How do you tell whether a bottleneck is CPU, memory, I/O, or network?

Question

Accepted Answer

**Look at resource utilization and saturation, then confirm with measurement.** A systematic approach — the **USE method** (Utilization, Saturation, Errors) — checks each resource so you don't blame the wrong one.

## The USE method

For every resource (CPU, memory, disk, network), ask three things:

```text
UTILIZATION → how busy is it (% time in use)?
SATURATION  → how much queued/waiting work (load, run queue, swap)?
ERRORS      → any errors (dropped packets, disk errors, OOM kills)?
→ a resource that is saturated, not just busy, is usually the bottleneck.
```

## Tools and symptoms per resource

Each resource has a distinct signature you can read with standard tools:

```text
CPU     → top/htop: high %us or load > #cores, run queue backed up
MEMORY  → free/vmstat: low free RAM + high SWAP in/out (si/so) → swapping = thrashing
DISK I/O→ iostat/vmstat: high %iowait, %util ~100%, high await → I/O-bound
NETWORK → iftop/sar/ss: bandwidth saturated, retransmits, high latency
```

The key distinctions: **high load with high %us** means CPU-bound; **swapping (si/so)** means memory pressure; **high %iowait** means the CPU is idle *waiting on disk*; high **latency/retransmits** points at the network.

```text
$ vmstat 1
 r  b   swpd  free   si  so   bi   bo  us sy id wa
 8  0      0  120m   0   0   12   20  92  6  2  0   ← CPU-bound (id=0, wa=0)
 1  3 800m   30m  40 120  900    8   5  3  2 90   ← I/O + swap (wa=90, so>0)
```

## Confirm with measurement

Never stop at one number. **Profile** the process (e.g. `perf`, a CPU/heap profiler) to find the hot function or allocation, change one thing, and **re-measure**. Symptoms can mislead — high CPU might really be a busy-wait caused by lock contention, not real work.

## Why it matters

Fixing the wrong resource (adding RAM to a CPU-bound box) wastes money and doesn't help. The USE method plus the right tool isolates the real constraint quickly, and re-measuring confirms the fix moved the needle.