Add check for invariant TSC on x86 and use TSC is default clock if reliable

[fio.git] / gettime.c

diff --git a/gettime.c b/gettime.c
index 35d685e1576149974d5e77f18db2b1fb180f3a9e..89a3a016e4c871a767365f96318392a65ec30ba6 100644 (file)
--- a/gettime.c
+++ b/gettime.c
@@ -15,11 +15,13 @@
 #ifdef ARCH_HAVE_CPU_CLOCK
 static unsigned long cycles_per_usec;
 static unsigned long last_cycles;
+int tsc_reliable = 0;
 #endif
 static struct timeval last_tv;
 static int last_tv_valid;
 
 enum fio_cs fio_clock_source = FIO_PREFERRED_CLOCK_SOURCE;
+int fio_clock_source_set = 0;
 
 #ifdef FIO_DEBUG_TIME
 
@@ -208,15 +210,17 @@ static unsigned long get_cycles_per_usec(void)
        return c_e - c_s;
 }
 
+#define NR_TIME_ITERS  50
+
 static void calibrate_cpu_clock(void)
 {
        double delta, mean, S;
-       unsigned long avg, cycles[10];
+       unsigned long avg, cycles[NR_TIME_ITERS];
        int i, samples;
 
        cycles[0] = get_cycles_per_usec();
        S = delta = mean = 0.0;
-       for (i = 0; i < 10; i++) {
+       for (i = 0; i < NR_TIME_ITERS; i++) {
                cycles[i] = get_cycles_per_usec();
                delta = cycles[i] - mean;
                if (delta) {
@@ -225,10 +229,10 @@ static void calibrate_cpu_clock(void)
                }
        }
 
-       S = sqrt(S / (10 - 1.0));
+       S = sqrt(S / (NR_TIME_ITERS - 1.0));
 
        samples = avg = 0;
-       for (i = 0; i < 10; i++) {
+       for (i = 0; i < NR_TIME_ITERS; i++) {
                double this = cycles[i];
 
                if ((fmax(this, mean) - fmin(this, mean)) > S)
@@ -237,10 +241,10 @@ static void calibrate_cpu_clock(void)
                avg += this;
        }
 
-       S /= 10.0;
-       mean /= 10.0;
+       S /= (double) NR_TIME_ITERS;
+       mean /= (double) NR_TIME_ITERS;
 
-       for (i = 0; i < 10; i++)
+       for (i = 0; i < NR_TIME_ITERS; i++)
                dprint(FD_TIME, "cycles[%d]=%lu\n", i, cycles[i] / 10);
 
        avg /= (samples * 10);
@@ -248,7 +252,6 @@ static void calibrate_cpu_clock(void)
        dprint(FD_TIME, "mean=%f, S=%f\n", mean, S);
 
        cycles_per_usec = avg;
-
 }
 #else
 static void calibrate_cpu_clock(void)
@@ -260,6 +263,17 @@ void fio_clock_init(void)
 {
        last_tv_valid = 0;
        calibrate_cpu_clock();
+
+       /*
+        * If the arch sets tsc_reliable != 0, then it must be good enough
+        * to use as THE clock source. For x86 CPUs, this means the TSC
+        * runs at a constant rate and is synced across CPU cores.
+        */
+       if (tsc_reliable) {
+               if (!fio_clock_source_set)
+                       fio_clock_source = CS_CPUCLOCK;
+       } else if (fio_clock_source == CS_CPUCLOCK)
+               log_info("fio: clocksource=cpu may not be reliable\n");
 }
 
 unsigned long long utime_since(struct timeval *s, struct timeval *e)