Intermittent system hangs on 7.2-RELEASE-p1

[Guy Helmer]

Linda Messerschmidt wrote:
> Well, this is interesting.  I got really frustrated with the other
> approach, so I thought I'd thin a machine down absolutely as far as I
> could, eliminate every possible source of delay, and see what happens.
>  I killed everything... cron, RPC, NFS, devd, gmon, nrpe, everything.
> The Apache and its exerciser are now the only things running on the
> machine, and the Apache is only touching an md0 swap device mounted on
> /mnt.  I *still* get the hangs.
>
> It hangs for all sorts of different periods, but the duration of the
> stall is approximately inversely proportional to the chance of seeing
> it.  To get a short delay, you need wait only a little bit.  If you
> want a 2-3 second delay, you may have to wait 15-20 minutes.
>   
On what sort of hardware is this hang occurring?  Several months ago I 
was trying to resolve an intermittent hang under FreeBSD 7.  I collected 
a large number of crashdumps I created using the kernel debugger when I 
caught the machine hanging, but the backtraces were very inconsistent, 
and the hang was only occurring on Xeons with multithreading (older 
2.8GHz and 3.6GHz Xeons).  I was able to prevent the hang by setting 
"mach.hyperthreading_enabled=0" in /boot/loader.conf, but I am still not 
sure why it worked.

Guy