Node.js cluster - optimal number of workers

Question

I have 4 cores and ran this code according to this example :

var cluster = require('cluster');
var http = require('http');
var numCPUs = require('os').cpus().length;

var id = 0;
if (cluster.isWorker) {
    id = cluster.worker.id;
}

var iterations = 1000000000;
console.time('Function #' + id);
for (var i = 0; i < iterations; i++) {
    var test = 0;
}
console.timeEnd('Function #' + id);

if (cluster.isMaster) {
    // Fork workers.
    for (var i = 0; i < numCPUs; i++) {
        cluster.fork();
    }
}

With 4 fork (the code above), I got :

Function #0: 1698.801ms

Function #1: 3282.679ms

Function #4: 3290.384ms

Function #3: 3425.090ms

Function #2: 3424.922ms

With 3 fork, I got :

Function #0: 1695.155ms

Function #2: 1822.867ms

Function #3: 2444.156ms

Function #1: 2606.680ms

With 2 fork, I got :

Function #0: 1684.929ms

Function #1: 1682.897ms

Function #2: 1686.123ms

I don't understand these results. Isn't 1 fork/core the optimal number ? Here I see that 4 fork is not better than 2 fork.

score 7 · Accepted Answer · answered Sep 15 '16 at 14:03

My guess is that your hardware actually only has 2 physical cores. However, because of hyper-threading (HT), the OS will say that there are 4 (logical) cores present.

The workers in your code keep a (physical) core entirely occupied, which is something that HT can't deal with very well, so the performance when keeping all 4 logical cores busy will be worse than when you keep only the 2 physical cores busy.

My hardware (quad core, so 4 physical and 8 logical cores) shows the same pattern:

8 workers:

Function #5: 926ms
Function #3: 916ms
Function #1: 928ms
Function #4: 895ms
Function #7: 934ms
Function #6: 905ms
Function #8: 928ms
Function #2: 928ms

4 workers:

Function #3: 467ms
Function #2: 467ms
Function #1: 473ms
Function #4: 472ms

That said, the rule of thumb of making the number of workers equivalent to the number of logical cores in your hardware still makes sense if your workers are I/O bound (which most Node apps are).

If you really want to perform heavy, blocking, calculations, count one physical core per worker.

Thanks for the information. Then for Node.js the other half of the cores are only used for the asynchronous code ? Does it requires cluster or does it means that Node.js uses only one execution thread but **also** others parallels threads (~ cores ?) for the asynchronous code ? Also, as I'm developping a game, it's mainly CPU bound. But I saw in a github work some `setTimeout(updateClients, 0);` in the mainloop because "asynchronous updates increase performance". Are they right ? — zbeyens, Sep 15 '16 at 16:21
Node itself doesn't schedule anything, that's up to the OS (and also the CPU itself). If you have CPU-bound code, `cluster` can help you because it'll allow you to run the CPU-heavy code in a separate process, that the OS can schedule onto a different code as the "main" part of the app. Using the `setTimeout()` trick is useful if you're mixing I/O-bound and CPU-bound core together in the same process. — robertklep, Sep 15 '16 at 18:38

Node.js cluster - optimal number of workers

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