Slow read(2) and write(2) syscalls

[Roland Illig <roland.illig%gmx.de@localhost>]

Hi,

some calls to read(2) or write(2) take almost exactly 0.5 seconds, while
most other syscalls are fast as usual. What could be the reason?

I'm running NetBSD 8.0 x86_64 inside VirtualBox 6.1 on a Windows 10
host. Add hard disks in that VM are added via a SCSI controller. One one
of the disks I created a file containing random data and read that via
cat(1). It looks like this:

~ $ time cat 80mb >/dev/zero

real    0m14.521s
user    0m0.000s
sys     0m14.521s

43.992198659 CALL  write(1,0x7e791cd01000,0x4000)
43.992255371 RET   write 16384/0x4000
43.992261517 CALL  read(3,0x7e791cd01000,0x4000)
44.492970628 RET   read 16384/0x4000
44.492981523 CALL  write(1,0x7e791cd01000,0x4000)
44.493084051 RET   write 16384/0x4000
44.493091315 CALL  read(3,0x7e791cd01000,0x4000)
44.493145233 RET   read 16384/0x4000

The VM has plenty of free memory, and the VM is otherwise unused. I ran
the "benchmark" in /var/shm as well and got mixed results:

/var/shm $ time cat 80mb >/dev/zero
sys     0m9.607s

/var/shm $ time cat 80mb >/dev/zero
sys     0m4.705s

/var/shm $ time cat 80mb >/dev/zero
sys     0m2.427s

/var/shm $ time cat 80mb >/dev/zero
sys     0m8.669s

/var/shm $ time cat 80mb >/dev/zero
sys     0m2.376s

How can I further research where this artificial delay comes from, and
finally fix it?

Roland