Varying WebSphere Application Server dynamic cache size

To study the impact of the size of the dynamic cache size from WebSphere® Application Server on throughput, we ran several tests varying the cache size from 2,000 to 20,000 statements.

The dynamic cache service is only applicable when Trade is configured to use either Command caching or DistributedMap cache. DistributedMap caching was selected because previous tests had shown this type of caching to produce the highest throughput.

By default, the dynamic cache size is set to 2,000 statements. Tests were then run at 2,000, 6,000, 10,000, and 20,000 statements. The WebSphere Application Server dynamic cache monitor application was used to check cache usage and it was observed that at 10,000 statements the cache was never full.

Figure 1 shows the impact of the size of the dynamic cache from WebSphere Application Server had on throughput. In the tests we ran, we varied the dynamic cache size from 2,000 to 20,000 statements.

Observations

With two CPUs the throughput increases linearly as the dynamic cache size is increased up to 10,000 statements, at which point the throughput hits a peak. An 8% decrease in throughput was seen at 20,000 statements. With four CPUs the throughput does not degrade. The WebSphere cache monitoring applet was used to examine dynamic cache usage. It showed that the dynamic cache never contained more than 8,600 statements at the same time.

Conclusions

Based on these tests, we used a dynamic cache size of 10,000 statements for all further test runs. The decrease in performance with two CPUs and 20,000 statements is related to the full CPU utilization of the WebSphere Application Server. The throughput stays stable when the WebSphere Application Server gets more virtual CPUs. This shows that when the WebSphere Application Server runs CPU constrained the throughput is not only limited, but modifications in the environment might lead to a degradation.