pg_test_timing pg_test_timing 1 Application pg_test_timing measure timing overhead pg_test_timing option pg_test_timing is a tool to measure the timing overhead on your system and confirm that the system time never moves backwards. It simply reads the system clock over and over again as fast as it can for a specified length of time, and then prints statistics about the observed differences in successive clock readings. Smaller (but not zero) differences are better, since they imply both more-precise clock hardware and less overhead to collect a clock reading. Systems that are slow to collect timing data can give less accurate EXPLAIN ANALYZE results. This tool is also helpful to determine if the track_io_timing configuration parameter is likely to produce useful results. pg_test_timing accepts the following command-line options: -d duration --duration=duration Specifies the test duration, in seconds. Longer durations give slightly better accuracy, and are more likely to discover problems with the system clock moving backwards. The default test duration is 3 seconds. -c cutoff --cutoff=cutoff Specifies the cutoff percentage for the list of exact observed timing durations (that is, the changes in the system clock value from one reading to the next). The list will end once the running percentage total reaches or exceeds this value, except that the largest observed duration will always be printed. The default cutoff is 99.99. -V --version Print the pg_test_timing version and exit. -? --help Show help about pg_test_timing command line arguments, and exit. The first block of output has four columns, with rows showing a shifted-by-one log2(ns) histogram of timing durations (that is, the differences between successive clock readings). This is not the classic log2(n+1) histogram as it counts zeros separately and then switches to log2(ns) starting from value 1. The columns are: nanosecond value that is >= the durations in this bucket percentage of durations in this bucket running-sum percentage of durations in this and previous buckets count of durations in this bucket The second block of output goes into more detail, showing the exact timing differences observed. For brevity this list is cut off when the running-sum percentage exceeds the user-selectable cutoff value. However, the largest observed difference is always shown. The example results below show that 99.99% of timing loops took between 8 and 31 nanoseconds, with the worst case somewhere between 32768 and 65535 nanoseconds. In the second block, we can see that typical loop time is 16 nanoseconds, and the readings appear to have full nanosecond precision. Wiki discussion about timing