Proposal: provide cas and barriers symbols even without -threaded
Ryan Newton
rrnewton at gmail.com
Sat Jul 20 08:47:49 CEST 2013
Sorry, "rewrite" was too overloaded a term to use here. I was just
referring to the proposal to "substitute the cas funcall with the right
llvm operation".
That is, the approach would pattern match for the CMM code "ccall cas" or
"foreign "C" cas" (I'm afraid I don't know the difference between those)
and replace it with the equivalent LLVM op, right?
I think the assumption there is that the native codegen would still have to
suffer the funcall overhead and use the C versions. I don't know exactly
what the changes would look like to make barriers/CAS all proper inline
primops, because it would have to reproduce in the code generator all the
platform-specific #ifdef'd C code that is currently in SMP.h. Which I
guess is doable, but probably only for someone who knows the native GHC
codegen properly...
On Sat, Jul 20, 2013 at 2:30 AM, Carter Schonwald <
carter.schonwald at gmail.com> wrote:
> Ryan, could you explain what you want more precisely? Specifically what
> you want in terms of exposed primops using the terminology / vocabulary in
> http://llvm.org/docs/LangRef.html#ordering and
> http://llvm.org/docs/Atomics.html ?
>
> I'll first do the work for just the LLVM backend, and I"ll likely need
> some active guidance / monitoring for the native codegen analogues
>
> (also asked this on ticket for documentation purposes)
>
>
> On Sat, Jul 20, 2013 at 2:18 AM, Ryan Newton <rrnewton at gmail.com> wrote:
>
>> Hi Carter,
>>
>> Yes, SMP.h is where I've copy pasted the duplicate functionality from
>> (since I can't presently rely on linking the symbols).
>>
>> Your proposal for the LLVM backend sounds **great**. But it also is
>> going to provide additional constraints for getting "atomic-primops" right.
>>
>> The goal of atomic-primops is to be a stable Haskell-level interface
>> into the relevant CAS and fetch-and-add stuff. The reason this is
>> important is that one has to be very careful to defeat the GHC optimizer in
>> all the relevant places and make pointer equality a reliable property. I
>> would like to get atomic-primops to work reliably in 7.4, 7.6 [and 7.8] and
>> have more "native" support in future GHC releases, where maybe the foreign
>> primops would become unecessary. (They are a pain and have already exposed
>> one blocking cabal bug, fixed in upcoming 1.17.)
>>
>> A couple additional suggestions for the proposal in ticket #7883:
>>
>> - we should use more unique symbols than "cas", especially for this
>> rewriting trick. How about "ghc_cas" or something?
>> - it would be great to get at least fetch-and-add in addition to CAS
>> and barriers
>> - if we reliably provide this set of special symbols, libraries like
>> atomic-primops may use them in the .cmm and benefit from the CMM->LLVM
>> substitutions
>> - if we include all the primops I need in GHC proper the previous
>> bullet will stop applying ;-)
>>
>> Cheers,
>> -Ryan
>>
>> P.S. Just as a bit of motivation, here are some recent performance
>> numbers. We often wonder about how close our "pure values in a box"
>> approach comes to efficient lock-free structures. Well here are some
>> numbers about using a proper unboxed counter in the Haskell heap, vs using
>> an IORef Int and atomicModifyIORef': Up to 100X performance difference
>> on some platforms for microbenchmarks that hammer a counter:
>>
>>
>> https://github.com/rrnewton/haskell-lockfree-queue/blob/fb12d1121690553e4f737af258848f279147ea24/AtomicPrimops/DEVLOG.md#20130718-timing-atomic-counter-ops
>>
>> And here are the performance and scaling advantages of using ChaseLev
>> (based on atomic-primops), over a traditional pure-in-a-box structure
>> (IORef Data.Seq). The following are timings of ChaseLev/traditional
>> respectively on a 32 core westmere:
>>
>> fib(42) 1 threads: 21s
>> fib(42) 2 threads: 10.1s
>> fib(42) 4 threads: 5.2s (100%prod)
>> fib(42) 8 threads: 2.7s - 3.2s (100%prod)
>> fib(42) 16 threads: 1.28s
>> fib(42) 24 threads: 1.85s
>> fib(42) 32 threads: 4.8s (high variance)
>>
>> (hive) fib(42) 1 threads: 41.8s (95% prod)
>> (hive) fib(42) 2 threads: 25.2s (66% prod)
>> (hive) fib(42) 4 threads: 14.6s (27% prod, 135GB alloc)
>> (hive) fib(42) 8 threads: 17.1s (26% prod)
>> (hive) fib(42) 16 threads: 16.3s (13% prod)
>> (hive) fib(42) 24 threads: 21.2s (30% prod)
>> (hive) fib(42) 32 threads: 29.3s (33% prod)
>>
>> And that is WITH the inefficiency of doing a "ccall" on every single
>> atomic operation.
>>
>> Notes on parfib performance are here:
>>
>>
>> https://github.com/rrnewton/haskell-lockfree-queue/blob/d6d3e9eda2a487a5f055b1f51423954bb6b6bdfa/ChaseLev/Test.hs#L158
>>
>>
>>
>>
>>
>>
>>
>> On Fri, Jul 19, 2013 at 5:05 PM, Carter Schonwald <
>> carter.schonwald at gmail.com> wrote:
>>
>>> ryan, the relevant machinery on the C side is here, see
>>> ./includes/stg/SMP.h :
>>> https://github.com/ghc/ghc/blob/7cc8a3cc5c2970009b83844ff9cc4e27913b8559/includes/stg/SMP.h
>>>
>>> (unless i'm missing something)
>>>
>>>
>>> On Fri, Jul 19, 2013 at 4:53 PM, Carter Schonwald <
>>> carter.schonwald at gmail.com> wrote:
>>>
>>>> Ryan,
>>>> if you look at line 270, you'll see the CAS is a C call
>>>> https://github.com/ghc/ghc/blob/95e6865ecf06b2bd80fa737e4fa4a24beaae25c5/rts/PrimOps.cmm#L270
>>>>
>>>>
>>>> What Simon is alluding to is some work I started (but need to finish)
>>>> http://ghc.haskell.org/trac/ghc/ticket/7883 is the relevant ticket,
>>>> and I'll need to sort out doing the same on the native code gen too
>>>>
>>>> there ARE no write barrier primops, they're baked into the CAS
>>>> machinery in ghc's rts
>>>>
>>>>
>>>> On Fri, Jul 19, 2013 at 1:02 PM, Ryan Newton <rrnewton at gmail.com>wrote:
>>>>
>>>>> Yes, I'd absolutely rather not suffer C call overhead for these
>>>>> functions (or the CAS functions). But isn't that how it's done currently
>>>>> for the casMutVar# primop?
>>>>>
>>>>>
>>>>> https://github.com/ghc/ghc/blob/95e6865ecf06b2bd80fa737e4fa4a24beaae25c5/rts/PrimOps.cmm#L265
>>>>>
>>>>> To avoid the overhead, is it necessary to make each primop in-line
>>>>> rather than out-of-line, or just to get rid of the "ccall"?
>>>>>
>>>>> Another reason it would be good to package these with GHC is that I'm
>>>>> having trouble building robust libraries of foreign primops that work under
>>>>> all "ways" (e.g. GHCI). For example, this bug:
>>>>>
>>>>> https://github.com/rrnewton/haskell-lockfree-queue/issues/10
>>>>>
>>>>> If I write .cmm code that depends on RTS functionality like
>>>>> stg_MUT_VAR_CLEAN_info, then it seems to work fine when in compiled mode
>>>>> (with/without threading, profiling), but I get link errors from GHCI where
>>>>> these symbols aren't defined.
>>>>>
>>>>> I've got a draft of the relevant primops here:
>>>>>
>>>>>
>>>>> https://github.com/rrnewton/haskell-lockfree-queue/blob/master/AtomicPrimops/cbits/primops.cmm
>>>>>
>>>>> Which includes:
>>>>>
>>>>> - variants of CAS for MutableArray# and MutableByteArray#
>>>>> - fetch-and-add for MutableByteArray#
>>>>>
>>>>> Also, there are some tweaks to support the new "ticketed" interface
>>>>> for safer CAS:
>>>>>
>>>>>
>>>>> http://hackage.haskell.org/packages/archive/atomic-primops/0.3/doc/html/Data-Atomics.html#g:3
>>>>>
>>>>> I started adding some of these primops to GHC proper (still as
>>>>> out-of-line), but not all of them. I had gone with the foreign primop
>>>>> route instead...
>>>>>
>>>>> https://github.com/rrnewton/ghc/commits/master
>>>>>
>>>>> -Ryan
>>>>>
>>>>> P.S. Where is the write barrier primop? I don't see it listed in
>>>>> prelude/primops.txt...
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> On Fri, Jul 19, 2013 at 11:41 AM, Carter Schonwald <
>>>>> carter.schonwald at gmail.com> wrote:
>>>>>
>>>>>> I guess I should find the time to finish the CAS primop work I
>>>>>> volunteered to do then. Ill look into in a few days.
>>>>>>
>>>>>>
>>>>>> On Friday, July 19, 2013, Simon Marlow wrote:
>>>>>>
>>>>>>> On 18/07/13 14:17, Ryan Newton wrote:
>>>>>>>
>>>>>>>> The "atomic-primops" library depends on symbols such as
>>>>>>>> store_load_barrier and "cas", which are defined in SMP.h. Thus the
>>>>>>>> result is that if the program is linked WITHOUT "-threaded", the
>>>>>>>> user
>>>>>>>> gets a linker error about undefined symbols.
>>>>>>>>
>>>>>>>> The specific place it's used is in the 'foreign "C"' bits of this
>>>>>>>> .cmm code:
>>>>>>>>
>>>>>>>> https://github.com/rrnewton/**haskell-lockfree-queue/blob/**
>>>>>>>> 87e63b21b2a6c375e93c30b98c28c1**d04f88781c/AtomicPrimops/**
>>>>>>>> cbits/primops.cmm<https://github.com/rrnewton/haskell-lockfree-queue/blob/87e63b21b2a6c375e93c30b98c28c1d04f88781c/AtomicPrimops/cbits/primops.cmm>
>>>>>>>>
>>>>>>>> I'm trying to explore hacks that will enable me to pull in those
>>>>>>>> functions during compile time, without duplicating a whole bunch of
>>>>>>>> code
>>>>>>>> from the RTS. But it's a fragile business.
>>>>>>>>
>>>>>>>> It seems to me that some of these routines have general utility. In
>>>>>>>> future versions of GHC, could we consider linking in those routines
>>>>>>>> irrespective of "-threaded"?
>>>>>>>>
>>>>>>>
>>>>>>> We should make the non-THREADED versions EXTERN_INLINE too, so that
>>>>>>> there will be (empty) functions to call in rts/Inlines.c. Want to submit a
>>>>>>> patch?
>>>>>>>
>>>>>>> A better solution would be to make them into primops. You don't
>>>>>>> really want to be calling out to a C function to implement a memory
>>>>>>> barrier. We have this for write_barrier(), but none of the others so far.
>>>>>>> Of couse that's a larger change.
>>>>>>>
>>>>>>> Cheers,
>>>>>>> Simon
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> ______________________________**_________________
>>>>>>> ghc-devs mailing list
>>>>>>> ghc-devs at haskell.org
>>>>>>> http://www.haskell.org/**mailman/listinfo/ghc-devs<http://www.haskell.org/mailman/listinfo/ghc-devs>
>>>>>>>
>>>>>>
>>>>>
>>>>
>>>
>>
>
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