Re: Hash join gets slower as work_mem increases?

Hi

> I ran operf on both backends, and they look quite similar, except that the
> number of samples is different (this is "opreport -c" output):
>
> CPU: Intel Sandy Bridge microarchitecture, speed 2899.8 MHz (estimated)
> Counted CPU_CLK_UNHALTED events (Clock cycles when not halted) with a unit
> mask of 0x00 (No unit mask) count 90000
> samples % image name symbol name
>
> -------------------------------------------------------------------------------
> 112 0.0019 postgres ExecProcNode
> 3020116 49.9904 postgres ExecScanHashBucket
> 3021162 50.0077 postgres ExecHashJoin
> 3020116 92.8440 postgres ExecScanHashBucket
> 3020116 49.9207 postgres ExecScanHashBucket [self]
> 3020116 49.9207 postgres ExecScanHashBucket
> 8190 0.1354 vmlinux apic_timer_interrupt
>
> What could be an explanation for this?
> Is this known behaviour?
>

one issue was fixed in 9.5

large hash table can introduce a lot of outs from L1, L2 caches.

Pavel

>
> Yours,
> Laurenz Albe
>
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