select() returning early from timeouts ? (on qemu, HEAD and RELENG_8)

Juergen Lock nox at
Sat Dec 26 20:33:41 UTC 2009

In article <20091226154604.GA6932 at> you write:

>while doing some tests with picobsd images on qemu, i noticed
>that "ping" runs too fast (approx every 0.4s instead of 1s)
>Upon investigation, the problem seems related to
>	retval =  select(..., &timeout)
>returning a retval of 0 way before the timeout has elapsed
>(at least, according to a gettimeofday which is run right after).
>My host system is FreeBSD 7.2-stable i386 with qemu.
>In the guest (which is built using picobsd 'bridge') i set
>	kern.timecounter.hardware=i8254
>A guest based on RELENG_7 works fine, whereas HEAD and RELENG_8
>exhibit the problem.
>Easy way to check:
>	time ping -c 21 localhost
>(or, on picobsd "date; ping -c 21 localhost; date" )
>you should see a real time of 20 seconds (I see 8s on my system).
>I was wondering if someone can try the above on real hardware
>and let me know.
>Even if just related to qemu or some ACPI/timecounter config, it
>would be good to know where this comes from.
>BTW I don't think this is related to the bug reported in september
>of timeouts firing one tick late.
>I am going to try various HEAD versions to see where the issue came
>out, but it might be a bit time-consuming so if you have suggestions
>on possible causes I'd be glad to know.

 Just in case this is not expected:  I can't reproduce this issue on a
	FreeBSD 7.2-STABLE FreeBSD 7.2-STABLE #1: Sun Jun 21 00:36:42 CEST 2009     nox at  amd64
host, at least with FreeBSD-9.0-HEAD-20091226-JPSNAP-i386-dvd1.iso,
9.0-HEAD-20091123-JPSNAP-i386-dvd1.iso and 8.0-RELEASE-i386-dvd1.iso
guests in livefs (fixit->cdrom/dvd), qemu 0.11.0, host kern.hz=1000,
and guest kern.hz=100 (which the guest kernel automagically goes down
to when running in a VM since 8.0.)

% qemu -m 256 -cdrom FreeBSD-9.0-HEAD-20091226-JPSNAP-i386-dvd1.iso -curses
Fixit# sysctl kern.timecounter.hardware=i8254
kern.timecounter.hardware: HPET -> i8254
Fixit# ifconfig lo0 127.1
Fixit# time ping -c5 127.1
PING 127.1 ( 56 data bytes
64 bytes from icmp_seq=0 ttl=64 time=5.095 ms
64 bytes from icmp_seq=1 ttl=64 time=0.380 ms
64 bytes from icmp_seq=2 ttl=64 time=0.370 ms
64 bytes from icmp_seq=3 ttl=64 time=0.342 ms
64 bytes from icmp_seq=4 ttl=64 time=0.327 ms

--- 127.1 ping statistics ---
5 packets transmitted, 5 packets received, 0.0% packet loss
round-trip min/avg/max/stddev = 0.327/1.303/5.095/1.896 ms
        4.06 real         0.02 user         0.08 sys
Fixit# time sleep 4
        4.04 real         0.00 user         0.03 sys
Fixit# vmstat -i
interrupt                          total       rate
irq1: atkbd0                         259          4
irq4: uart0                            1          0
irq15: ata1                          631         11
cpu0: timer                        21362        403
Total                              22253        419
Fixit# sysctl kern.hz
kern.hz: 100

 And If I boot the host with kern.hz=100 too I get the expected 1/4
guest timer slowdown (cpu0: timer only runs at 100 Hz when in fact it
wants 400 Hz) unless I disable the guest's apic.  (As also now noted
in the qemu port's pkg-message.)


PS: I got the snapshot isos from here in case you don't know it:

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