From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
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To: | Simon Riggs <simon(at)2ndQuadrant(dot)com> |
Cc: | Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, pgsql-hackers(at)postgresql(dot)org |
Subject: | Re: Re: [COMMITTERS] pgsql: Tune GetSnapshotData() during Hot Standby by avoiding loop |
Date: | 2010-04-19 14:55:32 |
Message-ID: | 14068.1271688932@sss.pgh.pa.us |
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Simon Riggs <simon(at)2ndQuadrant(dot)com> writes:
> On Mon, 2010-04-19 at 17:44 +0300, Heikki Linnakangas wrote:
>>> Choices are
>>> 1. Check RecoveryInProgress() once outside of lock, plus wild rumour of
>>> Murphy
>>> 2. Check RecoveryInProgress() before and after holding lock
>>> 3. Check RecoveryInProgress() while holding lock
>>
>> 4. Check RecoveryInProgress() once outside of lock, and scan the
>> ProcArray anyway, just in case. That's what we did before this patch.
>> Document that takenDuringRecovery == true means that the snapshot was
>> most likely taken during recovery, but there is some race conditions
>> where takenDuringRecovery is true even though the snapshot was taken
>> just after recovery finished. AFAICS all of the other current uses of
>> takenDuringRecovery work fine with that.
> Checking RecoveryInProgress() is much cheaper than scanning the whole
> ProcArray, so (4) is definitely worse than 1-3.
If the lock we're talking about is an LWLock, #3 is okay. If it's a
spinlock, not so much.
regards, tom lane
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