ugly locking corner cases ...

hello all ...

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 some tasty 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 we did a plain seq_scan in connection 2. instead of 262 ms (cached case) it started to head north linearily with the number of 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 profiling infos is coming from a seq scan on a 500.000 rows table - 400 mb or so).

i am thinking of doing a workaround for this problem

many thanks,

hans

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