diff --git a/arch/arch-x86-common.h b/arch/arch-x86-common.h
new file mode 100644
index 0000000..1e62354
--- /dev/null
+++ b/arch/arch-x86-common.h
@@ -0,0 +1,43 @@
+#ifndef FIO_ARCH_X86_COMMON
+#define FIO_ARCH_X86_COMMON
+
+static inline void do_cpuid(unsigned int *eax, unsigned int *ebx,
+			    unsigned int *ecx, unsigned int *edx)
+{
+	unsigned int id = *eax;
+
+	asm("movl %4, %%eax;"
+	    "cpuid;"
+	    "movl %%eax, %0;"
+	    "movl %%ebx, %1;"
+	    "movl %%ecx, %2;"
+	    "movl %%edx, %3;"
+		: "=r" (*eax), "=r" (*ebx), "=r" (*ecx), "=r" (*edx)
+		: "r" (id)
+		: "eax", "ebx", "ecx", "edx");
+}
+
+#define ARCH_HAVE_INIT
+extern int tsc_reliable;
+static inline int arch_init(char *envp[])
+{
+	unsigned int eax, ebx, ecx, edx;
+
+	/*
+	 * Check for TSC
+	 */
+	eax = 1;
+	do_cpuid(&eax, &ebx, &ecx, &edx);
+	if (!(edx & (1U << 4)))
+		return 0;
+
+	/*
+	 * Check for constant rate and synced (across cores) TSC
+	 */
+	eax = 0x80000007;
+	do_cpuid(&eax, &ebx, &ecx, &edx);
+	tsc_reliable = edx & (1U << 8);
+	return 0;
+}
+
+#endif
diff --git a/arch/arch-x86.h b/arch/arch-x86.h
index 1ededd8..4803006 100644
--- a/arch/arch-x86.h
+++ b/arch/arch-x86.h
@@ -1,6 +1,8 @@
 #ifndef ARCH_X86_H
 #define ARCH_X86_H
 
+#include "arch-x86-common.h"
+
 #define FIO_ARCH	(arch_i386)
 
 #ifndef __NR_ioprio_set
diff --git a/arch/arch-x86_64.h b/arch/arch-x86_64.h
index 29e681f..d8b0933 100644
--- a/arch/arch-x86_64.h
+++ b/arch/arch-x86_64.h
@@ -1,6 +1,8 @@
 #ifndef ARCH_X86_64_h
 #define ARCH_X86_64_h
 
+#include "arch-x86-common.h"
+
 #define FIO_ARCH	(arch_x86_64)
 
 #ifndef __NR_ioprio_set
diff --git a/crc/crc32c-intel.c b/crc/crc32c-intel.c
index 8a6e6dc..8e1cd58 100644
--- a/crc/crc32c-intel.c
+++ b/crc/crc32c-intel.c
@@ -78,22 +78,6 @@
 	return crc;
 }
 
-static void do_cpuid(unsigned int *eax, unsigned int *ebx, unsigned int *ecx,
-		     unsigned int *edx)
-{
-	int id = *eax;
-
-	asm("movl %4, %%eax;"
-	    "cpuid;"
-	    "movl %%eax, %0;"
-	    "movl %%ebx, %1;"
-	    "movl %%ecx, %2;"
-	    "movl %%edx, %3;"
-		: "=r" (*eax), "=r" (*ebx), "=r" (*ecx), "=r" (*edx)
-		: "r" (id)
-		: "eax", "ebx", "ecx", "edx");
-}
-
 void crc32c_intel_probe(void)
 {
 	if (!crc32c_probed) {
diff --git a/fio.c b/fio.c
index ac026fb..f44273f 100644
--- a/fio.c
+++ b/fio.c
@@ -104,6 +104,8 @@
 	if (parse_options(argc, argv))
 		return 1;
 
+	fio_time_init();
+
 	if (nr_clients)
 		return fio_handle_clients();
 	else
diff --git a/fio.h b/fio.h
index 3de3f00..de4ca4d 100644
--- a/fio.h
+++ b/fio.h
@@ -598,6 +598,7 @@
 extern int fio_gtod_offload;
 extern int fio_gtod_cpu;
 extern enum fio_cs fio_clock_source;
+extern int fio_clock_source_set;
 extern int warnings_fatal;
 extern int terse_version;
 extern int is_backend;
diff --git a/gettime.c b/gettime.c
index 35d685e..89a3a01 100644
--- 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 @@
 	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 @@
 		}
 	}
 
-	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 @@
 		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 @@
 	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 @@
 {
 	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)
diff --git a/options.c b/options.c
index d46bcbb..738c78b 100644
--- a/options.c
+++ b/options.c
@@ -370,7 +370,7 @@
 	struct thread_data *td = data;
 
 	fio_clock_source = td->o.clocksource;
-	fio_time_init();
+	fio_clock_source_set = 1;
 	return 0;
 }