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Subject: Re: Review Request 41915: Fixup TaskObserverTest to respect thread
 memory models.
From: "Maxim Khutornenko" <maxim@apache.org>
To: "Bill Farner" <wfarner@apache.org>, "Maxim Khutornenko" <maxim@apache.org>
Cc: "John Sirois" <john.sirois@gmail.com>,
 "Aurora" <reviews@aurora.apache.org>
Date: Tue, 05 Jan 2016 04:29:57 -0000
Message-ID: <20160105042957.26044.63884@reviews.apache.org>
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Sender: "Maxim Khutornenko" <noreply@reviews.apache.org>
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Reply-To: "Maxim Khutornenko" <maxim@apache.org>

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> On Jan. 5, 2016, 3:25 a.m., Maxim Khutornenko wrote:
> > src/main/python/apache/thermos/observer/task_observer.py, line 72
> > <https://reviews.apache.org/r/41915/diff/1/?file=1181509#file1181509line72>
> >
> >     We usually put this default initialization directly into the arg list, e.g:'stop_event=threading.Event()'.
> 
> John Sirois wrote:
>     I'm leery of that for mutable objects like an Event.  Surprising things happen if/when the containing object gets constructed a 2nd time and the single default Event has been mutated!
> 
> John Sirois wrote:
>     Maxim - your test originally so your call.  For a refresh - see here: https://reviews.apache.org/r/35527/
> 
> John Sirois wrote:
>     Disregard comment immediately above - now moved up under Bill's comment further above where it belongs.

Not sure how the current approach helps with anything. If a constructor is called once with `stop_event` and another time without it, wouldn't we end up in the same state with mutated field due to `stop_event` arg being None during the second call?


- Maxim


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On Jan. 5, 2016, 3:02 a.m., John Sirois wrote:
> 
> -----------------------------------------------------------
> This is an automatically generated e-mail. To reply, visit:
> https://reviews.apache.org/r/41915/
> -----------------------------------------------------------
> 
> (Updated Jan. 5, 2016, 3:02 a.m.)
> 
> 
> Review request for Aurora, Maxim Khutornenko and Bill Farner.
> 
> 
> Bugs: AURORA-1570
>     https://issues.apache.org/jira/browse/AURORA-1570
> 
> 
> Repository: aurora
> 
> 
> Description
> -------
> 
> Previously, a mock threading.Event was waited on in one thread
> and the count of waits was read in another thread.  Most thread
> memory models do not guaranty reads are fresh in this scenario
> unless there is a memory barrier of some sort forcing per-cpu
> caches to be flushed.
> 
> This change uses the underlying threading.Event as the memory
> barrier instead of mocking it and just wraps the event to record
> calls manually.
> 
>  src/main/python/apache/thermos/observer/task_observer.py      |  5 +++--
>  src/test/python/apache/thermos/observer/test_task_observer.py | 36 ++++++++++++++++++++++++------------
>  2 files changed, 27 insertions(+), 14 deletions(-)
> 
> 
> Diffs
> -----
> 
>   src/main/python/apache/thermos/observer/task_observer.py 1485de8faef52716f11b82a3556064de26c67427 
>   src/test/python/apache/thermos/observer/test_task_observer.py ace15c5305e75fac3a82971f4d71b92bcb37bafc 
> 
> Diff: https://reviews.apache.org/r/41915/diff/
> 
> 
> Testing
> -------
> 
> Before this change I got a failure between 1/5 and 1/10th of the
> time via:
> ```
> while true
> do
>   ./pants test src/test/python/apache/thermos/observer/ -- -kTaskObserverTest
> done
> ```
> 
> After the change I cannot trigger the failure.
> 
> 
> Thanks,
> 
> John Sirois
> 
>


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