Re: Performance bug in prepared statement binding in 9.2?

On 2013-09-09 20:38:09 -0400, Andrew Dunstan wrote:
>
> On 08/01/2013 03:20 PM, Jeff Janes wrote:
> >On Thu, Aug 1, 2013 at 10:58 AM, Josh Berkus <josh(at)agliodbs(dot)com> wrote:
> >>Amit, All:
> >>
> >>So we just retested this on 9.3b2. The performance is the same as 9.1
> >>and 9.2; that is, progressively worse as the test cycles go on, and
> >>unacceptably slow compared to 8.4.
> >>
> >>Some issue introduced in 9.1 is causing BINDs to get progressively
> >>slower as the PARSEs BINDs get run repeatedly. Per earlier on this
> >>thread, that can bloat to 200X time required for a BIND, and it's
> >>definitely PostgreSQL-side.
> >>
> >>I'm trying to produce a test case which doesn't involve the user's
> >>application. However, hints on other things to analyze would be keen.
> >Does it seem to be all CPU time (it is hard to imagine what else it
> >would be, but...)
> >
> >Could you use oprofile or perf or gprof to get a profile of the
> >backend during a run? That should quickly narrow it down to which C
> >function has the problem.
> >
> >Did you test 9.0 as well?
>
>
> This has been tested back to 9.0. What we have found is that the problem
> disappears if the database has come in via dump/restore, but is present if
> it is the result of pg_upgrade. There are some long-running transactions
> also running alongside this - we are currently planning a test where those
> are not present. We're also looking at constructing a self-contained test
> case.
>
> Here is some perf output from the bad case:
>
> + 14.67% postgres [.] heap_hot_search_buffer
> + 11.45% postgres [.] LWLockAcquire
> + 8.39% postgres [.] LWLockRelease
> + 6.60% postgres [.] _bt_checkkeys
> + 6.39% postgres [.] PinBuffer
> + 5.96% postgres [.] hash_search_with_hash_value
> + 5.43% postgres [.] hash_any
> + 5.14% postgres [.] UnpinBuffer
> + 3.43% postgres [.] ReadBuffer_common
> + 2.34% postgres [.] index_fetch_heap
> + 2.04% postgres [.] heap_page_prune_opt
> + 2.00% libc-2.15.so [.] 0x8041b
> + 1.94% postgres [.] _bt_next
> + 1.83% postgres [.] btgettuple
> + 1.76% postgres [.] index_getnext_tid
> + 1.70% postgres [.] LockBuffer
> + 1.54% postgres [.] ReadBufferExtended
> + 1.25% postgres [.] FunctionCall2Coll
> + 1.14% postgres [.] HeapTupleSatisfiesNow
> + 1.09% postgres [.] ReleaseAndReadBuffer
> + 0.94% postgres [.] ResourceOwnerForgetBuffer
> + 0.81% postgres [.] _bt_saveitem
> + 0.80% postgres [.] _bt_readpage
> + 0.79% [kernel.kallsyms] [k] 0xffffffff81170861
> + 0.64% postgres [.] CheckForSerializableConflictOut
> + 0.60% postgres [.] ResourceOwnerEnlargeBuffers
> + 0.59% postgres [.] BufTableLookup

After a second look at this, I very tentatively guess that you'll see
get_actual_variable_range() as the entry point here. Which would explain
why you're seing this during PARSE.

But there still is the question why we never actually seem to prune...

Greetings,

Andres Freund

--
Andres Freund http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services