VM: VM is much slower when timeout was specified

[mooyoul]

• Version:
  Node.js v6.7.0 / Node.js 4.4.2
• Platform:
  Ubuntu 14.04 LTS x64 (v6.7.0) / macOS Sierra (v4.4.2)
• Subsystem:
  VM

I just found there's unexpected behavior on timeout option of vm.runInContext.

When timeout option was specified, Script execution on VM is much slower then timeout options wasn't specified.

This is two of example snippets to reproduce that problem:

vm-normal.js

'use strict';

const
  vm = require('vm'),
  sandbox = {
    current: -1
  };

console.time('vm-normal');

vm.createContext(sandbox);

for (let i = 0 ; i < 100000 ; i++) {
  vm.runInContext(`current = ${i};`, sandbox, {
    displayErrors: false
  });
}

console.timeEnd('vm-normal');

vm-with-timeout.js

'use strict';

const
  vm = require('vm'),
  sandbox = {
    current: -1
  };

console.time('vm-with-timeout');

vm.createContext(sandbox);

for (let i = 0 ; i < 100000 ; i++) {
  vm.runInContext(`current = ${i};`, sandbox, {
    displayErrors: false,
    timeout: 1000
  });
}

console.timeEnd('vm-with-timeout');

I think maybe that's not a bug (timer for timeout is expansive?), but that's unexpected behavior. It would be nice if that limitation should be documented on VM API documentation.

[addaleax]

When timeout option was specified, Script execution on VM is much slower then timeout options wasn't specified.

The script execution isn’t slower, but of course there’s overhead associated with setting up the timer and stopping it again (especially because Node can’t use its usual facilities for that), which happens once for each runInContext call.

You can move the for loop into the script itself and the difference should vanish. You can feel free to open a PR against the documentation, of course.

[mscdex]

Also on an unrelated note, using the let in the for like that is slower (not sure about node v7).