Re: [HACKERS] Horrible CREATE DATABASE Performance in High Sierra

On Mon, Oct 2, 2017 at 9:06 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:

Andres Freund <andres@anarazel.de> writes:
> On 2017-10-02 19:50:51 -0400, Tom Lane wrote:
>> 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?

Interestingly, profiling with Activity Monitor seems to blame the problem
entirely on munmap() ... which squares with the place I hit every time
when randomly stopping the process with gdb^Hlldb, so I'm inclined to
believe it.

This still offers no insight as to why CREATE DATABASE is hitting the
problem while regular flush activity doesn't.

> 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".

Meh, the mapping is only 64K in this case vs. 8K in the other.  Hard
to credit that it breaks that easily.

                        regards, tom lane