Re: ugly locking corner cases ...

Excerpts from Hans-Jürgen Schönig's message of lun oct 04 07:02:04 -0400 2010:

> 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).

Isn't this really a problem with subtransaction rather than locks,
though? I wonder if the problem would go away if the subtransactions
were cachable in the PGPROC subxid cache (i.e. try enlarging
PGPROC_MAX_CACHED_SUBXIDS a bunch of orders of magnitude), or if the
pg_subtrans SLRU cache was bigger (i.e. try enlarging
NUM_SUBTRANS_BUFFERS).

--
Álvaro Herrera <alvherre(at)commandprompt(dot)com>
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