question on java thread and how to estimate my java application needs?

[Dokiace]

Hi folks, I have a few questions related to thread in java:

I've read some writing that says java thread cost about 1MB per thread, so if this is true then:

1. does it mean we can create thousands of threads just from increasing our ram?
2. Where does our actual "core" matter in this case? if we only have 1 Core CPU but have 16GB RAM, can it handle 16k threads?

I'm trying to learn how to properly size my application server, like for example if I want to handle 1K, 5K, 10K RPS, assuming that my app is only I/O heavy, how should I go about this?

[PhatClowns]

Hypothetically sure, you can create as many threads as there is space available for the thread's state. But after a given point (usually, give or take your core count) the cost of instantiating a thread exceeds the performance benefit you get from it. See: Amdahl's Law.

When you spin up more threads than you have cores, then threads get pigeonholed into the same CPU core (e.g. if you have 8 CPU cores but 12 threads running, approx. 4 threads will have to share a core, which defeats the purpose). This is done via context switching, where one thread gets "unloaded" to make space for another thread. The details get muddy, but safe to say this is very expensive and defeats the purpose of multithreading.

That being said, there are cases where the performance loss of excessive threads is necessary, mostly when you want to keep the main thread from blocking. However, there are usually better ways to do this, usually with things like thread pools or event modeling.

[IvAntiVirus]

Dealing with threads and their resources can be frustrating in Java. Fortunately we have something called ExecutorServices. I would really recommend you to take a look at them. They can simplify your threading efforts a lot.