Re: Horrible CREATE DATABASE Performance in High Sierra

On 2017-10-02 19:50:51 -0400, Tom Lane wrote:
> Andres Freund <andres(at)anarazel(dot)de> writes:
> > On 2017-10-02 18:33:17 -0400, Tom Lane wrote:
> >> I'm kind of surprised that machine B doesn't show obvious tanking in this
> >> test given that msync() makes it suck so badly at copying a database.
> >> I wonder what is different from the kernel's standpoint ... maybe the
> >> sheer number of different files mmap'd by a single process during the
> >> copy?
>
> > Yea, that's curious. I've really no clue about OSX, so pardon my
> > question: With HEAD and CREATE DATABASE, is it IO blocked or kernel cpu
> > blocked?
>
> What I saw was that the backend process was consuming 100% of (one) CPU,
> while the I/O transaction rate viewed by "iostat 1" started pretty low
> --- under 10% of what the machine is capable of --- and dropped from
> there as the copy proceeded. I did not think to check if that was user
> or kernel-space CPU, but I imagine it has to be the latter.

So that's pretty clearly a kernel bug... Hm. I wonder if it's mmap() or
msync() that's the problem here. I guess you didn't run a profile?

One interesting thing here is that in the CREATE DATABASE case there'll
probably be a lot larger contiguous mappings than in *_flush_after
cases. So it might be related to the size of the mapping / flush "unit".

Greetings,

Andres Freund