![]() ![]() ![]() Then suppose we take just 2 stack samples, and we see instruction I on both samples, designated observation o=2/2. 0.9, 1.0, and the prior probability of each of these possibilities is 0.1, so all of these costs are equally likely a-priori. For simplicity, suppose we don't know what f is, but assume it is either 0.1, 0.2, 0.3. ![]() Suppose there is some instruction I (call or otherwise) which is on the call stack some fraction f of the time (and thus costs that much). That is natural, but you can be sure if it finds a problem it is real, and vice-versa.Īdded: Let me make a Bayesian explanation of how it works. Sampling tells you something is a problem, and your first reaction is disbelief. This comes from having a prior concept of what the real problem is.Ī key property of performance problems is that they defy expectations. The reason is that the time that would be saved by removal of an instruction is approximated by the fraction of samples containing it, regardless of the number of times it may occur within a sample.Īnother objection I often hear is: " It will stop someplace random, and it will miss the real problem". P.P.S As a rough generality, the more layers of abstraction you have in your software, the more likely you are to find that that is the cause of performance problems (and the opportunity to get speedup).Īdded: It might not be obvious, but the stack sampling technique works equally well in the presence of recursion. This can also be done on multi-thread programs if there is a way to collect call-stack samples of the thread pool at a point in time, as there is in Java. If you see a problem on more than one sample, it is real. They will say it sometimes finds things that aren't problems, but that is only true if you see something once. They will also say it only works on toy programs, when actually it works on any program, and it seems to work better on bigger programs, because they tend to have more problems to find. They give confusing summaries in the presence of recursion.They don't summarize at the instruction level, and.(The summaries are where the insight is lost.) Call graphs don't give you the same information, because They will say that profilers give you this information, but that is only true if they sample the entire call stack, and then let you examine a random set of samples. This magnification effect, when compounded over multiple problems, can lead to truly massive speedup factors.Ĭaveat: Programmers tend to be skeptical of this technique unless they've used it themselves. If you clean out any one of them, the remaining ones will take a larger percentage, and be easier to spot, on subsequent passes. You may have multiple performance problems of different sizes. If you do have a guess as to what the problem is, this will prove or disprove it. So, that is roughly the percentage of samples on which you will see it. ![]() If there is some code that is wasting some percentage of the time, 20% or 50% or whatever, that is the probability that you will catch it in the act on each sample. Just halt it several times, and each time look at the call stack. However, if you're in a hurry and you can manually interrupt your program under the debugger while it's being subjectively slow, there's a simple way to find performance problems. If your goal is to use a profiler, use one of the suggested ones. Obviously other communication strings remain untranslated here (Install, Answer) which would need to be addressed in a more fully completed translation, but even a partial translation would be helpful in many cases.įinally, please check out the excellent answer by F. Adapting my first example to better serve multiple languages might look like this: set - $(locale LC_MESSAGES) It also loops automatically, so there's no need for a while true loop to retry if they give invalid input.Īlso, Léa Gris demonstrated a way to make the request language agnostic in her answer. With select you don't need to sanitize the input – it displays the available choices, and you type a number corresponding to your choice. Here is the same example using select: echo "Do you wish to install this program?" Read -p "Do you wish to install this program?" ynĪnother method, pointed out by Steven Huwig, is Bash's select command. The best way to illustrate its use is a simple demonstration: while true do The simplest and most widely available method to get user input at a shell prompt is the read command. ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |