Ryan, would you please file a ticket on GHC trac explaining WHY ghc makes it difficult to support CAS for boxed vectors? And you would like to be able to do vs what you have had to do? Understanding why its currently difficult to write new primitives for lockfree concurrency in user land seems like something worth digging deeper into! (and perhaps trying to fix!) as much as CAS on Storable / Unboxed would be cool, It seems like it'd only work on instances that are actually 4,8,16 byte locations, right? 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. On Wed, Nov 20, 2013 at 10:57 AM, Ryan Newton 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...

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@alum.mit.edu> wrote:

...

On Wed, Nov 20, 2013 at 10:06 AM, Ryan Newton 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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nothing blocking adding the primops right now from a technical standpoint (though it'd be too late for the 7.8 merge window). The main blocker is i'm moving over the next month + I'm amidst sorting transitioning from consulting to some other work. (life and all). i want to also add the inline primops to the native code gen and -fvia-c in tandem with baking in the LLVM support. Which will take a teeny bit of extra work, but really very much worth it I think. Glad you agree about the constructors being handy! Should make is possible to have the CAS vector stuff working as a selfcontained experiment that just works ™ with current vector. On Thu, Nov 21, 2013 at 2:05 PM, Ryan Newton wrote:

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@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 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...

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@alum.mit.edu> wrote:

...

On Wed, Nov 20, 2013 at 10:06 AM, Ryan Newton 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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[@Aleksey] Very good point! I'd missed that vector does the SOA->AOS business too. (I thought that was just Repa & Accelerate.) In any case, an unboxed vector of tuples can't *actually* support atomic CAS anyway, so not supporting it is inevitable. It does mean that exposeMutableByteArray can't be a method of the Unbox class. It would need to be another class capturing the subset of vector types that are "Atomic" (support concurrent/atomic memory ops).