Lazy ST vs concurrency

[Simon Peyton Jones]

We don’t want to do this on a per-module basis do we, as -fatomic-eager-blackholing would suggest.  Rather, on per-thunk basis, no?  Which thunks, precisely?   I think perhaps precisely thunks one of whose free variables has type (Sttate# s) for some s.  These are thunks that consume a state token, and must do so no more than once.

If entering such thunks was atomic, could we kill off noDuplicate#?

I still don’t understand exactly what noDuplicate# does, what problem it solves, and how the problem it solves relates to this LazyST problem.

We need some kind of fix for 8.2.  Simon what do you suggest?

Simon

From: Simon Marlow [mailto:marlowsd at gmail.com]
Sent: 30 January 2017 21:51
To: David Feuer <david at well-typed.com>
Cc: Simon Peyton Jones <simonpj at microsoft.com>; ghc-devs at haskell.org
Subject: Re: Lazy ST vs concurrency

On 30 January 2017 at 16:18, David Feuer <david at well-typed.com<mailto:david at well-typed.com>> wrote:
I forgot to CC ghc-devs the first time, so here's another copy.

I was working on #11760 this weekend, which has to do with concurrency
breaking lazy ST. I came up with what I thought was a pretty decent solution (
https://phabricator.haskell.org/D3038 ). Simon Peyton Jones, however, is quite
unhappy about the idea of sticking this weird unsafePerformIO-like code
(noDup, which I originally implemented as (unsafePerformIO . evaluate), but
which he finds ugly regardless of the details) into fmap and (>>=).  He's also
concerned that the noDuplicate# applications will kill performance in the
multi-threaded case, and suggests he would rather leave lazy ST broken, or
even remove it altogether, than use a fix that will make it slow sometimes,
particularly since there haven't been a lot of reports of problems in the
wild.

In a nutshell, I think we have to fix this despite the cost - the implementation is incorrect and unsafe.

Unfortunately the mechanisms we have right now to fix it aren't ideal - noDuplicate# is a bigger hammer than we need.  All we really need is some way to make a thunk atomic, it would require some special entry code to the thunk which did atomic eager-blackholing.  Hmm, now that I think about it, perhaps we could just have a flag, -fatomic-eager-blackholing.  We already do this for CAFs, incidentally. The idea is to compare-and-swap the blackhole info pointer into the thunk, and if we didn't win the race, just re-enter the thunk (which is now a blackhole).  We already have the cmpxchg MachOp, so It shouldn't be more than a few lines in the code generator to implement it.  It would be too expensive to do by default, but doing it just for Control.Monad.ST.Lazy should be ok and would fix the unsafety.

(I haven't really thought this through, just an idea off the top of my head, so there could well be something I'm overlooking here...)

Cheers
Simon


My view is that leaving it broken, even if it only causes trouble
occasionally, is simply not an option. If users can't rely on it to always
give correct answers, then it's effectively useless. And for the sake of
backwards compatibility, I think it's a lot better to keep it around, even if
it runs slowly multithreaded, than to remove it altogether.

Note to Simon PJ: Yes, it's ugly to stick that noDup in there. But lazy ST has
always been a bit of deep magic. You can't *really* carry a moment of time
around in your pocket and make its history happen only if necessary. We can
make it work in GHC because its execution model is entirely based around graph
reduction, so evaluation is capable of driving execution. Whereas lazy IO is
extremely tricky because it causes effects observable in the real world, lazy
ST is only *moderately* tricky, causing effects that we have to make sure
don't lead to weird interactions between threads. I don't think it's terribly
surprising that it needs to do a few more weird things to work properly.

David

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