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66 lines
3 KiB
C
66 lines
3 KiB
C
#ident "Copyright (c) 2007-2011 Tokutek Inc. All rights reserved."
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#ifndef TOKU_TIME_H
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#define TOKU_TIME_H
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#include <time.h>
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#include <sys/time.h>
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#if defined(__cplusplus) || defined(__cilkplusplus)
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extern "C" {
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#endif
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static inline float toku_tdiff (struct timeval *a, struct timeval *b) {
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return (float)((a->tv_sec - b->tv_sec) + 1e-6 * (a->tv_usec - b->tv_usec));
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}
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// *************** Performance timers ************************
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// What do you really want from a performance timer:
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// (1) Can determine actual time of day from the performance time.
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// (2) Time goes forward, never backward.
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// (3) Same time on different processors (or even different machines).
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// (4) Time goes forward at a constant rate (doesn't get faster and slower)
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// (5) Portable.
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// (6) Getting the time is cheap.
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// Unfortuately it seems tough to get Properties 1-5. So we go for Property 6,, but we abstract it.
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// We offer a type tokutime_t which can hold the time.
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// This type can be subtracted to get a time difference.
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// We can get the present time cheaply.
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// We can convert this type to seconds (but that can be expensive).
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// The implementation is to use RDTSC (hence we lose property 3: not portable).
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// Recent machines have constant_tsc in which case we get property (4).
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// Recent OSs on recent machines (that have RDTSCP) fix the per-processor clock skew, so we get property (3).
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// We get property 2 with RDTSC (as long as there's not any skew).
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// We don't even try to get propety 1, since we don't need it.
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// The decision here is that these times are really accurate only on modern machines with modern OSs.
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typedef u_int64_t tokutime_t; // Time type used in by tokutek timers.
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// The value of tokutime_t is not specified here.
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// It might be microseconds since 1/1/1970 (if gettimeofday() is
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// used), or clock cycles since boot (if rdtsc is used). Or something
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// else.
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// Two tokutime_t values can be subtracted to get a time difference.
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// Use tokutime_to_seconds to that convert difference to seconds.
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// We want get_tokutime() to be fast, but don't care so much about tokutime_to_seconds();
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//
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// For accurate time calculations do the subtraction in the right order:
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// Right: tokutime_to_seconds(t1-t2);
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// Wrong tokutime_to_seconds(t1)-toku_time_to_seconds(t2);
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// Doing it the wrong way is likely to result in loss of precision.
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// A double can hold numbers up to about 53 bits. RDTSC which uses about 33 bits every second, so that leaves
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// 2^20 seconds from booting (about 2 weeks) before the RDTSC value cannot be represented accurately as a double.
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//
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double tokutime_to_seconds(tokutime_t) __attribute__((__visibility__("default"))); // Convert tokutime to seconds.
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// Get tokutime. We want this to be fast, so we expose the implementation as RDTSC.
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static inline tokutime_t get_tokutime (void) {
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u_int32_t lo, hi;
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__asm__ __volatile__ ("rdtsc" : "=a" (lo), "=d" (hi));
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return (uint64_t)hi << 32 | lo;
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}
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#if defined(__cplusplus) || defined(__cilkplusplus)
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}
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#endif
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#endif
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