Tomcat maxthreads not increasing after 300, CPU under-utilised for high loads

Manas Saxena 0
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I have a microservice using spring boot 2.7.0 with embedded NIO tomcat. The application is responsible for receiving requests and for each request it makes 6 parallel remote calls waits at most 2 seconds for response from any of the 6 requests.

While performance testing this microservice using jmeter I observed that the CPU remains under-utilised around 14-15% but the microservice's response time increases to more than a minute. Typically it shouldn't be more than 2-3 seconds.

There are 3 thread configurations in my microservice:

  1. Tomcat threads here I tried various configuration of maxthreads, maxconnection,accept-like (5000,30000,2000), (500,10000,2000), (200,5000,2000) but the CPU is always under-utilised. Here are the properties I am changing
server.tomcat.max-threads=200
server.tomcat.max-connections=5000
server.tomcat.accept-count=2000
server.connection-timeout=3000

  1. For each request received we create a ForkJoinPool with parallelism as 6 to make the 6 remote calls. We tried using an ExecutorService too with different configuration like newSingleThreadExecutor,newCachedThreadPool,newWorkStealingPool. Also increased pool size to around same as maxThreads of tomcat and beyond but the result was same CPU still underutilized but microservice taking more than a minute to respond. On logging the active thread count here we saw that no matter how much thread pool size or tomcat maxthreads we increased the, active thread count went upto 300 then start declining. We tried with a 4core 8GB system and 8core 16GB system results were exactly same

  2. For making remote calls we use spring rest template with maxConnTotal and maxConnTotalPerRoute same as maxthreads of tomcat. maxConnTotal and maxConnTotalPerRoute are same because all 6 remote calls are to the same server.

Here are the jmeter parameters used -GTHREADS=1000 -GRAMP_UP=180 -GDURATION=300 There are 3 instances of this microservice running, roughly after 2-2.5 minutes after jmeter starts, all 3 instance's response time goes beyond a minute for all requests while CPU remains at 14-15% only. Could someone please help figure out what CPU is not spiking if CPU would spike to 35% then autoscaling would kick in but since CPU is under-utilised no scaling is happening

tomcat microservices spring-boot jmeter
2 Answers
  1. Use a profiler tool like VisualVM, YourKit or JProfiler to see where your application spends the most time
  2. CPU is not the only possible bottleneck, check Tomcat's connection pool utilization as it might be the case the requests are queuing up, memory usage, network usage, database pool usage, DB slow queries log and so on. If you don't have a better monitoring software or an APM tool in place you can consider using JMeter PerfMon Plugin
Dmitri T0

We replaced RestTemplate for remote calls with WebClient and introducted WebFlux Mono to make the complete request non-blocking. The request itself now returns our response wrapped in Mono. It solved our issue now there is no idle time as threads are not blocked on IO rather they are busy serving other requests.

Manas Saxena0
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