On Mon, Apr 24, 2023 at 6:13 PM Andres Freund <andres@anarazel.de> wrote:
Hi,
On 2023-04-24 17:37:48 -0400, Melanie Plageman wrote: > On Mon, Apr 24, 2023 at 02:14:32PM -0700, Andres Freund wrote: > > It starts blocking once "enough" IO is in flight. For things like an immediate > > checkpoint, that can happen fairly quickly, unless you have a very fast IO > > subsystem. So often it'll not matter whether we track smgrwriteback(), but > > when it matter, it can matter a lot. > > I see. So, it sounds like this is most likely to happen for checkpointer > and not likely to happen for other backends who call > ScheduleBufferTagForWriteback().
It's more likely, but once the IO subsystem is busy, it'll also happen for other users ScheduleBufferTagForWriteback().
> Also, it seems like this (given the current code) is only reachable for > permanent relations (i.e. not for IO object temp relation). If other backend > types than checkpointer may call smgrwriteback(), we likely have to consider > the IO context.
I think we should take it into account - it'd e.g. interesting to see a COPY is bottlenecked on smgrwriteback() rather than just writing the data.
With the quick and dirty attached patch and using your example but with a pgbench -T200 on my rather fast local NVMe SSD, you can still see quite a difference. This is with a stats reset before the checkpoint.
