When timing how long a quick process runs, how many runs should be used?_问答_开发者

Lets say I am going to run process X and see how long it takes. I am going to save into a database a date I ra开发者_运维技巧n this process, and the time it took. I want to know what to put into the DB.

Process X almost always runs under 1500ms, so this is a short process. It usually runs between 500 and 1500ms, quite a range (3x difference).

My question is, how many "runs" should be saved into the DB as a single run?

Every run saved into the DB as its own row?
5 Runs, averaged, then save that time?
10 Runs averaged?
20 Runs, remove anything more than 2 std deviations away, and save everything inside that range?

Does anyone have any good info backing them up on this?

Save the data for every run into its own row. Then later you can use and analyze the data however you like... ie, all you the other options you listed can be performed after the fact. It's not really possible for someone else to draw meaningful conclusions about how to average/analyze the data without knowing more about what's going on.

The fastest run is the one that most accurately times only your code.

All slower runs are slower because of noise introduced by the operating system scheduler.

The variance you experience is going to differ from machine to machine, and even on identical machines, the set of runnable processes will introduce noise.

None of the above. Bran is close though. You should save every measurment. But don't average them. The average (arithmetic mean) can be very misleading in this type of analysis. The reason is that some of your measurments will be much longer than the others. This will happen becuse things can interfere with your process - even on 'clean' test systems. It can also happen becuse your process may not be as deterministic as you might thing.

Some people think that simply taking more samples (running more iterations) and averaging the measurmetns will give them better data. It doesn't. The more you run, the more likelty it is that you will encounter a perturbing event, thus making the average overly high.

A better way to do this is to run as many measurments as you can (time permitting). 100 is not a bad number, but 30-ish can be enough.

Then, sort these by magnitude and graph them. Note that this is not a standard distribution. Compute compute some simple statistics: mean, median, min, max, lower quaertile, upper quartile.

Contrary to some guidance, do not 'throw away' outside vaulues or 'outliers'. These are often the most intersting measurments. For example, you may establish a nice baseline, then look for departures. Understanding these departures will help you fully understand how your process works, how the sytsem affecdts your process, and what can interfere with your process. It will often readily expose bugs.

Depends what kind of data you want. I'd say one line per run initially, then analyze the data, go from there. Maybe store a min/max/average of X runs if you want to consolidate it.

http://en.wikipedia.org/wiki/Sample_size

Bryan is right - you need to investigate more. if your code has that much variance even "most" of the time then you might have a lot of fluctuation in your test environment because of other processes, os paging or other factors. If not it seems that you have code paths doing wildly varying amount of work and coming up with a single number/run data to describe the performance of such a multi-modal system is not going to tell you much. So i'd say isolate your setup as much as possible, run at least 30 trials and get a feel for what your performance curve looks like. Once you have that, you can use that wikipedia page to come up with a number that will tell you how many trials you need to run per code-change to see if the performance has increased/decreased with some level of statistical significance.

While saying, "Save every run," is nice, it might not be practical in your case. However, I do think that storing only the average eliminates too much data. I like storing the average of ten runs, but instead of storing just the average, I'd also store the max and min values, so that I can get a feel for the spread of the data in addition to its center.

The max and min information in particular will tell you how often corner cases arise. Is the 1500ms case a one-in-1000 outlier? Or is it something that recurs on a regular basis?