Thanks Chris.
Yes, I did see that earlier. Two things about it:
1. It is for the linux kernel.
Interestingly, the linux kernel is almost close enough to the BSD
kernel, particularly in the choice of constants and usage, although in
atypical convolutions. The main difference is in how it is an unfolded
expression, which makes it hard to follow immediately, and in the formula
itself, for which, ...
2. The linux formula though is:
67 #define CALC_LOAD(load,exp,n) \
68 load *= exp; \
69 load += n*(FIXED_1-exp); \
70 load = FSHIFT;
Which isload = ((load * exp) + n * (f_1 - exp)) FSHIFT.
Exp is really (in BSD speak) FSCALE * \alpha, f_1 is FSCALE, so this formula
boils down to (\alpha * load) + nrun * (1 - \alpha), a nice clean formula.
Thanks,
Kannan
On 8/19/05 1:58 PM, Chris St Denis [EMAIL PROTECTED] wrote:
This may help
http://www.teamquest.com/resources/gunther/ldavg1.shtml
-Original Message-
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] On Behalf Of Kannan Varadhan
Sent: Friday, August 19, 2005 12:08 PM
To: freebsd-questions@freebsd.org
Subject: Calculating the load average in the freebsd kernel
Hello,
I am staring at the code in kern/kern_synch.c that calculates the load
average of the system, and I cannot fully understand how the freebsd version
works. Specifically, it looks as:
/*
* Constants for averages over 1, 5, and 15 minutes
* when sampling at 5 second intervals.
*/
static fixpt_t cexp[3] = {
0.9200444146293232 * FSCALE,/* exp(-1/12) */
0.9834714538216174 * FSCALE,/* exp(-1/60) */
0.9944598480048967 * FSCALE,/* exp(-1/180) */
};
...
/*
* Compute a tenex style load average of a quantity on
* 1, 5 and 15 minute intervals.
* XXXKSE Needs complete rewrite when correct info is available.
* Completely Bogus.. only works with 1:1 (but compiles ok now :-)
*/
static void
loadav(void *arg)
{
int i, nrun;
struct loadavg *avg;
nrun = sched_load();
avg = averunnable;
for (i = 0; i 3; i++)
avg-ldavg[i] = (cexp[i] * avg-ldavg[i] +
nrun * FSCALE * (FSCALE - cexp[i])) FSHIFT;
...
And elsewhere, FSCALE is defined as 1FSHIFT, and FSHIFT is 11.
Focusing only the formula, then
avg-ldavg[i] = (cexp[i] * avg-ldavg[i] +
nrun * FSCALE * (FSCALE - cexp[i])) FSHIFT;
^^
Why do we have that extra FSCALE multiplier in the second term? If I do
some logical simplifications, this seems to get me:
(\alpha * FSCALE * ldavg[I] + nrum * FSCALE * FSCALE (1 - \alpha)) FSHIFT
I.e.
\alpha * ldavg[I] + nrun * FSCALE * (1 - \alpha)
What am I missing in this arithmetic?
Thanks,
Kannan
___
freebsd-questions@freebsd.org mailing list
http://lists.freebsd.org/mailman/listinfo/freebsd-questions
To unsubscribe, send any mail to [EMAIL PROTECTED]
___
freebsd-questions@freebsd.org mailing list
http://lists.freebsd.org/mailman/listinfo/freebsd-questions
To unsubscribe, send any mail to [EMAIL PROTECTED]
