Nov 20, 2013 at 10:21 AM, Jan-Willem Maessen <
>> jmaessen at alum.mit.edu> wrote:
>>
>>> On Wed, Nov 20, 2013 at 10:06 AM, Ryan Newton <rrnewton at gmail.com>wrote:
>>>
>>>> (3) The most invasive (but best for the user) change would be to extend
>>>> the basic MVector interface to include notions of concurrency-related
>>>> operations right inside the vector package (CAS, etc). But they should
>>>> still probably go in a separate class (not class MVector), just because
>>>> they will be specific to Unboxed and Storable vectors rather than boxed
>>>> ones....
>>>>
>>>
>>> Any reason we shouldn't be able to CAS on boxed vectors? Every
>>> architecture supports a pointer-sized CAS. What "equality" means for CAS,
>>> of course, has to be rather carefully defined, but that's equally true for
>>> the unboxed types.
>>>
>>> -Jan
>>>
>>>
>>>>
>>>> -Ryan
>>>>
>>>>
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>>>
>>
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