Increasing Script Performance, Part III
How do the different languages compare? Because you cannot use MetaBench for this comparison, here are the (rough) results.put x + 1 into x
Pascal: x := x+1 --> 1The relative speeds might be different for different computers or operating systems of course. Nevertheless, it can easily be seen that HyperCard is much slower than MetaCard. MetaCard however is much slower than Pascal or C. This difference may suggest that writing a simple extension that adds up numbers might considerably enhance the speed of calculations. Unfortunately this is not true. The overhead of calling an extension and sending the result back to MC outweighs the gain. In the table above you can see that calling an "empty" extension (an extension that just does nothing) would take 3500 time units. Extensions can enhance the speed, but only if the total amount of calculations performed by the extension outweighs calling the extension and sending the result back. It is easy to see that if your extension performs an adding operation 100 times, you have a significant speed gain (about 3600 versus 8000 units). More about extensions in a forthcoming part. Hence we need different approaches to enhance the speed of calculations. Don't expect to learn how to add up numbers faster. But you can expect how to prevent that scripts manipulating numbers may run 10 or even 50 times (!) or so slower than necessary. First some fairly simple things to get the most out of calculations.
C++: x = x+1 --> 1
MetaCard: add 1 to x --> 80
MetaCard: put x + 1 into x --> 130
HyperCard: add 1 to x --> 2800
HyperCard: put x + 1 into x --> 3600
Empty extension: --> 3500
You most likely know that
executes faster then:
add 1 to x
The reason is that in 'add 1 to x', the variable x is accessed only once, whereas in 'put x + 1 into x' it is accessed two times. The computer doesn't know that the result of 'x + 1' should be put in the same place x and has to figure out twice where in memory x resides; execution of the statement is similar to the execution of 'put x + 1 into y'.
put x + 1 into x
Probably you already preferred to write 'add 1 to x' instead of 'put x + 1 into x': you have less characters to type. Be honest, do you also always write 'subtract 1 from x' instead of 'put x - 1 into x'? The speed gain is the same, for the very same reason. And this also holds of course for the multiply and divide operators; or for x^2 versus x*x.
Accessing variables as least as possible is a general principle to enhance the speed. Suppose you have a complex calculation:
The statement (the formula for the Pearson Correlation, if you really want to know) is hardly readable and probably you run into trouble in putting the parentheses at the right places. So the next script looks much better:put (Ncases*covXY-sumX*sumY)/ sqrt((Ncases*sum2X-(sumX)^2)*(Ncases*sum2Y-(sumY)^2)) into r
Much nicer, but the script is 30% slower because of the overhead of putting and getting the variables c, r1 and r2. The general rule is: minimize accessing variables, either to get them, or to put something in them.put Ncases * covXY - sumX * sumY into c put Ncases * sum2X - (sumX)^2 into r1 put Ncases * sum2Y - (sumY)^2 into r2 put c / sqrt(r1 * r2) into r
The div and mod operators are often used for particular transformations of numbers. For example, to transform number 0 to 7 into 7, 8 to 15 into 15, 16 to 23 into 23, etc. you may use:
(i is the number to transform, x signifies the range, 8 in this case) To transform 0 to 7 into 0 to 7, 8 to 15 into 0 to 7, 16 to 23 into 0 to 7, etc. you may use:
get (i div x + 1) * x - 1
For particular numbers (i.e. 2, 4, 8, 16, etc.) it is faster to use the bitwise operators:
(i mod x + 1) - 1
i bitor (x-1)
to obtain the same result, but more then 50 % faster.
i bitand (x-1)
To determine whether a number is odd or even, you can use:
(true if even, false if odd)
(x bitand 2) = 0
If you wonder what bitwise operators are (the documentation in MC is minimal), see the text at the end of this contribution.
Now for something completely different. Suppose you have read a real number x, e.g. "0.345678" from a field. Compare these two scripts:
script D:get x/(1-x)
Believe it or not, but script D will run considerably faster. If the content of a field (or a file) is read, MC hasn't the faintest idea that it's meant to be a number, and stores it as a string of characters. Thus it has to convert the string into a number, including a check whether or not x is a number before it actually starts calculating. In executing script C, this conversion takes place two times. In script D however, the conversion takes place only once. If 'add 0 to x' is executed, the string x is converted to a number, and the number is stored into x: now x no longer contains the string "0.345678" but (a binary representation of) the number 0.345678. Thus it saves a lot of time in performing calculations on it. The more x's in the equation, e.g. x*(1+x)/(1-x), the more you gain. It's a bit tricky to perform a speed test on script D, to compare it with C. Consider script E: script E:add 0 to x get x/(1-x)
After the first loop x is converted to a number. Hence the conversion takes place only once, instead of 100000 times. You should be very well aware of such peculiarities in performing speed tests. But you can safely try:put fld "inputFld" into x repeat 100000 add 0 to x get x/(1-x) end repeat
It will run nearly 10 times as fast compared to a script without 'add 0 to x', excluding the overhead of the repeat loop.put fld "inputFld" into x add 0 to x -- convert it to a real number repeat 100000 get x/(1-x) end repeat
Generally, if you store a variable, and MC has become aware that this variable is a number, it will be stored as a binary number: no conversion is needed if it is used for further calculations. You may note that if your are doing speed tests on numbers, and test numbers are read from a field, results will be completely unreliable, if you assume that the numbers read, are binary numbers.
If a variable is stored as a binary number, and you use statements like 'get length (x)' or 'get char 3 of x', it has to be converted to a string, before the statement can be executed. For some reason, converting a number to a string is much slower then converting a string to a number. Converting an integer to a string takes about 3 times as much as the other way around, whereas converting a real number is even slower, because of the more complex formatting (by the way, MC does not distinguish internally between integers and real numbers; all numbers are stored as reals). Suppose variable x is read from a field, containing "0.345678". Compare:
script H:add 0 to x -- convert it to a real number repeat 100000 get last char of x end repeat
Script G may take about 50 times as long as script H...repeat 100000 get last char of x end repeat
If variable x is binary stored, and you are going to perform a number of string operations on it (like if char 1 of x = "2"; get offSet (".", x); put length (x) into s), you better copy it into another variable and convert the copy to a string first, for example:
You should leave the original variable x untouched. Compare script I and J:put x into s put empty after s
script J:put 1/3 into x put last char of x into last char of x put 3*x
Script I yields 0.999999 (if the local numberFormat has the default value), script J yields 1. Because you converted the number to a string, you lost precision. To illustrate things, here is a nice example from Xavier, in a reaction to Part 1 of How to speed up MC. He wrote:put 1/3 into x put 3*x
Although his observations are correct, his solution is not (sorry Xavier, but I love your example because it illustrates that we really can gain a lot). The variable x is a binary one, not a string one (it's produced by MC itself). So each time 'if char -3 of x is not oldx then' is executed, x is converted to a string, to be able to get character -3: it takes an awful lot of time. To be precise, x itself remains intact, the converted copy is put in some internal temporary variable. The script takes 370 ticks on my computer, neglecting 'showStatus' of course. Alternatively, take a look at script J:> Another 2 things to consider in your loops: > - set cursor to busy > will slow down scripts considerably. > > - If you have a "progress" bar, dont change it at every loop, > do so for each percent change or at every 100th... > this will also save major time... > > for example > repeat with x = 1 to 100000 > if char -3 of x is not oldx then > set cursor to busy > showstatus > put char -3 of x into oldx > end if
It takes 160 ticks. Not that much difference? Please note the difference in the number of repeats: hundred and thousand versus one million! This script has some of other advantages too:put the ticks into myTestTime repeat with x = 1 to 1000000 if the ticks - myTestTime > 5 then put the ticks into myTestTime set cursor to busy end if -- do something end repeat
If analyses take more then 10 ticks, case 0 will occur the least, hence this case is the last one; if the analysis takes less then 10 ticks -- well, why bother? The five ticks delay before a progress bar appears will not be observed by the user. Take the switch part (not the ticks check!) out of the script to create a function handler, returning myProgresStatus; this will hardly add time, because it is only called each five ticks.put 60 into myDelayPref -- but better let user decide on whether progress bar will be -- shown if analysis is expected to take more then 1 second, -- 2 seconds, 5 seconds or whatever put 0 into myProgresStatus put the number of lines of myData into nLines put the ticks into myTestTime repeat with x = 1 to nLines if the ticks - myTestTime > 5 then switch myProgresStatus case 2 -- analysis takes more then one second myAdjustProgressBar -- adjusts the progress bar break case 1 -- analysis will take less then 1 second set cursor to busy break case 0 -- not checked yet if (5*nLines)/x > myDelayPref then -- take 5 ticks as criterion for updating myShowProgressBar -- show a progress bar put 2 into myProgresStatus else put 1 into myProgresStatus end if break end switch put the ticks into myTestTime end if -- do something end repeat
Posted on 4/14/03 by Will Dijkstra to the MetaCard list (See the complete post/thread)