kern/55268: tmpfs is slow

[gson%gson.org@localhost (Andreas Gustafsson)]

>Number:         55268
>Category:       kern
>Synopsis:       tmpfs is slow
>Confidential:   no
>Severity:       non-critical
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Fri May 15 15:55:00 +0000 2020
>Originator:     Andreas Gustafsson
>Release:        NetBSD-current, source date 2020.05.14.07.59.03
>Organization:

>Environment:
System: NetBSD
Architecture: x86_64
Machine: amd64
>Description:

Doing a "build.sh -j 24 release" on a tmpfs takes more real time than
on an ffs mounted with "-o async", and uses much more system time.

This is building NetBSD-8 on a 24-core Dell PowerEdge R630 running a
-current from 2020.05.14.07.59.03 with DIAGNOSTIC and acpicpu
disabled.  The timings are not directly comparable to those I have
posted earlier as the RAM has now been increased to 64 GB and node
interleaving has been enabled in the BIOS.

Building on ffs on a WD Green 120 GB SATA SSD:
2319.70 real      9354.58 user      4449.16 sys

Building on ffs with "-o async" on a Samsung 850 EVO 1 TB SATA SSD:
1417.57 real      9452.17 user      4606.12 sys

Building on tmpfs:
2167.63 real     10102.86 user     14655.71 sys

>How-To-Repeat:

>Fix: