Caching Script/Module instances

[sr229]

There seems to be no clear documentation on how to cache Script/Module instances made by Esprima. It would be nice if I could be pointed to the right direction on how to get the script/module instance from Jint to cache.

[lahma]

There are static functions for this, Engine.PrepareScript() and Engine.PrepareModule().

[lahma]

You can keep the output in memory. The engine API has methods that accept the Script/Module instance. See Execute, Evaluate and AddModule.

[unruledboy]

@lahma what is the complication / right usage of script?

I tried to run the following test logic:

using System.Diagnostics;
using Jint;

var engine = new Engine();

var script = Engine.PrepareScript(@"
    const result = Math.min(value, 3);

    return result;
");

var result = new List<double>();
var t = Stopwatch.StartNew();
for (int i = 0; i < 1000; i++)
{
    engine.SetValue("value", i);
    var x = engine.Evaluate(script);
    result.Add(x.AsNumber());
}

Console.WriteLine(t.Elapsed.TotalMilliseconds);
Console.WriteLine($"{result.Min()} {result.Max()}");

And it gave this error:

Jint.Runtime.JavaScriptException: 'Identifier 'result' has already been declared'

Looks like the script is kept in a closure and when it is executed more than once, it will have above issue?

[unruledboy]

Right, after more research and according to your answer to #1398 , it works fine if I move the engine instantiation into the loop:

using System.Diagnostics;
using Jint;

var script = Engine.PrepareScript(@"
    const result = Math.min(value, 3);

    return result;
");

var result = new List<double>();
var t = Stopwatch.StartNew();
for (int i = 0; i < 100000; i++)
{
    var engine = new Engine();
    engine.SetValue("value", i);
    var x = engine.Evaluate(script);
    result.Add(x.AsNumber());
}

Console.WriteLine(t.Elapsed.TotalMilliseconds);
Console.WriteLine($"{result.Min()} {result.Max()}");

So, does it mean the cost of instantiating Engine object is low?

We have a use case where it would evaluate like 10 scripts, and each script could run 100,000 times.

What is the best practice WRT to performance as well memory cost?

[lahma]

• There's always a cost to instantiate engine, but it will give you a clean starting point if your scripts pollute the global state like in your example, we've tried to make the engine fast to construct
• If you have static set of scripts which will be run multiple times, you should cache the results from Engine.PrepareScript and always feed that to the engine
• You need to measure, it all depends on the use case

[unruledboy]

Thanks @lahma , we will cache and reuse the script from Engine.PrepareScript as much as possible.

Initial performance tests are looking good, and it works like a charm! Well done!