javascript - Slow delete of object properties in JS in V8

Question

Just to train myself a bit of Typescript I wrote a simple ES6 Map+Set-like implementation based on plain JS Object. It works only for primitive keys, so no buckets, no hash-codes, etc. The problem I encountered is implementing delete method. Using plain delete is just unacceptably slow. For large maps it's about 300-400x slower than ES6 Map delete. I noticed the huge performance degradation if size of the object is large. On Node JS 7.9.0 (and Chrome 57 for example) if object has 50855 properties delete performance is the same as ES6 Map. But for 50856 properties the ES6 Map is faster on 2 orders of magnitude. Here is the simple code to reproduce:

// for node 6: 76300
// for node 7: 50855
const N0 = 50855;

function fast() {
const N = N0

const o = {}
for ( let i = 0; i < N; i++ ) {
o[i] = i
}

const t1 = Date.now()
for ( let i = 0; i < N; i++ ) {
delete o[i]
}
const t2 = Date.now()

console.log( N / (t2 - t1) + ' KOP/S' )
}

function slow() {
const N = N0 + 1 // adding just 1

const o = {}
for ( let i = 0; i < N; i++ ) {
o[i] = i
}

const t1 = Date.now()
for ( let i = 0; i < N; i++ ) {
delete o[i]
}
const t2 = Date.now()

console.log( N / (t2 - t1) + ' KOP/S' )
}

fast()

slow()

Answer 1

(V8 developer here.) Yes, this is a known issue. The underlying problem is that objects should switch their elements backing store from a flat array to a dictionary when they become too sparse, and the way this has historically been implemented was for every delete operation to check if enough elements were still present for that transition not to happen yet. The bigger the array, the more time this check took. Under certain conditions (recently created objects below a certain size), the check was skipped -- the resulting impressive speedup is what you're observing in the fast() case.

I've taken this opportunity to fix the (frankly quite silly) behavior of the regular/slow path. It should be sufficient to check every now and then, not on every single delete. The fix will be in V8 6.0, which should be picked up by Node in a few months (I believe Node 8 is supposed to get it at some point).

That said, using delete causes various forms and magnitudes of slowdown in many situations, because it tends to make things more complicated, forcing the engine (any engine) to perform more checks and/or fall off various fast paths. It is generally recommended to avoid using delete whenever possible. Since you have ES6 maps/sets, use them! :-)