I accidentally replied to Herbert privately. I'm forwarding the message to the list. - Takano Akio ---------- Forwarded message ---------- From: Akio Takano Date: Mon, Feb 25, 2013 at 6:15 PM Subject: Re: Race-condition in alternative 'System.Timeout.timeout' implementation To: Herbert Valerio Riedel Hi, I think the problem is that E.unregisterTimeout doesn't guarantee that no timeout will be delivered after it returns. This allows an execution sequence like: 0. Thread A calls timeout2 1. Thread A registers a callback to the event manager 2. Thread A unregisters the callback 3. Thread A exits from the handleJust 4. The callback is triggered, killing thread A If I understand correctly this can be worked around with an extra mutex in timeout2. I'll attach my implementation. It's called timeout3 and is a bit slower than timeout2, but I haven't seen a leaking exception with it. On Mon, Feb 25, 2013 at 7:31 AM, Herbert Valerio Riedel wrote:

Hello *,

I've been experimenting with an alternative implementation of 'System.Timeout.timeout'[1] which avoids the overhead of spawning a new thread for each invocation.

Part of my motivation is to see if I can implement a faster version of

threadWaitReadTimeout :: Int -> Fd -> IO Bool threadWaitReadTimeout to = liftM (maybe False (const True)) . timeout to . threadWaitRead

and thus exploit GHC's event notification system instead of having to reimplement a timeout-manager myself (like popular HTTP server libraries such as Snap or Warp do currently).

The following Haskell program shows a proof-of-concept implementation derived directly from 'System.Timeout.timeout' together with a Criterion benchmark comparing the performance between the original and the alternative 'timeout' function wrapping a 'readMVar' call.

On a i7-3770 with GHC-7.6.2/Linux/64bit ran with "+RTS -A4m -N4", the benchmark shows a 15x improvement for the new implementation (below 1 uS) compared to the original implementation (~13 uS):

,---- | benchmarking id | mean: 22.60933 ns, lb 22.50331 ns, ub 22.73515 ns, ci 0.950 | std dev: 591.0383 ps, lb 509.6189 ps, ub 663.2670 ps, ci 0.950 | found 17 outliers among 100 samples (17.0%) | 17 (17.0%) high mild | variance introduced by outliers: 19.992% | variance is moderately inflated by outliers | | benchmarking timeout_1ms | mean: 13.79584 us, lb 13.62939 us, ub 13.92814 us, ci 0.950 | std dev: 756.3080 ns, lb 524.7628 ns, ub 1.068547 us, ci 0.950 | found 14 outliers among 100 samples (14.0%) | 4 (4.0%) low severe | 5 (5.0%) high mild | 5 (5.0%) high severe | variance introduced by outliers: 52.484% | variance is severely inflated by outliers | | benchmarking timeout2_1ms | mean: 879.8152 ns, lb 874.5223 ns, ub 885.9759 ns, ci 0.950 | std dev: 29.31963 ns, lb 25.65941 ns, ub 32.98116 ns, ci 0.950 | found 9 outliers among 100 samples (9.0%) | 9 (9.0%) high mild | variance introduced by outliers: 28.734% | variance is moderately inflated by outliers | ... `----

Alas there's a race-condition hidden somewhere I'm struggling with; When the timeout is set low enough, the internal 'Timeout2' exceptions leaks outside the 'timeout2' wrapper:

,---- | ... | benchmarking timeout2_10us | newtimeout: <<timeout2>> `----

I've tried rewriting the code but couldn't figure out a way to keep the exception from escaping 'timeout2'. Does the race-condition actually lie in the 'timeout2' implementation -- and if so, is it possible to rewrite 'timeout2' to solve it?

[1]: http://hackage.haskell.org/packages/archive/base/latest/doc/html/System-Time...

cheers, hvr

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