I wrote a small micro-benchmark utility[1] to test various time
syscalls and the results were a bit surprising to me. The results
were from a UP machine and I believe that the difference between
gettimeofday(2) and clock_gettime(CLOCK_REALTIME_FAST) would've
been bigger on an SMP system and performance would've degraded
further with each additional core.
I wouldn't expect SMP to make much difference between CLOCK_REALTIME
and
CLOCK_REALTIME_FAST. The only difference is that the former calls
nanotime() where the latter calls getnanotime(). nanotime() always
does
more, but it doesn't have any extra SMP overheads in most cases (in
rare
cases like i386 using the i8254 timecounter, it needs to lock
accesses to
the timecounter hardware). gettimeofday() always does more than
CLOCK_REALTIME, but again no more for SMP.
You may be right, I can only speculate. Going off of phk@'s
rhetorical questions regarding gettimeofday(2) working across cores/
threads, I assumed there would be some kind of synchronization.
http://lists.freebsd.org/mailman/htdig/freebsd-current/2005-October/057280.html
clock_gettime(CLOCK_REALTIME_FAST) is likely the ideal function for
most authors (CLOCK_REALTIME_FAST is supposed to be precise to +/-
10ms of CLOCK_REALTIME's value[2]). In fact, I'd assume that
CLOCK_REALTIME_FAST is just as accurate as Linux's gettimeofday(2)
(a statement I can't back up, but believe is likely to be correct)
and therefore there isn't much harm (if any) in seeing
clock_gettime(2) + CLOCK_REALTIME_FAST receive more widespread use
vs. gettimeofday(2). FYI. -sc
The existence of most of CLOCK_* is a bug. I wouldn't use
CLOCK_REALTIME_FAST
for anything (if only because it doesn't exist in most kernels that I
run.
I think that's debatable, actually. I modified my little micro-
benchmark program to test the realtime values returned from each
execution and found that CLOCK_REALTIME_FAST likely updates itself
sufficiently frequently for most applications (not all, but most). My
test ensures that time doesn't go backwards and tally's the number of
times that the values are identical. It'd be nice of
CLOCK_REALTIME_FAST incremented by a small and reasonable fudge factor
every time it's invoked that way the values aren't identical.
On my machine, I can make 100K gettimeofday(2) calls compared to 3M
CLOCK_REALTIME_FAST calls, which is a significantly large delta when
you're aiming for software that's handling around ~40-50Kpps and want
to include time information periodically (see above comment about a
fudge factor being included after every call *grin* ).
http://sean.chittenden.org/pubfiles/freebsd/bench_clock_realtime.c
% ./bench_clock_realtime 9079882 | sort -rnk1
clock realtime micro-benchmark. 9079882 syscall iterations.
Avg. us/call Elapsed Name
9.317078 84.597968 gettimeofday(2)
8.960372 81.359120 time(3)
8.776467 79.689287 clock_gettime(2/CLOCK_REALTIME)
0.332357 3.017763 clock_gettime(2/CLOCK_REALTIME_FAST)
0.311705 2.830246 clock_gettime(2/CLOCK_SECOND)
Value from time(3): 1212427374
Last value from gettimeofday(2): 1212427293.590511 Equal: 0
Last value from clock_gettime(2/CLOCK_SECOND): 1212427460.000000000
Equal: 9079878
Last value from clock_gettime(2/CLOCK_REALTIME_FAST):
1212427457.656410126 Equal: 9078198
Last value from clock_gettime(2/CLOCK_REALTIME): 1212427454.639076390
Equal: 0
% irb
>> tot = 9079882
=> 9079882
>> eq = 9078198
=> 9078198
>> tot - eq
=> 1684
>> time = 3.017763
=> 3.017763
>> (tot - eq) / time
=> 558.029242190324
>> tot / time
=> 3008812.15655437 # number of CLOCK_REALTIME_FAST calls per second
>> tot / 84.597968
=> 107329.788346689 # number of gettimeofday(2) calls per second
I switched from using gettimeofday() to CLOCK_REALTIME many years
ago when syscalls started taking less than 1 usec and still
occasionally
have problems from this running old kernels, because old i386 kernels
don't support CLOCK_REALTIME and old amd64 kernels have a broken
CLOCK_REALTIME in 32-bit mode).
Entirely possible that's why things are more expensive on my test
machine.
% sysctl hw.model
hw.model: AMD Athlon(tm) 64 Processor 3500+
% uname -a
FreeBSD dev2.office.chittenden.org 7.0-RELEASE FreeBSD 7.0-RELEASE #0:
Sun Feb 24 10:35:36 UTC 2008 [EMAIL PROTECTED]:/usr/
obj/usr/src/sys/GENERIC amd64
PS Is there a reason that time(3) can't be implemented in terms of
clock_gettime(CLOCK_SECOND)? 10ms seems precise enough compared to
time_t's whole second resolution.
I might use CLOCK_SECOND (unlike CLOCK_REALTIME_FAST), since the low
accuracy timers provided by the get*time() family are accurate enough
to give the time in seconds. Unfortunately, they are still broken --
they are all incoherent relative to nanotime() and some are incoherent
relative to each other. CLOCK_SECOND can lag the time in seconds
given
by up to tc_tick/HZ seconds. This is because CLOCK_SECOND returns the
time in seconds at the last tc_windup(), so it misses seeing rollovers
of the second in the interval between the rollover and the next
tc_windup(), while nanotime() doesn't miss seeing these rollovers so
it gives incoherent times, with nanotime()/CLOCK_REALTIME being
correct
and time_second/CLOCK_SECOND broken.
Interesting. Incoherent, but accurate enough? We're talking about a
<10ms window of incoherency, right?
% ./bench_time 9079882 | sort -rnk1
Timing micro-benchmark. 9079882 syscall iterations.
Avg. us/call Elapsed Name
9.322484 84.647053 gettimeofday(2)
8.955324 81.313291 time(3)
8.648315 78.525684 clock_gettime(2/CLOCK_REALTIME)
8.598495 78.073325 clock_gettime(2/CLOCK_MONOTONIC)
0.674194 6.121600 clock_gettime(2/CLOCK_PROF)
0.648083 5.884515 clock_gettime(2/CLOCK_VIRTUAL)
0.330556 3.001412 clock_gettime(2/CLOCK_REALTIME_FAST)
0.306514 2.783111 clock_gettime(2/CLOCK_SECOND)
0.262788 2.386085 clock_gettime(2/CLOCK_MONOTONIC_FAST)
These are very slow. Are they on a 486? :-) I get about 262 ns for
CLOCK_REALTIME using the TSC timecounter on all ~2GHz UP systems.
The syscall overhead is about 200 nsec (170 nsec for a simpler syscall
and maybe 30 nsec extra for copyin/out for clock_gettime()) and
reading
the TSC timecounter adds another 60 nsec, including a whole 6 nsec for
the hardware part of the read (perhaps more like 30 nsec than 60 for
the
whoe read). The TSC doesn't work on all machines (never for SMP), but
this will hopefully change. (Phenom is supposed to have TSCs that are
coherent across CPUs, and rdtsc has slowed down from 12 cycles to 40+
to implement this :-(. Core2 already has a 40+ cycles rdtsc, but
AFAIK
it doesn't have coherent TSCs.) Other timecounters are much slower
than
the TSC, but I haven't seen one take 8000 nsec since 486 days.
*shrug* elapsed / number of calls. Not doing anything fancy here.
Some of my benchmark results:
Can I run this same test/see how this was written?
This system has a fairly fast ACPI and i8254 timecounters. 1500-800
nsec is more typical for ACPI-fast, and 4000-5000 is more typical
for i8254. ACPI-fast should be named ACPI-not-very-slow. ACPI-safe
is very slow, perhaps slower than i8254. i8254 could be made about
twice as fast if anyone cared.
Hrm.
% sysctl -a | grep -i acpi_timer
machdep.acpi_timer_freq: 3579545
dev.acpi_timer.0.%desc: 24-bit timer at 3.579545MHz
dev.acpi_timer.0.%driver: acpi_timer
dev.acpi_timer.0.%location: unknown
dev.acpi_timer.0.%pnpinfo: unknown
dev.acpi_timer.0.%parent: acpi0
% sysctl -a | grep -i tsc
kern.timecounter.choice: TSC(800) ACPI-safe(850) i8254(0)
dummy(-1000000)
kern.timecounter.tc.TSC.mask: 4294967295
kern.timecounter.tc.TSC.counter: 2749242907
kern.timecounter.tc.TSC.frequency: 2222000000
kern.timecounter.tc.TSC.quality: 800
kern.timecounter.smp_tsc: 0
machdep.tsc_freq: 2222000000
Other implementation bugs (all in clock_getres()):
- all of the clock ids that use getnanotime() claim a resolution of 1
nsec, but that us bogus. The actual resolution is more like
tc_tick/HZ.
The extra resolution in a struct timespec is only used to return
garbage related to the incoherency of the clocks. (If it could be
arranged that tc_windup() always ran on a tc_tick/HZ boundary, then
the clocks would be coherent and the times would always be a multiple
of tc_tick/HZ, with no garbage in low bits.)
- CLOCK_VIRTUAL and CLOCK_PROF claim a resolution of 1/hz, but that is
bogus. The actual resolution is more like 1/stathz, or perhaps 1
microsecond. hz is irrelevant here since statclock ticks are used.
statclock ticks only have a resolution of 1/stathz, but if 1 nsec is
correct for CLOCK_REALTIME_FAST, then 1 usec is correct here since
caclru() calculates the time to a resolution of 1 usec; it is just
very inaccurate at that resolution.
"Resolution" is a poor term for the functionality needed here. I
think
a hint about the accuracy is more important. In simple
implementations
using interrupts and ticks, the accuracy would be about the the same
as
the resolution, but FreeBSD is more complicated.
Is there any reason that the garbage resolution can't be zero'ed out
to indicate confidence of the kernel in the precision of the
information? -sc
--
Sean Chittenden
[EMAIL PROTECTED]
http://sean.chittenden.org/
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