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<?xml version="1.0" encoding="ISO-8859-1" standalone="yes" ?>
<content>
<pagetitle>SLoad</pagetitle>
<textheader>Calculating server CPU loads</pagetitle>
<text>
<rimg href="?document=images/haskell.png" title="Haskell" />
<noop>In order to learn functional programming and to calculate server loads at work I programmed in my spare time this small tool using the purely functional programming language </noop>
<namedlink href="http://www.haskell.org">Haskell!</namedlink>
<noop>SLoad is an abbreviation for serverload. SLoad uses basic parameters (current CPU peak, current requests per second) for approximating the need of additional servers.</noop>
</text>
<text>In order to get started check it out from the Subversion repository. You also need to install GHC (the Haskell compiler).</noop>
<code>
pbuetow@earth:~/svn/sload/trunk$ make
ghc SLoad.hs -o sload
</code>
<text>Lets imagine you own an apache server cluster which consists of 12 physical servers. The current maximum CPU usage (CPU peak) is 70%. A single server of the cluster handles at the moment 323 requests per seconds (request peak). And you want to know "does my farm handle 5000000 more requests per day?":</text>
<code>
pbuetow@earth:~/svn/sload/trunk$ ./sload --name=serverfarm.example.net --servers=12 \
--cpuPeak=70 --peak=323 --requests=5000000
Requests : 5000000.0
Percentage of req. to use: 100.0
Approximated peak: 277.77777 requests per second (Peak = 2 * (5000000.0/(60 * 60 * 10)))
serverfarm.example.net (12.0 Servers):
currentPeakPerServer: 323.0
currentCpuPeakPerServer: 70.0
currentPeakPerFarm: 3876.0
possiblePeakPerFarm: 5038.8
possibleAddPeak (possiblePeakPerFarm - currentPeakPerFarm): 1162.7998
addPeakPerServer: 23.148148
addPeakPerFarm: 277.77777
leftPeak (possibleAddPeak - addPeakPerFarm): 885.02203
currentCpuPerPeak (currentCpuPeakPerServer / currentPeakPerServer): 0.21671827
neededCpuPeakPerServer (currentCpuPeakPerServer + addPeakPerServer * currentCpuPerPeak): \
75.016624
</code>
<text>The approximated peak is being calculated using 2 * (requestsPerDay/10h). This formular has been well-tried and tested. It is not proven that this formular is always right, but it is more a 'worst case' approach. Which is ok for calculating server loads as long as you don't do a 'best case' calculation which may lead to a server overload. The value of neededCpuPeakPerServer shows if the cluster will handle the additional amount of requests. If neededCpuPeakPerSerer is less than 85 it should be ok. A value between 85 and 100 may be critical. A value more than 100 shows that there is definitive not enough power behind your cluster.</text>
<text>
<enumeration>
<enumitem>Requests: Requests per day your server should handle additionally to the current server load. You want to find out if your server cluster will handle this.</enumitem>
<enumitem>Percentage of req. to use: If you use 50, only half of "Requests" will be used in the following calculations.</enumitem>.
<enumitem>currentPeakPerServer: Requests per seconds a single server of the cluster handles (peak)</enumitem>
<enumitem>currentCpuPeakPerServer: The maximum (peak) CPU load of one of your servers.</enumitem>
<enumitem>currentPeakPerFarm: currentPeakPerServer * number of servers in the cluster.</enumitem>
<enumitem>possiblePeakPerFarm: How many requests per seconds your farm may handle (depending on the CPU % which is left)</enumitem>
<enumitem>possibleAddPeak: How many requests per seconds your farm may handle additionally to the CPU usage (depending on the CPU % which is left)</enumitem>
<enumitem>addPeakPerServer: How many requests per seconds a single server of your cluster may handle additionally.</enumitem>
<enumitem>addPeakPerFarm: addPeakPerServer * number of servers in the cluster.</enumitem>
<enumitem>leftPeak: How many requests per second can be handled more even after adding "Requests" to the cluster.</enumitem>
<enumitem>currentCpuPerPeak: How many CPU % is needed for a single request.</enumitem>
<enumitem>neededCpuPeakPerServer: How many CPU % (peak) is needed for a single server of the cluster after adding "Requests".</enumitem>
</enumeration>
</text>
</content>
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