Add ability to configure numCache about mqtt-packet

[mcollina]

I would not merge any PRs that have any form of performance degradation.

How did you measured the 14MB?

2 * 65536 = 131072 bytes = 128 KB.

Something else should be going on. Maybe we need to allocate a big buffer, and then slice through it.

[kirilknysh]

I just took the existing number.js, added setInterval(() => {}, 9999); (just not to finish the process immediately) and run node number.js. Then I measured memory with just regular top command (MacOS Sierra, 10.12.4, node v6.5.0).

Node process with the original number.js - 22Mb
Node process with patched code - 9Mb

Keep in mind that this is not JS heap but node process itself.

[mcollina]

@kirilknysh so, how do you think we can make this configurable? Also, we might check if using an array improve things.

[kirilknysh]

I've tested with an array. Regarding memory - no big difference in both: memory usage and performance.

How to make it configurable - not really sure. Unfortunately, I'm not closely familiar with the overall architecture of the mqttjs. From the first sight - the best candidate is connect packet in writeToStream.js (assuming that connect command is always the first in a flow). Then it could be done similar to settings like protocolId, protocolVersion etc.

[mcollina]

I don't think it's feasible in that way, and I would not like to change the API.

How about we do:

mqttPacket.cacheNumbers = false

which then replaces the current system? We can do that with a getter I think.

Would you like to attempt a PR?

[kirilknysh]

Sure, I'd love to attempt. One thing: how do you expect to get access to mqttPacket.cacheNumbers from writeToStream.js?

[mcollina]

I think we might call a method in writeToStream and replace it? Be careful to lazy-load numbers.js only when needed.

[kirilknysh]

How about API like:

var mqttPacket = require('mqtt-packet');
mqttPacket.writeToStream.cacheNumbers = false;
...
mqttPacket.writeToStream(packet, client.stream)
...

numCache would be initialized in the first call of writeToStream based on cacheNumbers prop (either generate cache or not). Potential draw back - a bit slower first call.

[mcollina]

that'd be good for me.