Why We Test

Despite design principles, despite best intentions, things can go wrong.  This is true when all of the design parameters are quantifiable, as exhibited by this spectacular engine failure:F1EngineFail What happens when some of the design constraints are not fully understood?  Specifically, consider the following excerpt taken from Amazon’s page detailing instance specifications:

Instance Family Instance Type Processor Arch vCPU ECU Memory (GiB) Instance Storage (GB) EBS-optimized Available Network Performance
Storage optimized i2.xlarge 64-bit 4 14 30.5 1 x 800 SSD Yes Moderate
Storage optimized i2.2xlarge 64-bit 8 27 61 2 x 800 SSD Yes High
Storage optimized i2.4xlarge 64-bit 16 53 122 4 x 800 SSD Yes High
Storage optimized i2.8xlarge 64-bit 32 104 244 8 x 800 SSD 10 Gigabit*4
Storage optimized hs1.8xlarge 64-bit 16 35 117 24 x 2,048*3 10 Gigabit*4
Storage optimized hi1.4xlarge 64-bit 16 35 60.5 2 x 1,024
10 Gigabit*4
Micro instances t1.micro 32-bit or
1 Variable*5 0.615 EBS only Very Low
10 Gigabit is a measure we can understand.  But what does Moderate mean?  How does it compare to High, or Very Low?  It is possible to find out, but only if we test.

According to Amazon’s page on instance types (emphasis added):

Amazon EC2 allows you to provision a variety of instances types, which provide different combinations of CPU, memory, disk, and networking. Launching new instances and running tests in parallel is easy, and we recommend measuring the performance of applications to identify appropriate instance types and validate application architecture. We also recommend rigorous load/scale testing to ensure that your applications can scale as you intend.

Using LoadUIWeb by Smartbear, we were able to take a given workload and run it on different instance types to gain a better understanding of the performance differences.  Running 100 simultaneous virtual users with a 9 millisecond think time between pages, we ran against an M1.small and an M1.medium.  Here is are some pictures to illustrate what we discovered:

M1.small performance test output:


M1.medium performance test output:




Looking at the numbers, we see that the the response time of the small is roughly twice that of the medium.  As neither CPU, memory, or disk activity were constraints, this serves as a warning to us – the quality of service limitations placed on different instance types must be taken into consideration when doing instance size analysis.

When in doubt, test it out!

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