Date: Sun, 5 Apr 1998 23:06:05 +1000 From: Bruce Evans <bde@zeta.org.au> To: bde@zeta.org.au, phk@critter.freebsd.dk Cc: peter@netplex.com.au, smp@FreeBSD.ORG Subject: Re: more SMP stuff Message-ID: <199804051306.XAA05313@godzilla.zeta.org.au>
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>>The scaling needs to be more robust since the h/w counter might be
>>slower than 10^6 hz.
>
>No, that is a design decision. Timecounter must be >= 1MHz.
Why? The i8254 is only marginally faster than 1MHz. There's no
reason to expect a faster clock unless the CPU clock can be read.
>>>This isn't it anyway, the dummy counter is only used by syscons
>>>screensaver calls to getmicroruntime();
>>
>>It's used by my version of the floppy driver which calls microtime()
>>to timestamp certain events.
>
>before the i8254 is initialized ?
In fdprobe(), before the i8254 is completely initialized.
>>I don't think I call nanotime() so early. My rtcintr() calls
>>nanotime() to gather statistics, and nanotime() was reentered.
>>I don't see any reentrancy problems. There will be one when
>>nanotime updates tc->nanotime like it should.
>
>No it shouldn't, I'm sorry but I cannot find a way where that makes
>fewer problems than it fixes...
It is essential for POSIX.1 conformance and probably for `make' E.g.,
the following shows chmod()+stat() sometimes updating the ctime of
a fifo apparently-before either is called. (POSIX.1 doesn't have
gettimeofday(); this example uses it just to find a bad time and to
show how close to the bad time we managed to do the chmod(). argv[1]
can be set to a small integer to manually search for the window of
bad times.)
---
#include <sys/stat.h>
#include <sys/time.h>
int
main(int argc, char **argv)
{
unsigned delay;
int try;
struct stat sb;
struct timeval tv0, tv1, tv2;
unlink("f");
mkfifo("f", 0777);
delay = argc <= 1 ? 0 : atoi(argv[1]);
for (try = 0;; try++) {
gettimeofday(&tv0, 0);
while (1) {
gettimeofday(&tv1, 0);
if (tv1.tv_sec != tv0.tv_sec)
break;
}
if (delay != 0) {
while (--delay != 0)
;
gettimeofday(&tv1, 0);
}
chmod("f", 0777);
stat("f", &sb);
gettimeofday(&tv2, 0);
if (tv1.tv_sec > sb.st_ctime || tv2.tv_sec < sb.st_ctime)
break;
}
printf("%d: %ld.%06ld %ld %ld.%06ld\n",
try, tv1.tv_sec, tv1.tv_usec, sb.st_ctime, tv2.tv_sec, tv2.tv_usec);
}
---
Bruce
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