Date: Fri, 22 Sep 2000 16:28:31 -0400 (EDT) From: Darrell Anderson <anderson@cs.duke.edu> To: freebsd-alpha@freebsd.org Cc: gallatin@cs.duke.edu (Andrew Gallatin) Subject: FreeBSD alpha cycle counter wackiness Message-ID: <200009222028.QAA16630@cold.cs.duke.edu>
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Can someone explain this cycle counter behavior to me? The attached
program reads the cycle counter from userspace, spitting out cycle counter
values and deltas.
On x86, I see consistent deltas (~8000 cycles). On alpha, I usually see a
reasonable delta (~6000 cycles), with occassional jumps (~2^64). The jumps
appear at consistent intervals. Sometimes there are long sequences where
every delta is a jump. Before you tell me it's just wrapping, the delta is
~2^56. Deltas before and after the jumps seem quite reasonable.
Here is some sample output from a 500MHz XP1000. I get similar results for
4.0-RELEASE and 5.0-CURRENT.
0xc5657103cc1d06a3 delta 0x1602
0xc5657103cc1d1ae5 delta 0x1442
0xc5657103cc1d2f41 delta 0x145c
0xc5652696cc1d9bc3 delta 0xffffb59300006c82
0xc5652696cc1db76e delta 0x1bab
0xc5652696cc1dcf85 delta 0x1817
0xc5652696cc1de53c delta 0x15b7
...
0xc5652696cc1fda5e delta 0x13bc
0xc5652696cc1fedba delta 0x135c
0xc5652696cc200266 delta 0x14ac
0xc564dfc7cc206c65 delta 0xffffb931000069ff
0xc564dfc7cc208930 delta 0x1ccb
0xc564dfc7cc20a033 delta 0x1703
0xc564dfc7cc20b646 delta 0x1613
...
0xc564dfc7cc22b111 delta 0x1374
0xc564dfc7cc22c690 delta 0x157f
0xc564dfc7cc22d9ed delta 0x135d
0xc564900ecc234d57 delta 0xffffb0470000736a
0xc564900ecc236b22 delta 0x1dcb
0xc564900ecc2382fb delta 0x17d9
0xc564900ecc239aae delta 0x17b3
here's the code:
--begin--
#include <stdio.h>
#include <unistd.h>
static __inline u_int64_t
read_cc(void)
{
u_int64_t rv = 0;
#ifdef __i386__
__asm__ __volatile__ (".byte 0x0f, 0x31" : "=A" (rv));
#endif
#ifdef __alpha__
__asm__ __volatile__ ("rpcc %0" : "=r" (rv));
#endif
return rv;
}
int
main(int argc, char *argv[])
{
unsigned long then = read_cc(), now;
while (1) {
now = read_cc();
printf("0x%lx delta 0x%lx\n", now, now - then);
then = now;
}
return 0;
}
--end--
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