Micro-benchmark for various time syscalls...

[Bruce Evans]

On Mon, 2 Jun 2008, Gary Stanley wrote:

> At 06:19 AM 6/2/2008, Bruce Evans wrote:
>
>> 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.
>
> Phenom's don't have TSCs that are coherent, as least on a few machines here:

According to the amd64 arch manual (volume 3 3.14 Sep 2007):

If CPUID 8000_0007.edx[8] = 1, then [details about hardware states...]
then the TSC is suitable for use as a source of time.  Google shows
support for this feature in at least Linux and Xen.

Phenom also has a rdtscp instruction which is serializing.

> 4 CPUs, running 4 parallel test-tasks.
> checking for time-warps via:
> - read time stamp counter (RDTSC) instruction (cycle resolution)
> - gettimeofday (TOD) syscall (usec resolution)
> - clock_gettime(CLOCK_MONOTONIC) syscall (nsec resolution)
>
> new TSC-warp maximum: -4294919263 cycles, 00000000ffffe11b -> 
> 0000000000009cbc
> new TSC-warp maximum: -4294919300 cycles, 00000000ffff74e4 -> 
> 0000000000003060
> | TSC: 2.24us, fail:3 | TOD: 2.24us, fail:0 | CLK: 2.24us, fail:0 |

The difference seems to be only about -0x6000, with an overflow bug in
the test giving a value near -2^32.

> The code to test the TSC to check for warping:
>
> http://leaf.dragonflybsd.org/~gary/tests/time-warp-test.c

> However, it seems that Core2's don't have any warping of the TSC. I tested 
> that code on a core2quad for 8 hours with no TSC failures.

Interesting.  Please check the manual.  I don't have current Intel arch
manuals handy

Bruce