[GHC] #16065: Stack squeezing during context switches makes for non-determinism in allocations

[GHC]

#16065: Stack squeezing during context switches makes for non-determinism in
allocations
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        Reporter:  sgraf             |                Owner:  (none)
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:  ⊥
       Component:  Runtime System    |              Version:  8.6.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #4450, #8861      |  Differential Rev(s):
       Wiki Page:                    |
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Old description:

> As #4450 and #8611 show, stack squeezing in the RTS makes allocation
> numbers between two different runs of the same binary non-deterministic,
> because its effect is depending on when context switches are bound to
> happen.
>
> A short-term solution might be to deactivate stack squeezing for
> vulnerable benchmarks with `+RTS -Z` like in Phab:D5460, but IMO a more
> elegant solution would be to only deactivate stack squeezing in
> `threadPause` calls that happen due to context switches. Would you agree?

New description:

 As #4450 and #8611 show, stack squeezing in the RTS makes allocation
 numbers between two different runs of the same binary non-deterministic,
 because its effect is depending on when context switches are bound to
 happen.

 A short-term solution might be to deactivate stack squeezing for
 vulnerable benchmarks with ~~`+RTS -Z`~~ `+RTS -V0` like in Phab:D5460,
 but IMO a more elegant solution would be to only deactivate stack
 squeezing in `threadPause` calls that happen due to context switches.
 Would you agree?

--

Comment (by sgraf):

 Updated the patch for #8611 to reflect this.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16065#comment:8>
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