Re: [Postgres-xc-general] failed to find proc - increasing numProcs

[Aaron J. <aja...@re...>]

CFLAGS="-O2"
gcc version 4.8.1 (Ubuntu/Linaro 4.8.1-10ubuntu9)

The failed evaluation occurs on line 421 of backend/storage/ipc/procarray.c

The test portion of the clause fails.  I'm not entirely sure why gcc specifically fails, but if I were taking an educated guess, it would be that arrayP->numProcs was volatile and the resultant value of the test was optimized and cached.  I've used several techniques (none of which I like) to fool gcc into believing the value is volatile and discarding the value of arrayP->numProcs.  It concerns me more because the ProcArrayLock should be locked during this sequence.

Aaron

________________________________
From: 鈴木 幸市 [ko...@in...]
Sent: Sunday, April 13, 2014 7:55 PM
To: Aaron Jackson
Cc: pos...@li...
Subject: Re: [Postgres-xc-general] failed to find proc - increasing numProcs

Thank you Aaron for the detailed analysis.   As long as the issue is just for XC, we need a fix for it to work correctly regardless the compiler optimization.

Did to locate where such wrong estimation takes place?   And what compilation option did you use?

They are very helpful.

Best;
---
Koichi Suzuki

2014/04/12 11:40、Aaron Jackson <aja...@re...<mailto:aja...@re...>> のメール：

It appears that problem is a compiler optimization issue.  I narrowed the issue down to the loop at the end of the ProcArrayRemove method.  I'm not entirely sure why, but the compiler generated code that evaluates the test block of the loop improperly.  Since changing the compiler options, the problem has been resolved.

Aaron

________________________________
From: Aaron Jackson [aja...@re...<mailto:aja...@re...>]
Sent: Friday, April 11, 2014 1:07 AM
To: pos...@li...<mailto:pos...@li...>
Subject: Re: [Postgres-xc-general] failed to find proc - increasing numProcs

I forgot to mention that if I injected a context switch (sleep(0) did the trick as did an elog statement) during the test in the ProcArrayRemove, that it no longer failed.  Hopefully that will help in understanding the reasons why that may have triggered the ProcArrayRemove to succeed.
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