I have stabilized and accelerated things a lot with clocksource=hpet on
the kernel line; but I am wondering if I could get rid of my very very
few remaining xruns with "-c hpet" in the jackd command line.  Only
thing is, jackd is reporting hpet as unsupported / not implemented.
Thoughts, anyone?

HPET has as about as close to zero effect on xruns as is possible.

More info on this can be found in a thread on LAD about a year ago:
http://linuxaudio.org/mailarchive/lad/2009/11/10/161854

To use it your kernel and hardware must support the HPET timer. If one
or both of them do not you will see that message.

Just for completeness: the message ("This system has no accessible HPET
device (Device or resource busy") also shows if the user has no
permission to access /dev/hpet

best,
robin

[jeb@youngdavid realtimeconfigquickscan]$ ls -l /dev/hpet
crw-rw---- 1 root audio 10, 228 Sep 11 09:30 /dev/hpet
[jeb@youngdavid realtimeconfigquickscan]$ grep audio /etc/group
audio:x:63:jeb
[jeb@youngdavid realtimeconfigquickscan]$ dmesg | grep hpet
Command line: ro root=/dev/mapper/vg_youngdavid-lv_root rd_LVM_LV=vg_youngdavid/lv_root rd_LVM_LV=vg_youngdavid/lv_swap rd_NO_LUKS rd_NO_MD rd_NO_DM LANG=en_US.UTF-8 SYSFONT=latarcyrheb-sun16 KEYBOARDTYPE=pc KEYTABLE=us rhgb hpet=enable clocksource=hpet quiet selinux=0 rdblacklist=nouveau vga=0×318 nouveau.modeset=0
Kernel command line: ro root=/dev/mapper/vg_youngdavid-lv_root rd_LVM_LV=vg_youngdavid/lv_root rd_LVM_LV=vg_youngdavid/lv_swap rd_NO_LUKS rd_NO_MD rd_NO_DM LANG=en_US.UTF-8 SYSFONT=latarcyrheb-sun16 KEYBOARDTYPE=pc KEYTABLE=us rhgb hpet=enable clocksource=hpet quiet selinux=0 rdblacklist=nouveau vga=0×318 nouveau.modeset=0
hpet clockevent registered
hpet0: at MMIO 0xfeff0000, IRQs 2, 8, 31
hpet0: 3 comparators, 32-bit 25.000000 MHz counter
Switching to clocksource hpet
rtc0: alarms up to one year, y3k, 114 bytes nvram, hpet irqs
[jeb@youngdavid realtimeconfigquickscan]$