How to properly make method async?

Question

I have method for computing Levenshtein distance

public static LevenshteinMatches LevenshteinSingleThread(this string str, string expression, int maxDistance) { if (str.Length > expression.Length + 1) { int len = expression.Length; long strLen = str.Length - len + 1; int[] results = new int[strLen]; int[][,] dimension = new int[strLen][,]; for (int i = 0; i < strLen; i++) { dimension[i] = new int[len + 1, len + 1]; } string source = str; source = source.ToUpper(); expression = expression.ToUpper(); for (int i = 0; i < strLen; i++) { results[i] = SqueareLevenshtein(ref dimension[i], str.Substring(i, len).ToUpper(), expression, len); } LevenshteinMatches matches = new LevenshteinMatches(); for (int i = 0; i < strLen; i++) { if (results[i] <= maxDistance) { matches.addMatch(str.Substring(i, len), Math.Round((1.0 - ((double)results[i] / len)) * 100.0, 2), i, len, results[i]); } } return matches; } else { LevenshteinMatch match = str.LevenshteinCPU(expression, maxDistance); if (match != null) return new LevenshteinMatches(match); else return new LevenshteinMatches(); } }

What should I do to make it async?

Or should i leave this method and just call it some different way?

Here is my attempt to make it async; Not sure what is wrong but I can't get any results - thread is inf working but it should take only few ms.

public static async Task<LevenshteinMatches> LevenshteinSingleThread(this string str, string expression, int maxDistance) { return await Task.Factory.StartNew(() => { if (str.Length > expression.Length + 1) { int len = expression.Length; long strLen = str.Length - len + 1; int[] results = new int[strLen]; int[][,] dimension = new int[strLen][,]; for (int i = 0; i < strLen; i++) { dimension[i] = new int[len + 1, len + 1]; } string source = str; source = source.ToUpper(); expression = expression.ToUpper(); for (int i = 0; i < strLen; i++) { results[i] = SqueareLevenshtein(ref dimension[i], str.Substring(i, len).ToUpper(), expression, len); } LevenshteinMatches matches = new LevenshteinMatches(); for (int i = 0; i < strLen; i++) { if (results[i] <= maxDistance) { matches.addMatch(str.Substring(i, len), Math.Round((1.0 - ((double)results[i] / len)) * 100.0, 2), i, len, results[i]); } } return matches; } else { LevenshteinMatch match = str.LevenshteinCPU(expression, maxDistance); if (match != null) return new LevenshteinMatches(match); else return new LevenshteinMatches(); } }); }

rest of the code link

And that's how I call it:

string s = "xcjavxzcbvmrmummuuutmtumuumtryumtryumtrutryumtryumtrymutryumtyumtryumtrmutyumtrurtmutymurtmyutrymut"; s = string.Concat(Enumerable.Repeat(s, 4000)); var watch = System.Diagnostics.Stopwatch.StartNew(); var ret = s.LevenshteinSingleThread("jas", 1); var res = ret.Result; watch.Stop(); var elapsedMs = watch.ElapsedMilliseconds;

With your second example you're likely deadlocking if your calling the code like that from a UI context. You need to await ret instead of .Result. — JSteward
– JSteward, Commented Nov 16, 2017 at 20:18
You don't make the method asynchronous. Nothing about what that method is doing is actually asynchronous. — Servy
– Servy, Commented Nov 16, 2017 at 20:18
It would seem that your code is CPU bound and unsuited to async await. You may however gain some benefit by parallelizing certain hotspots? — StuartLC
– StuartLC, Commented Nov 16, 2017 at 20:24

Scott Chamberlain · Accepted Answer · 2017-11-16 20:30:55Z

There is nothing async about your function, it is entirely CPU bound work. Changing the signature to async Task<LevenshteinMatches> and never using await in the function should have caused a warning in the compiler.

Instead of "making it async" if the thing you are really after is making it work in parallel then just call the code in parallel.

string s = "xcjavxzcbvmrmummuuutmtumuumtryumtryumtrutryumtryumtrymutryumtyumtryumtrmutyumtrurtmutymurtmyutrymut"; s = string.Concat(Enumerable.Repeat(s, 4000)); var expressions = new[] {"jas", "cbv"} var tasks = new List<Task<LevenshteinMatches>>() foreach(var expression in expressions) { var task = new Task.Run(()=> message.LevenshteinSingleThread(expression, 1)); //Start multiple threads tasks.Add(task); } LevenshteinMatches[] results = Task.WaitAll(tasks); //Wait for all the threads to end.

You could even make parts of the internal function multi-threaded by using Parallel.For( to make some of the for loops parallel, just be careful that any collections you call Add on are either synchronized inside a lock or are thread safe collections.

For example if SqueareLevenshtein is thread safe internally you could do

Parallel.For(0, strLen, i => { //Are you sure ref is needed here? results[i] = SqueareLevenshtein(ref dimension[i], str.Substring(i, len).ToUpper(), expression, len); }); LevenshteinMatches matches = new LevenshteinMatches(); Parallel.For(0, strLen; i => { if (results[i] <= maxDistance) { lock(matches) { matches.addMatch(str.Substring(i, len), Math.Round((1.0 - ((double)results[i] / len)) * 100.0, 2), i, len, results[i]); } } }); return matches;

I used ref because i wanted multiple threads to work on separate slices, but can be removed now

Collectives™ on Stack Overflow

How to properly make method async?

1 Answer 1

1 Comment

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Related