[Nix-dev] Haskell packages in the binary cache broken?

[Vladimír Čunát]

On 05/27/2013 06:02 PM, Karn Kallio wrote:
>> I don't know
>> where the non-determinism comes from, but if they just use a random
>> generator, it would be enough to seed it e.g. from the hash of $out
>> instead of the standard sources.
>
> As I understand, GHC has many uses of unsafeInterleaveIO in its
> implementation, so the order of effects such as the generation of fresh names
> will depend on how the IO manager schedules evaluation of those thunks, which
> could depend on eg time for receiving a filesystem response, which may depend
> on installed hardware (eg amount of RAM for filesystem buffer) and state of
> the machine (eg how many other processes are using IO at the moment).  So the
> nondeterminism arises not via a controlled mechanism like a random generator
> but from races influenced by the configuration and state of the wider machine.

I see, but then passing +RTS -V0 to the compiler might make it 
deterministic. I suppose it will impact build performance by increasing 
I/O wait time, but IMHO builders are usually so loaded that total 
rebuild performance won't suffer so much (all these are pure 
speculations, I've never tried).

[-V option]: 
http://www.haskell.org/ghc/docs/latest/html/users_guide/runtime-control.html#rts-options-misc

I would suggest this only for nix builds, *not* for user-run GHC from store.


Vlada


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