不错的文章,帮助理解load average
上一篇 / 下一篇 2010-02-08 14:55:13 / 个人分类:linux
Understanding Linux CPU Load - when should you be worried?
([C^z;M tI,ec0翻译:http://www.gracecode.com/archives/2973/51Testing软件测试网g;m6h&H O!jKdk
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1v-h.FeT-P0You might be familiar with Linux load averages already. Load averages are the three numbers shown with theuptime
andtop
commands - they look like this:51Testing软件测试网Q
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Most people have an inkling of what the load averages mean: the three numbers represent averages over progressively 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? What constitutes "good" and "bad" load average values? When should you be concerned over a load average value, and when should you scramble to fix it ASAP?51Testing软件测试网GRu@9O-Yx
w-u$Cg]Y)j3L0First, a little background on what the load average values mean. We'll start out with the simplest case: a machine with one single-core processor.
)Ci'Y8?T f;w}0The traffic analogy
51Testing软件测试网7E [J%J5NA single-core CPU is like a single lane of traffic. Imagine you are a bridge operator ... sometimes your bridge is so busy there are cars lined up to cross. You want to let folks know how traffic is moving on your bridge. A decent metric would behow many cars are waitingat a particular time. If no cars are waiting, incoming drivers know they can drive across right away. If cars are backed up, drivers know they're in for delays.
my!iMbs#e](Dp0S w!C#H:f}/O/}0So, Bridge Operator, what numbering system are you going to use? How about:
S)I"b1k5z0- 0.00 means there's no traffic on the bridge at all. In fact, between 0.00 and 1.00 means there's no backup, and an arriving car will just go right on.
- 1.00 means the bridge isexactlyat capacity.All is still good, but if traffic gets a little heavier, things are going to slow down.
- over 1.00 means there's backup.How much? Well, 2.00 means that there are two lanes worth of cars total -- one lane's worth on the bridge, and one lane's worth waiting. 3.00 means there are three lane's worth total -- one lane's worth on the bridge, and two lanes' worth waiting. Etc.
O`c(ZM}(i.y3C0= load of 1.0051Testing软件测试网PS(i.]r3W-CMD
51Testing软件测试网(c$e7jV|Q)tA)H= load of 0.50
$Z m#K+N8WQ(M2R051Testing软件测试网#?Ui:e2\/_4m4z6e` t= load of 1.7051Testing软件测试网:`y uZ#sIP3N0j;H
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"Rk7r!U8E,h0This is basically what CPU load is. "Cars" are processes using a slice of CPU time ("crossing the bridge") or queued up to use the CPU. Unix refers to this as therun-queue length: the sum of the number of processes that are currently running plus the number that are waiting (queued) to run.
#@Pw-zUa0}Dj7[V$Z0Like the bridge operator, you'd like your cars/processes to never be waiting. So, your CPU load should ideally stay below 1.00. Also like the bridge operator, you are still ok if you get some temporary spikes above 1.00 ... but when you're consistently above 1.00, you need to worry.51Testing软件测试网5n-b0r\ _!R#g,]
So you're saying the ideal load is 1.00?
t2nU;y,hX0Well, not exactly. The problem with a load of 1.00 is that you have no headroom. In practice, many sysadmins will draw a line at 0.70:51Testing软件测试网1yGZ_-Aj-P)t
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The"Need to Look into it"Rule of Thumb:0.70If your load average is staying above > 0.70, it's time to investigate before things get worse.51Testing软件测试网kU0U I0`+s,i"^
i Nrox B#r w0The"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.51Testing软件测试网?Q(VA*AI1ac
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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.51Testing软件测试网 a1o ?|o'{*cu7l%\
What about Multi-processors? My load says 3.00, but things are running fine!
51Testing软件测试网%ov L,D%v)o;p_Got a quad-processor system? It's still healthy with a load of 3.00.51Testing软件测试网 U1[qN#j4Nt"Ix7r
)dx$[FC0On multi-processor system, the load is relative to the number of processor cores available. The "100% utilization" mark is 1.00 on a single-core system, 2.00, on a dual-core, 4.00 on a quad-core, etc.51Testing软件测试网6e1c5W0H7SZ
m0\l Ov+Z0If 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.
}8|t&YfF{-}051Testing软件测试网7La K,{I Y~w4S= load of 2.00 on two-lane road
+W2cN'kus:m+p&e051Testing软件测试网o:LK|&pSame 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.51Testing软件测试网1GtQa)a3M|
Multicore vs. multiprocessor
P`6y_i%m6YX0While 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 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, thetotal number of coresis what matters, regardless of how many physical processors those cores are spread across.51Testing软件测试网2c:d'n5x&Hsf8E yk
51Testing软件测试网 zPr3bbO,~Jk]Which leads us to a two new Rules of Thumb:51Testing软件测试网Jzx3A1L3y7vo l0R5}.N
jR)L6}zN}ga0The "number of cores = max load"Rule of Thumb: on a multicore system, your load should not exceed the number of cores available.
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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.
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Bringing It Home
51Testing软件测试网lt0s4?1f}Let's take a look at the load averages output fromuptime
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23:05 up 14 days, 6:08, 7 users, load averages: 0.65 0.42 0.36