Tuesday, July 28, 2015

Tuning Tomcat For A High Throughput, Fail Fast System

Problem

Netflix has a number of high throughput, low latency mid tier services. In one of these services, it was observed that in case there is a huge surge in traffic in a very short span of time, the machines became cpu starved and would become unresponsive. This would lead to a bad experience for the clients of this service. They would get a mix of read and connect timeouts. Read timeouts can be particularly bad if the read timeouts are set to be very high. The client machines will wait to hear from the server for a long time. In case of SOA, this can lead to a ripple effect as the clients of these clients will also start getting read timeouts and all services can slow down. Under normal circumstances, the machines had ample amount of cpu free and the service was not cpu intensive. So, why does this happen? In order to understand that, let's first look at the high level stack for this service. The request flow would look like this




On simulating the traffic surge in the test environment it was found that the reason for cpu starvation was improper apache and tomcat configuration. On a sudden increase in traffic, multiple apache workers became busy and a very large number of tomcat threads also got busy. There was a huge jump in system cpu as none of the threads could do any meaningful work since most of the time cpu would be context switching.

Solution

Since this was a mid tier service, there was not much use of apache. So, instead of tuning two systems (apache and tomcat), it was decided to simplify the stack and get rid of apache. To understand why too many tomcat threads got busy, let's understand the tomcat threading model.

High Level Threading Model for Tomcat Http Connector

Tomcat has an acceptor thread to accept connections. In addition, there is a pool of worker threads which do the real work. The high level flow for an incoming request is:
  1. TCP handshake between OS and client for establishing a connection. Depending on the OS implementation there can be a single queue for holding the connections or there can be multiple queues. In case of multiple queues, one holds incomplete connections which have not yet completed the tcp handshake. Once completed, connections are moved to the completed connection queue for consumption by the application. "acceptCount" parameter in tomcat configuration is used to control the size of these queues.
  2. Tomcat acceptor thread accepts connections from the completed connection queue.
  3. Checks if a worker thread is available in the free thread pool. If not, creates a worker thread if the number of active threads < maxThreads. Else wait for a worker thread to become free.
  4. Once a free worker thread is found, acceptor thread hands the connection to it and gets back to listening for new connections.
  5. Worker thread does the actual job of reading input from the connection, processing the request and sending the response to the client. If the connection was not keep alive then it closes the connection and places itself in the free thread pool. For a keep alive connection, waits for more data to be available on the connection. In case data does not become available until keepAliveTimeout, closes the connection and makes itself available in the free thread pool.
In case the number of tomcat threads and acceptCount values are set to be too high, a sudden increase in traffic will fill up the OS queues and make all the worker threads busy. When more requests than that can be handled by the system are sent to the machines, this "queuing" of requests is inevitable and will lead to increased busy threads, causing cpu starvation eventually.  Hence, the crux of the solution is to avoid too much queuing of requests at multiple points (OS and tomcat threads) and fail fast (return http status 503) as soon the application's maximum capacity is reached. Here is a recommendation for doing this in practice:

Fail fast in case the system capacity for a machine is hit

Estimate the number of threads expected to be busy at peak load. If the server responds back in 5 ms on avg for a request, then a single thread can do a max of 200 requests per second (rps). In case the machine has a quad core cpu, it can do max 800 rps. Now assume that 4 requests (since the assumption is that the machine is a quad core) come in parallel and hit the machine. This will make 4 worker threads busy. For the next 5 ms all these threads will be busy. The total rps to the system is the max value of 800, so in next 5 ms, 4 more requests will come and make another 4 threads busy. Subsequent requests will pick up one of the already busy threads which has become free. So, on an average there should not be more than 8 threads busy at 800 rps. The behavior will be a little different in practice because all system resources like cpu will be shared. Hence one should experiment for the total throughput the system can sustain and do a calculation for expected number of busy threads. This will provide a base line for the number of threads needed to sustain peak load. In order to provide some buffer lets more than triple the number of max threads needed to 30. This buffer is arbitrary and can be further tuned if needed. In our experiments we used a slightly more than 3 times buffer and it worked well.

Track the number of active concurrent requests in memory and use it for fast failing. If the number of concurrent requests is near the estimated active threads (8 in our example) then return an http status code of 503. This will prevent too many worker threads becoming busy because once the peak throughput is hit, any extra threads which become active will be doing a very light weight job of returning 503 and then be available for further processing.

Configure Operating System parameters

The acceptCount parameter for tomcat dictates the length of the queues at the OS level for completing tcp handshake operations (details are OS specific). It's important to tune this parameter, otherwise one can have issues with establishing connections to the machine or it can lead to excessive queuing of connections in OS queues which will lead to read timeouts. The implementation details of handling incomplete and complete connections vary across OS. There can be a single queue of connections or multiple queues for incomplete and complete connections (please refer to the References section for details). So, a nice way to tune the acceptCount parameter is to start with a small value and keep increasing it unless the connection errors get removed.

Having too large a value for acceptCount means that the incoming requests can get accepted at the OS level. However, if the incoming rps is more than what a machine can handle, all the worker threads will eventually become busy and then the acceptor thread will wait for a worker thread to become free. More requests will continue to pile up in the OS queues since acceptor thread will consume them only when a worker thread becomes available. In the worst case, these requests will timeout while waiting in the OS queues, but will still be processed by the server once they get picked by the tomcat's acceptor thread. This is a complete waste of processing resources as a client will never receive any response.

If the value of acceptCount is too small, then in case of a high rps there will not be enough space for OS to accept connections and make it available for the acceptor thread. In this case, connect timeout errors will be returned to the client way below the actual throughput for the server is reached.

Hence experiment by starting with a small value like 10 for acceptCount and keep increasing it until there are are no connection errors from the server.

On doing both the changes above, even if all the worker threads become busy in the worst case, the servers will not be cpu starved and will be able to do as much work as possible (max throughput).

Other considerations

As explained above, each incoming connection is ultimately handled to a worker tomcat thread. In case http keep alive is turned on, a worker thread will continue to listen on a connection and will not be available in the free thread pool. So, if the clients are not smart to close the connection once it's not being actively used, the server can very easily run out of worker threads. If keep alive is turned on then one has to size the server farm by keeping this constraint in mind.

Alternatively, if keep alive is turned off then one does not have to worry about the problem of inactive connections using worker threads. However, in this case on each call one has to pay the price of opening and closing the connection. Further, this will also create a lot of sockets in the TIME_WAIT state which can put pressure on the servers.

Its best to pick the choice based on the use cases for the application and to test the performance by running experiments.

Results

Multiple experiments were run with different configurations. The results are shown below. The dark blue line is the original configuration with apache and tomcat. All the other are different configurations for the stack with only tomcat


Throughput
Note the drop after a sustained period of traffic higher than what can be served by server.


Busy Apache Workers


Idle cpu
Note that the original configuration got so busy that it was not even able to publish the stats for idle cpu on a continuous basis. The stats were published (valued 0) for the base configuration intermittently as highlighted in the red circles


Server average latency to process a request

Note

Its possible to achieve the same results by tuning the combination of apache and tomcat to work together. However, since there was not much use of apache for our service, we found the above model simpler with one less moving part. It's best to make choices by a combination of understanding the system and use of experimentation and testing in a real-world environment to verify hypothesis.

References

  1. https://books.google.com/books/about/UNIX_Network_Programming.html?id=ptSC4LpwGA0C&source=kp_cover&hl=en
  2. http://www.sean.de/Solaris/soltune.html
  3. https://tomcat.apache.org/tomcat-7.0-doc/config/http.html
  4. http://grepcode.com/project/repository.springsource.com/org.apache.coyote/com.springsource.org.apache.coyote/

Acknowledgment

I would like to thank Mohan Doraiswamy for his suggestions in this effort.