Date: Tue, 12 Nov 2002 12:13:58 -0500 (EST) From: Andrew Gallatin <gallatin@cs.duke.edu> To: "Dr. Michael Mackey" <mackey@engineering.uiowa.edu> Cc: John Baldwin <jhb@FreeBSD.ORG>, freebsd-alpha@FreeBSD.ORG Subject: Re: Extreme time drift in SMP mode Message-ID: <15825.14038.294916.650698@grasshopper.cs.duke.edu> In-Reply-To: <15825.12436.199011.915697@grasshopper.cs.duke.edu> References: <XFMail.20021112104816.jhb@FreeBSD.org> <3DD12AA2.8D84B3E2@engineering.uiowa.edu> <3DD12CE7.10103@spock.cl> <15825.12436.199011.915697@grasshopper.cs.duke.edu>
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Andrew Gallatin writes:
>
> Roberto de Iriarte writes:
>
> > Hmmm. my dmesg reads differently
> > BTW, my system has only one CPU, and the clock has never drifted.
> >
>
> That's the thing. The 2100 is unlike all other alphas, and handles
> clocks differently in MP mode.
To elaborate on the above, I think most MP alphas tie the clock
interrupt to the boot cpu, and it ticks at hz.
I think the 2100 round-robins the clock interrupt to all running cpus,
therefore the boot cpu only ticks at hz/mp_ncpus.
Currently, we pay attention only to the boot CPU. From
alpha_clock_interrupt() in sys/alpha/alpha/interrupt.c
<...>
critical_enter();
#ifdef SMP
/*
* Only one processor drives the actual timer.
*/
if (PCPU_GET(cpuid) == boot_cpu_id) {
#endif
(*platform.clockintr)(framep);
/* divide hz (1024) by 8 to get stathz (128) */
if ((++schedclk2 & 0x7) == 0)
statclock((struct clockframe *)framep);
#ifdef SMP
} else {
mtx_lock_spin(&sched_lock);
hardclock_process(curthread, TRAPF_USERMODE(framep));
if ((schedclk2 & 0x7) == 0)
statclock_process(curkse, TRAPF_PC(framep),
TRAPF_USERMODE(framep));
mtx_unlock_spin(&sched_lock);
}
#endif
critical_exit();
<...>
Given that I know zippo about how timekeeping works, I don't know
what a good fix might be. If I had a machine in front of me,
I'd be tempted to try dividing the a frequency by 2 in sys/alpha/alpha/clock.c,
but I'm not sure which one, or if it can be adjusted as additional processors
are found and started. (which would be slightly less wrong..).
Drew
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