Re: Slow standby snapshot

Hello, Alexander.

Thanks for your review.

> It looks like KnownAssignedXidsNext doesn't have to be
> pg_atomic_uint32. I see it only gets read with pg_atomic_read_u32()
> and written with pg_atomic_write_u32(). Existing code believes that
> read/write of 32-bit values is atomic. So, you can use just uint32
> instead of pg_atomic_uint32.

Fixed. Looks better now, yeah.

Also, I added an additional (not depending on KnownAssignedXidsNext
feature) commit replacing the spinlock with a memory barrier. It goes
to Simon Riggs and Tom Lane at 2010:

> (We could dispense with the spinlock if we were to
> create suitable memory access barrier primitives and use those instead.)

Now it is possible to avoid additional spinlock on each
KnownAssignedXidsGetAndSetXmin. I have not measured the performance
impact of this particular change yet (and it is not easy to reliable
measure impact less than 0.5% probably), but I think it is worth
adding. We need to protect only the head pointer because the tail is
updated only with exclusive ProcArrayLock. BTW should I provide a
separate patch for this?

So, now we have a pretty successful benchmark for the typical use-case
and some additional investigation had been done. So, I think I’ll
re-add the patch to the commitfest app.

Thanks,
Michail