Hi, Mian, The two timing values are printed from two timing functions. They are about the same time period. The CPU seconds is measured function clock (or a similar function depending on system). This measurement is based on a sampling performed by CPU, therefore reflects how long the program is in the CPU, hence the name. The elapsed time is simply from the wall-clock time from the begin to the end of the time period. It is measured by gettimeofday (or a similar function if not available). The elapsed time can also be viewed as the real time (or total time). The difference between elapsed time and the CPU time can be regarded as the I/O time, however, this is not exactly accurate since the I/O operations also require CPU time.
Typically gettimeofday returns a value that is of higher precision than that returned by the function clock. In some cases, this can cause the elapsed time to appear smaller than the CPU time. This is simply a precision problem. Another way that CPU time can appear larger than the elapsed time is when running the program with multiple threads. For relatively small test datasets, most of the OSes can cache the content of the files being used and therefore significantly diminish the differences between the CPU time and the elapsed time. In many of our timing measurements, we explicitly unmount the file system before each query to make sure nothing is cached. The time we report in our publications are usually the elapsed time, unless we are specifically comparing CPU time. Hope this helps. John On 10/22/2009 9:36 PM, Mian Lu wrote: > Hi John: > > I want to confirm a question about the time measurements in the Fastbit. I > notice that after a query done, there are outputs: took X CPU seconds, Y > elapsed seconds. Then the X is the in-memory processing time, right? > Therefore the I/O access time actually is (Y-X) seconds, am I right? > Thanks in advance. > > Regards, > Mian > _______________________________________________ FastBit-users mailing list [email protected] https://hpcrdm.lbl.gov/cgi-bin/mailman/listinfo/fastbit-users
