Re: autovacuum truncate exclusive lock round two

On 12/3/2012 5:42 PM, Kevin Grittner wrote:
> Jan Wieck wrote:
>
>> Attached is a new patch that addresses most of the points raised
>> in discussion before.
>>
>> 1) Most of the configuration variables are derived from
>> deadlock_timeout now. The "check for conflicting lock request"
>> interval is deadlock_timeout/10, clamped to 10ms. The "try to
>> acquire exclusive lock" interval is deadlock_timeout/20, also
>> clamped to 10ms. The only GUC variable remaining is
>> autovacuum_truncate_lock_try=2000ms with a range from 0 (just try
>> once) to 20000ms.
>
> If we're going to keep this GUC, we need docs for it.

Certainly. But since we're still debating which and how many GUC 
variables we want, I don't think doc-time has come yet.

>
>> I'd like to point out that this is a significant change in
>> functionality as without the config option for the check
>> interval, there is no longer any possibility to disable the call
>> to LockHasWaiters() and return to the original (deadlock code
>> kills autovacuum) behavior.
>
> Arguably we could simplify the deadlock resolution code a little,
> but it seems like it is probably safer to leave it as a failsafe,
> at least for now.

Thinking about it, I'm not really happy with removing the 
autovacuum_truncate_lock_check GUC at all.

Fact is that the deadlock detection code and the configuration parameter 
for it should IMHO have nothing to do with all this in the first place. 
A properly implemented application does not deadlock. Someone running 
such a properly implemented application should be able to safely set 
deadlock_timeout to minutes without the slightest ill side effect, but 
with the benefit that the deadlock detection code itself does not add to 
the lock contention. The only reason one cannot do so today is because 
autovacuum's truncate phase could then freeze the application with an 
exclusive lock for that long.

I believe the check interval needs to be decoupled from the 
deadlock_timeout again. This will leave us with 2 GUCs at least.


Jan

-- 
Anyone who trades liberty for security deserves neither
liberty nor security. -- Benjamin Franklin