Message-ID: <4B57501F.8030804@enterprisedb.com>
Date: Wed, 20 Jan 2010 20:49:03 +0200
From: Heikki Linnakangas <heikki.linnakangas@enterprisedb.com>
Organization: EnterpriseDB
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To: Tom Lane <tgl@sss.pgh.pa.us>
CC: Andres Freund <andres@anarazel.de>, Simon Riggs <simon@2ndquadrant.com>,
	pgsql-hackers@postgresql.org, Hiroyuki Yamada <yamada@kokolink.net>
Subject: Re: Synchronization primitives (Was: Re: An example of
	bugs for Hot Standby)
References: <200912151111.AA00168@silver.kokolink.net>
	<201001201740.22069.andres@anarazel.de>
	<14578.1264006776@sss.pgh.pa.us>
	<201001201841.15760.andres@anarazel.de>
	<4B5744C1.40806@enterprisedb.com> <16135.1264011365@sss.pgh.pa.us>
In-Reply-To: <16135.1264011365@sss.pgh.pa.us>
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Tom Lane wrote:
> Heikki Linnakangas <heikki.linnakangas@enterprisedb.com> writes:
>> Streaming Replication introduces a few places with a polling pattern
>> like this (in pseudocode):
> 
>> while()
>> {
>>   /* Check if variable in shared has advanced beoynd X */
>>   SpinLockAcquire()
>>   localvar = sharedvar;
>>   SpinLockRelease()
>>   if (localvar > X)
>>     break;
> 
>>   /* Not yet. Sleep
>>   pg_usleep(100);
>> }
> 
> I trust there's a CHECK_FOR_INTERRUPTS in there ...
> 
>> It would be nice to have a new synchronization primitive for that.
> 
> Maybe.  The lock, the variable, the comparison operation, and the sleep
> time all seem rather specific to each application.  Not sure that it'd
> really buy much to try to turn it into a generic subroutine.

My point is that we should replace such polling loops with something
non-polling, using wait/signal or semaphores or something. That gets
quite a bit more complex. You'd probably still have the loop, but
instead of pg_usleep() you'd call some new primitive function that waits
until the shared variable changes.

-- 
  Heikki Linnakangas
  EnterpriseDB   http://www.enterprisedb.com