Re: autovacuum truncate exclusive lock round two - Mailing list pgsql-hackers

On 10/25/2012 10:12 AM, Stephen Frost wrote:
> Jan,
>
> * Jan Wieck (JanWieck@Yahoo.com) wrote:
>> The problem case this patch is dealing with is rolling window tables
>> that experienced some bloat. The typical example is a log table,
>> that has new data constantly added and the oldest data constantly
>> purged out. This data normally rotates through some blocks like a
>> rolling window. If for some reason (purging turned off for example)
>> this table bloats by several GB and later shrinks back to its normal
>> content, soon all the used blocks are at the beginning of the heap
>> and we find tens of thousands of empty pages at the end. Only now
>> does the second scan take more than 1000ms and autovacuum is at risk
>> to get killed while at it.
>
> My concern is that this could certainly also happen to a heavily updated
> table in an OLTP type of environment where the requirement to take a
> heavy lock to clean it up might prevent it from ever happening..  I was
> simply hoping we could find a mechanism to lock just those pages we're
> getting ready to nuke rather than the entire relation.  Perhaps we can
> consider how to make those changes alongside of changes to eliminate or
> reduce the extent locking that has been painful (for me at least) when
> doing massive parallel loads into a table.

I've been testing this with loads of 20 writes/s to that bloated table. 
Preventing not only the clean up, but the following ANALYZE as well is 
precisely what happens. There may be multiple ways how to get into this 
situation, but once you're there the symptoms are the same. Vacuum fails 
to truncate it and causing a 1 second hiccup every minute, while vacuum 
is holding the exclusive lock until the deadlock detection code of 
another transaction kills it.

My patch doesn't change the logic how we ensure that we don't zap any 
data by accident with the truncate and Tom's comments suggest we should 
stick to it. It only makes autovacuum check frequently if the 
AccessExclusiveLock is actually blocking anyone and then get out of the 
way.

I would rather like to discuss any ideas how to do all this without 3 
new GUCs.

In the original code, the maximum delay that autovacuum can cause by 
holding the exclusive lock is one deadlock_timeout (default 1s). It 
would appear reasonable to me to use max(deadlock_timeout/10,10ms) as 
the interval to check for a conflicting lock request. For another 
transaction that needs to access the table this is 10 times faster than 
it is now and still guarantees that autovacuum will make some progress 
with the truncate.

The other two GUCs control how often and how fast autovacuum tries to 
acquire the exclusive lock in the first place. Since we actively release 
the lock *because someone needs it* it is pretty much guaranteed that 
the immediate next lock attempt fails. We on purpose do a 
ConditionalLockRelation() because there is a chance to deadlock. The 
current code only tries one lock attempt and gives up immediately. I 
don't know from what to derive a good value for how long to retry, but 
the nap time in between tries could be a hardcoded 20ms or using the 
cost based vacuum nap time (which defaults to 20ms).

Any other ideas are welcome.


Thanks,
Jan

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