Date: Tue, 01 May 2001 11:35:39 -0700 (PDT) From: John Baldwin <jhb@FreeBSD.org> To: Terry Lambert <tlambert@primenet.com> Cc: freebsd-alpha@FreeBSD.org, (Rolf Neugebauer) <neugebar@dcs.gla.ac.uk> Subject: Re: determine cycle counter frequency in user space Message-ID: <XFMail.010501113539.jhb@FreeBSD.org> In-Reply-To: <200105011758.KAA17133@usr01.primenet.com>
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On 01-May-01 Terry Lambert wrote:
> Minimally, you will require periodic resynchornization with
> the kernel drift information stored in timer structure. You
> should see /sys/i386/isa/clock.c and the timer code in the
> file /sys/kern/kern_time.c.
You have _got_ to be kidding me Terry. This is freebsd-_alpha_@FreeBSD.org.
/sys/i386/anything isn't very relevant here. :) /sys/alpha would be a better
place to investigate. Using microtime or microuptime in the kernel in a
critical section (or at splhigh for 4.x I would guess) in conjuction with each
rpcc would be perfectly sufficient:
struct timeval *start_tv, end_tv;
register_t start_cc, end_cc;
critical_t s;
s = critical_enter();
microuptime(&start_tv);
start_cc = alpha_rpcc();
critical_exit(s);
tsleep(&foo, PUSER, "ccsec", 10);
s = critical_enter();
microuptime(&end_tv);
end_cc = alpha_rpcc();
critical_exit(s);
/* do calculations here */
There is still the problem on an SMP system that you may spin on a lock in
microuptime() while talking to the i8254. Also, I probably fubb'd the priority
to tsleep(), and some different timeouts might be more optimal. To be truly
pedantic, one could make the clock lock allow recursion and do something along
these lines:
mtx_lock_spin(&sclock_lock);
microuptime(&start_tv);
start_cc = alpha_rpcc();
for (count = 0; count < 1000000000; count ++)
; /* nothing */
microuptime(&end_tv);
end_cc = alpha_rpcc();
mtx_unlock_spin(&clock_lock);
/* do calculations here */
(In -current of course, in -stable splhigh() instead of the mutex ops would
probably work as well.)
This isn't cheap though. :)
--
John Baldwin <jhb@FreeBSD.org> -- http://www.FreeBSD.org/~jhb/
PGP Key: http://www.baldwin.cx/~john/pgpkey.asc
"Power Users Use the Power to Serve!" - http://www.FreeBSD.org/
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