The true delight is in the finding out rather than in the knowing. — Isaac Asimov

The Promises/A+ specification is a fresh and very interesting way of dealing with the asynchronous nature of Javascript. It also provides a sensible way to deal with error handling and exceptions. In this article we will go through the performance hits you should be aware of and as a side-effect do a comparison between the two most popular Promises/A+ implementations, When and Q and how they compare to Async, the lowest abstraction you can get on asynchronicity.

 

Basic nodejs single thread architecture:

 

The Case

My motivation for looking deeper into the performance of Promises/A+ was a Job Queuing system i’ve been working on named Kickq. It is expected that the system will get hammered when used on production so stress testing was warranted. After stubbing all the database interactions, essentially making the operation of job creation synchronous, I was getting odd performance results.

The test was simple, create 500 jobs in a loop and measure how long it takes for all the jobs to finish.

The measurements were in the ~550ms range and my eyeballs started to roll. “That’s a synchronous operation, it should finish in less than 3ms, WHAT THE????!?!”. After taking a few moments to let it sip in the suspect was found, it was Promises. I used them as the only pattern to handle asynchronous ops and callbacks throughout the whole project. Brian Cavalier, one of the authors of When.js, helped me pinpoint the real culprit, it was the tick:

Promises/A+ Specification, Note 4.1 In practical terms, an implementation must use a mechanism such as setTimeout, setImmediate, or process.nextTick to ensure that onFulfilled and onRejected are not invoked in the same turn of the event loop as the call to then to which they are passed.

In other words, Promises, per the specification, must be resolved Asynchronously! That comes with a cost, a heavy one apparently.

In the process of studying performance I had to create a performance library, poor mans profiling. And a benchmark test for Promises/A+ implementations that’s already used to optimize the future versions of When.

 

Creating The Promises/A+ Benchmark

I tried to broaden the definition of the test case. If an application uses the Promises pattern as the only way to manage how the internal parts interact, we can make a few assumptions:

• There will be a series of promises chained together, representing the various operations that will be performed by your application.
• The Deferred Object is used on each link of the chain to control resolution and how the promise object is exposed.
• Throughout the whole chain of promises there can be operations that are actually synchronous, we will measure all cases.

Difference to First Resolved Promise, 500 Loops

Perf Type	Async	When 2.1.0	Q 0.9.5	Promise 3.0.1
Sync Diff	0.01ms	36.62ms	186.43ms	63.96ms
Mixed Diff	5.37ms	41.78ms	226.34ms	83.83ms
Async Diff	22.42ms	58.18ms	241.80ms	93.68ms
Sync Diff vs AsyncLib	1x	3,662x	18,643x	6,396x
Mixed Diff vs AsyncLib	1x	7.78x	42.15x	15.61x
Async Diff vs AsyncLib	1x	2.60x	10.79x	4.18x

Libraries When.js v1.8.1 and Deferred are not included in this table because they resolve promises synchronously. This difference makes the Diff metric inapplicable.

Total Time of execution, 500 Loops

Perf Type	Async	When 1.8.1	When 2.1.0	Q 0.9.5	Deferred 0.6.3	Promise 3.0.1
Sync Total	5.15ms	12.35ms	72.35ms	301.47ms	71.25ms	80.50ms
Mixed Total	18.94ms	40.57ms	80.21ms	325.49ms	94.58ms	95.67ms
Async Total	35.70ms	50.63ms	90.52ms	337.82ms	105.87ms	107.01ms
Sync Total vs AsyncLib	1x	2.40x	14.05x	58.54x	13.83x	15.63x
Mixed Total vs AsyncLib	1x	2.14x	4.23x	17.19x	4.99x	5.05x
Async Total vs AsyncLib	1x	1.42x	2.54x	9.46x	2.97x	3.00x

Average Memory Difference – Single 500 Loop Runs

Pert Type	Async	When 1.8.1	When 2.0.1	When 2.1.x	Q	Q longStack=0	Deferred
Sync	113.29%	160.98%	840.21%	866.88%	1106.67%	684.56%	354.07%
Async	159.29%	458.44%	811.32%	834.63%	1110.21%	691.41%	429.18%

 

via http://thanpol.as/javascript/promises-a-performance-hits-you-should-be-aware-of/