Re: Re: [Xen-users] About profiling xen

[Jeremy Fitzhardinge <jeremy@goop.org>]

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On 10/01/09 00:35, Marco Tizzoni wrote:
> On Thu, Oct 1, 2009 at 1:23 AM, Jeremy Fitzhardinge <jeremy@goop.org> wrote:
>   
>> Your clocksource is "xen", which is the best possible for a PV xen guest.
>>
>> However, a clocksource is for measuring elapsed time, not triggering
>> timers.
>>     
> Why? How would you solve the problem of raising events at a fixed rate?
>   

I'm not sure I follow you.  In the kernel, the clock*source* subsystem
is for measuring time: its used to implement gettimeofday, as well as
internal time accounting.  It isn't directly related to time events.

The clock*event* mechanism is all about setting up timers to raise events.

When running paravirtualized, we use Xen-specific versions of both.

>> Unfortunately there doesn't seem to be a /sys file to show the
>> current clockevent source in use, but if you have "xen" clocksource it's
>> almost certainly the xen clockevent.
>>
>> However, this is only relevent if you have CONFIG_NO_HZ and
>> CONFIG_HIGHRES_TIMERS enabled.
>>     
> I've tried both set/unset but nothing change.
>   

Hm.  Its best to leave both enabled either way.

Try the attached program to get some info about how well timers are
working.  Compile with:
$ gcc -o testtimer testtimer.c -O -lrt

And test various results:
$ ./testtimer .1 > xen-0.1.out
$ ./testtimer .01 > xen-0.01.out
$ ./testtimer .001 > xen-0.001.out

You can plot the results with:
$ gnuplot
gnuplot> plot "xen-0.001.out" with lines

The plot is delta from the expected time, so the ideal is that they all
be 0.

On my system in dom0 I see about 10% overhead at .001 and .0005 sec, so
the timers aren't being quantized to 100/250Hz.

    J

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#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <sys/time.h>

#define SAMPLES	1000

static double samples[SAMPLES];
static const char prog[] = "/-\\|";

#define NS 1000000000

int main(int argc, char **argv)
{
	int i, iter;
	struct timespec start, end;
	struct timespec sleep;
	double base;

	if (argc < 2) 
		base = .1;
	else
		base = strtod(argv[1], NULL);

	iter = 20 / base;
	if (iter > SAMPLES)
		iter = SAMPLES;

	fprintf(stderr, "%d iterations at %f sec\n", iter, base);

	sleep.tv_sec = 0;
	sleep.tv_nsec = base * NS;

	while (sleep.tv_nsec >= NS) {
		sleep.tv_sec++;
		sleep.tv_nsec -= NS;
	}

	for(i = 0; i < iter; i++) {
		double delta;

		fprintf(stderr, "%c\b", prog[i % (sizeof(prog)-1)]);
		clock_gettime(CLOCK_MONOTONIC, &start);
		nanosleep(&sleep, NULL);
		clock_gettime(CLOCK_MONOTONIC, &end);

		delta = (end.tv_sec * NS + end.tv_nsec) -
			(start.tv_sec * NS + start.tv_nsec);

		samples[i] = delta - (base * NS);
	}

	for(i = 0; i < iter; i++)
		printf("%f\n", samples[i] / NS);

	return 0;
}

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