Compare-and-swap semantics in GHC
Hi, java.util.concurrent.atomic package provides two flavors of compare and set operations: compareAndSet and weakCompareAndSet. The latter does not create any happens-before orderings, so we can use it where no guarantees with respect to previous or subsequent reads and writes of any variables other than the target of the weakCompareAndSet are required. I'd like to ask if the compare-and-swap function provided by GHC runtime (cas() in includes/stg/SMP.h) is strong or weak. If it is strong, do all use cases of cas() require this semantics? Regards, Kwang Yul Seo
cas in ghc has sequential semantics.
i'm not sure if every use of it needs that semantics, if you identify
examples where weaker operations may suffice, please share!
On Thu, Dec 5, 2013 at 2:58 AM, KwangYul Seo
Hi,
java.util.concurrent.atomic package provides two flavors of compare and set operations: compareAndSet and weakCompareAndSet. The latter does not create any happens-before orderings, so we can use it where no guarantees with respect to previous or subsequent reads and writes of any variables other than the target of the weakCompareAndSet are required.
I'd like to ask if the compare-and-swap function provided by GHC runtime (cas() in includes/stg/SMP.h) is strong or weak. If it is strong, do all use cases of cas() require this semantics?
Regards, Kwang Yul Seo
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hrm, i'm reading your mesage a bit more closely, and i'm not sure i
understand the distinction you mean by strong vs weak.
Do you mean "strong" as in STM style semantics for reads and writes? (ie if
i'm doing a CAS on memory location x, it totally orders all reads and
writes to ANY location y!=x too?).
if you mean something like that, I think CAS isn't strong, though perhaps
the best way to answer you question is to read the source for CAS and look
up the semantics of the assembly instructions on various architectures!
cheers (and apologies for the confusion on my part)
-Carter
On Thu, Dec 5, 2013 at 3:48 AM, Carter Schonwald wrote: cas in ghc has sequential semantics. i'm not sure if every use of it needs that semantics, if you identify
examples where weaker operations may suffice, please share! On Thu, Dec 5, 2013 at 2:58 AM, KwangYul Seo Hi, java.util.concurrent.atomic package provides two flavors of compare and
set operations: compareAndSet and weakCompareAndSet. The latter does not
create any happens-before orderings, so we can use it where no guarantees
with respect to previous or subsequent reads and writes of any variables
other than the target of the weakCompareAndSet are required. I'd like to ask if the compare-and-swap function provided by GHC runtime
(cas() in includes/stg/SMP.h) is strong or weak. If it is strong, do all
use cases of cas() require this semantics? Regards,
Kwang Yul Seo _______________________________________________
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It's strong. How to tell: x86 doesn't support really have any kind of weak CAS implementation; and our PowerPC implementation loops over LL/SC. (oh, and our asm is marked volatile) As for whether or not there are any uses of CAS that don't need these guarantees, probably the best way to figure this out is to look for any uses of CAS, for which better algorithms using LL/SC are known (since that's really the only well known implementation of this "weak" compare and swap). Of course, it's worth noting that any of these improvements would not apply to today's x86, so they'd be of somewhat limited applicability. Cheers, Edward Excerpts from KwangYul Seo's message of 2013-12-04 23:58:50 -0800:
Hi,
java.util.concurrent.atomic package provides two flavors of compare and set operations: compareAndSet and weakCompareAndSet. The latter does not create any happens-before orderings, so we can use it where no guarantees with respect to previous or subsequent reads and writes of any variables other than the target of the weakCompareAndSet are required.
I'd like to ask if the compare-and-swap function provided by GHC runtime (cas() in includes/stg/SMP.h) is strong or weak. If it is strong, do all use cases of cas() require this semantics?
Regards, Kwang Yul Seo
participants (3)
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Carter Schonwald -
Edward Z. Yang -
KwangYul Seo