Hi,
On 2020-06-04 14:50:40 -0400, Tom Lane wrote:
> Actually ... we could probably use this design with a uint32 counter
> as well, on machines where the 64-bit operations would be slow.
On skylake-x even a 32bit [i]div is still 26 cycles. That's more than an
atomic operation 18 cycles.
> 2. The computed completePasses value would go backwards. I bet
> that wouldn't matter too much either, or at least we could teach
> BgBufferSync to cope. (I notice the comments therein suggest that
> it is already designed to cope with completePasses wrapping around,
> so maybe nothing needs to be done.)
If we're not concerned about that, then we can remove the
atomic-inside-spinlock, I think. The only reason for that right now is
to avoid assuming a wrong pass number.
I don't think completePasses wrapping around is comparable in frequency
to wrapping around nextVictimBuffer. It's not really worth worrying
about bgwriter wrongly assuming it lapped the clock sweep once ever
UINT32_MAX * NBuffers ticks, but there being a risk every NBuffers seems
worth worrying about.
Greetings,
Andres Freund
