Re: ugly locking corner cases ...

On Oct 4, 2010, at 1:23 PM, Heikki Linnakangas wrote:

> On 04.10.2010 14:02, Hans-Jürgen Schönig wrote:
>> it seems we have found a fairly nasty problem.
>> imagine a long transaction which piles up XX.XXX of locks (count on pg_locks) inside the same transaction by doing
sometasty savepoints, with hold cursors and so on. 
>> in this case we see that a normal count issued in a second database connection will take ages. in a practical case
wedid a plain seq_scan in connection 2. instead of 262 ms (cached case) it started to head north linearily with the
numberof locks taken by connection 1. in an extreme case it took around 1.5 hours or so (on XXX.XXX pg_locks entries). 
>>
>> i tracked down the issue quickly and make the following profile (in 10k locks or so):
>>
>> Flat profile:
>>
>> Each sample counts as 0.01 seconds.
>>   %   cumulative   self              self     total
>>  time   seconds   seconds    calls   s/call   s/call  name
>>  32.49      6.01     6.01 98809118     0.00     0.00  SimpleLruReadPage_ReadOnly
>>  26.97     11.00     4.99 98837761     0.00     0.00  LWLockAcquire
>>  21.89     15.05     4.05 98837761     0.00     0.00  LWLockRelease
>>   8.70     16.66     1.61 98789370     0.00     0.00  SubTransGetParent
>>   4.38     17.47     0.81    19748     0.00     0.00  SubTransGetTopmostTransaction
>>   2.41     17.92     0.45 98851951     0.00     0.00  TransactionIdPrecedes
>>   0.59     18.03     0.11                             LWLockAssign
>>   0.54     18.13     0.10                             LWLockConditionalAcquire
>>   0.46     18.21     0.09    19748     0.00     0.00  TransactionLogFetch
>>   0.38     18.28     0.07                             SimpleLruReadPage
>>   0.27     18.33     0.05                             SubTransSetParent
>>   0.05     18.34     0.01   136778     0.00     0.00  AllocSetAlloc
>>   0.05     18.35     0.01    42996     0.00     0.00  slot_deform_tuple
>>   0.05     18.36     0.01    42660     0.00     0.00  TransactionIdIsCurrentTransactionId
>>
>> it seems we are running into a nice shared buffer / locking contention here and the number of calls explodes (this
profilinginfos is coming from a seq scan on a 500.000 rows table - 400 mb or so). 
>
> That doesn't seem related to the lock manager. Is the long-running transaction inserting a lot of tuples (by INSERT
orUPDATE) to the same table that the seqscan scans? With a lot of different subtransaction xids. That profile looks
likethe seqscan is spending a lot of time swapping pg_subtrans pages in and out of the slru buffers. 
>
> Increasing NUM_SUBTRANS_BUFFERS should help. A more sophisticated solution would be to allocate slru buffers (for
clogand other slru caches as well) dynamically from shared_buffers. That's been discussed before but no-one has gotten
aroundto it. 
>
> --
>  Heikki Linnakangas
>  EnterpriseDB   http://www.enterprisedb.com



hello ...

yeah, it seems this solves the problem.
i had a closer look at the SQL trace and did some more profiling.
this was the case.
many thanks for the quick hint.
hans






--
Cybertec Schönig & Schönig GmbH
Gröhrmühlgasse 26
A-2700 Wiener Neustadt
Web: http://www.postgresql-support.de