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
