What is the mutator?
jason.dusek at gmail.com
Thu Aug 6 20:35:32 EDT 2009
2009/08/06 Jost Berthold <berthold at mathematik.uni-marburg.de>:
> as Malcolm already said, the "mutator" in this text is the/a
> thread evaluating some Haskell expression.
I want to thank everyone for taking the time to clarify that
to me; I'm now much more able to follow discussions of Haskell
> 1. Garbage collection and mutator running concurrently: while
> they usually do, they do not _have_ to exclude each other,
> but not doing so means that the objects they are treating
> have to be locked.
So this is the part that actually lead me here. Say you are
implementing a network server, for example -- you don't want
to have big spikes in the request latency due to GC. Not that
Haskell is so much worse off relative to Java, say; Erlang is
the only language I'm aware of that takes concurrent GC
seriously. However, it seems that this problem is hard to
solve for Haskell:
Parallel GC is when the whole system stops and performs
multi-threaded GC, as opposed to "concurrent GC", which is
when the GC runs concurrently with the program. We think
concurrent GC is unlikely to be practical in the Haskell
setting, due to the extra synchronisation needed in the
mutator. However, there may always be clever techniques that
we haven't discovered, and synchronisation might become less
expensive, so the balance may change in the future.
-- Simon Marlow
So I wonder, to what degree is GC latency controllable in
Haskell? It seems that, pending further research, we can not
hope for concurrent GC.
> 2. About "Blackholing": in the sequential evaluation (where
> hitting a blackhole indeed means to have a loop), some
> better performance can be gained by not blackholing a thunk
> immediately, so this was done in GHC earlier. However, it
> increases the chance for 2 mutator threads to evaluate the
> same thunk (double work), and we got better performance by
> blackholing immediately.
Can blackholing too early could result in non-termination
("...hitting a blackhole indeed means to have a loop")? Then
it's not just a matter of performance when we do it?
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