Understanding Linux CPU Load – when should you be worried?
You might be familiar with Linux load averages already. Load averages are the three numbers shown with the uptime and top commands – they look like this:
load average: 0.09, 0.05, 0.01
Most people have an inkling(模糊概念) of what the load averages mean: the three numbers represent averages over progressively(渐进地) longer periods of time (one, five, and fifteen minute averages), and that lower numbers are better. Higher numbers represent a problem or an overloaded machine. But, what's the the threshold(临界值)? What constitutes "good" and "bad" load average values? When should you be concerned over a load average value, and when should you scramble(仓促行动) to fix it ASAP?
First, a little background on what the load average values mean. We'll start out with the simplest case: a machine with one single-core processor.
The traffic analogy(类比)
- A single-core CPU is like a single lane(单行道) of traffic. Imagine you are a bridge operator … sometimes your bridge is so busy there are cars lined up to cross. You want to let folks know how traffic is moving on your bridge. A decent metric would be how many cars are waiting at a particular time. If no cars are waiting, incoming drivers know they can drive across right away. If cars are backed up（阻塞）, drivers know they're in for delays.
So, Bridge Operator, what numbering system(计算系统) are you going to use? How about:
- 0.00 means there's no traffic on the bridge at all. In fact, between 0.00 and 1.00 means there's no backup(阻塞), and an arriving car will just go right on.
- 1.00 means the bridge is exactly at capacity(以最大程度). All is still good, but if traffic gets a little heavier, things are going to slow down.
- over 1.00 means there's backup. How much? Well, 2.00 means that there are two lanes worth of cars total — one lane's worth on the bridge, and one lane's worth waiting. 3.00 means there are three lane's worth total — one lane's worth on the bridge, and two lanes' worth waiting. Etc.
This is basically what CPU load is. "Cars" are processes using a slice of CPU time ("crossing the bridge") or queued up(排队等候) to use the CPU. Unix refers to(参考) this as the run-queue length(运行队列的长度): the sum of the number of processes that are currently running plus the number that are waiting (queued) to run.
Like the bridge operator, you'd like your cars/processes to never be waiting. So, your CPU load should ideally( 理想地) stay below 1.00. Also like the bridge operator, you are still ok if you get some temporary spikes(峰值) above 1.00 … but when you're consistently（始终） above 1.00, you need to worry.
So you're saying the ideal load is 1.00?
Well, not exactly. The problem with a load of 1.00 is that you have no headroom（剩余空间）. In practice（在实践中）, many sysadmins will draw a line at 0.70:
- The "Need to Look into it" Rule of Thumb（经验法则）: 0.70 If your load average is staying above > 0.70, it's time to investigate before things get worse.
- The "Fix this now" Rule of Thumb: 1.00. If your load average stays above 1.00, find the problem and fix it now. Otherwise, you're going to get woken up in the middle of the night, and it's not going to be fun.
- The "Arrgh, it's 3AM WTF?" Rule of Thumb: 5.0. If your load average is above 5.00, you could be in serious trouble, your box is either hanging or slowing way down, and this will (inexplicably) happen in the worst possible time like in the middle of the night or when you're presenting at a conference. Don't let it get there.
What about Multi-processors? My load says 3.00, but things are running fine!
Got a quad-processor（四核处理器） system? It's still healthy with a load of 3.00.
On multi-processor system, the load is relative to the number of processor cores available. The "100% utilization（利用）" mark is 1.00 on a single-core system, 2.00, on a dual-core, 4.00 on a quad-core, etc.
If we go back to the bridge analogy, the "1.00" really means "one lane's worth of traffic". On a one-lane bridge, that means it's filled up. On a two-late bridge, a load of 1.00 means its at 50% capacity — only one lane is full, so there's another whole lane that can be filled.
Same with CPUs: a load of 1.00 is 100% CPU utilization on single-core box. On a dual-core box, a load of 2.00 is 100% CPU utilization.
Multicore vs. multiprocessor
While we're on the topic, let's talk about multicore vs. multiprocessor. For performance purposes, is a machine with a single dual-core processor basically equivalent to a machine with two processors with one core each? Yes. Roughly. There are lots of subtleties（微妙） here concerning amount of cache, frequency of process hand-offs between processors, etc. Despite those finer points, for the purposes of sizing up the CPU load value, the total number of cores is what matters, regardless of how many physical processors those cores are spread across.
Which leads us to a two new Rules of Thumb:
- The "number of cores = max load" Rule of Thumb: on a multicore system, your load should not exceed the number of cores available.
- The "cores is cores" Rule of Thumb: How the cores are spread out over CPUs doesn't matter. Two quad-cores == four dual-cores == eight single-cores. It's all eight cores for these purposes.