Re: plan with result cache is very slow when work_mem is not enough

pá 7. 5. 2021 v 21:06 odesílatel Yura Sokolov <y(dot)sokolov(at)postgrespro(dot)ru>
napsal:

> Pavel Stehule писал 2021-05-07 21:45:
> >>
> >> Samples: 119K of event 'cycles', 4000 Hz, Event count (approx.):
> >> Overhead Shared Object Symbol
> >> 79.20% postgres [.] cache_reduce_memory
> >> 1.94% [kernel] [k] native_write_msr_safe
> >> 1.63% [kernel] [k] update_cfs_shares
> >> 1.00% [kernel] [k] trigger_load_balance
> >> 0.97% [kernel] [k] timerqueue_add
> >> 0.51% [kernel] [k] task_tick_fair
> >> 0.51% [kernel] [k] task_cputime
> >> 0.50% [kernel] [k] perf_event_task_tick
> >> 0.50% [kernel] [k] update_curr
> >> 0.49% [kernel] [k] hrtimer_active
> >>
> >> Regards
> >>
> >> Pavel
>
> It is strange to see cache_reduce_memory itself consumes a lot of CPU.
> It doesn't contain CPU hungry code.
> It calls prepare_probe_slot, that calls some tuple forming. Then
> it calls resultcache_lookup that may call to ResultCacheHash_hash
> and ResultCacheHash_equal. And finally it calls remove_cache_entry.
> I suppose remove_cache_entry should consume most of CPU time since
> it does deallocations.
> And if you compile with --enable-cassert, then remove_cache_entry
> iterates through whole cache hashtable, therefore it reaches
> quadratic complexity easily (or more correct M*N, where M - size
> of a table, N - eviction count).
>

yes, the slowdown is related to debug assertions

Pavel

> regards,
> Yura Sokolov
>