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From Michael McCandless <luc...@mikemccandless.com>
Subject Re: Excessive use of ensureOpen()
Date Sat, 20 Jun 2009 23:07:33 GMT
I agree we should only promise best-effort, not immediate detection on
close, so we shouldn't be checking volatile refCount.

Mike

On Sat, Jun 20, 2009 at 2:32 PM, Yonik Seeley<yonik@lucidimagination.com> wrote:
> On Sat, Jun 20, 2009 at 2:10 PM, Shai Erera<serera@gmail.com> wrote:
>> I noticed both IndexReader and IndexWriter call ensureOpen in almost every
>> method. How critical is this check? Why would someone call any of these when
>> the writer or reader are close?
>
> It's to catch user errors, calling methods after the reader is closed.
>
> I do have a slight issue with it in that it's a penalty to correctly
> implemented programs.  The original implementation checked a
> non-volatile "boolean closed" variable (from what I remember).  I
> argued against ensureOpen() for maxDoc() and numDocs() for performance
> reasons.  At some point, ensureOpen() was changed to check the
> volatile refCount.  On current x86 systems, this volatile read simply
> prevents optimizations and instruction reordering across the volatile
> read, but the penalty on some other systems is greater since it can
> cause/include L1/L2 cache flushes.  It's even worse for something like
> ParallelReader which has multiple levels of checks.
>
> So, IMO: ensureOpen() should not consult volatile variables. open and
> isOpen variables should not be volatile.  Throwing of AlreadyClosed
> exceptions should be on a best-effort basis (not immediately
> guaranteed).
>
> -Yonik
> http://www.lucidimagination.com
>
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