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How Many Threads Does It Take to Fill a Pool?

Updated: Jun 18, 2018

In recent months we have seen a small but persistent percentage of our operations fail with a strange exception—

org.springframework.jdbc.CannotGetJdbcConnectionException—“Could not get JDBC Connection; nested exception is java.sql.SQLException: An attempt by a client to checkout a Connection has timed out.”

Our natural assumption was that we had some sort of contention on our C3P0 connection pool, that clients trying to acquire a connection have to wait for one to be available. Our best guess was that it was this contention that caused the timeouts.

So, of course, the first thing we did was increase the max number of connections in the connection pool. However, no matter how high we set the limit, it did not help. Then, we tried changing the timeout parameters of the connection, which did not produce any better results.

At this point intelligence settled in, and since guessing did not seem to work, we decided to measure. Using a simple wrapper on the connection pool, we saw that even when we had free connections in the connection pool, we still got checkout timeouts.

Investigating Connection Pool Overhead To investigate connection pool overhead, we performed a benchmark consisting of 6 rounds, each including 20,000 SQL operations (with a read/write ratio of 1:10), performed using 20 threads with a connection pool of 20 connections. Having 20 threads using a pool with 20 connections means that there is no contention on the resources (the connections). Therefore, any overhead is caused by the connection pool itself.

We disregarded results from the first (warm-up) run, taking the statistics from the subsequent 5 runs. From this data, we gathered the connection checkout time, connection release time, and total pool overhead.

The benchmark project code can be found on GitHub.

We tested 3 different connection pools:

  1. C3P0: com.mchange:c3p0:0.9.5-pre3 – class C3P0DataSourceBenchmark

  2. BoneCP: com.jolbox:bonecp:0.8.0-rc1 – class BoneDataSourceBenchmark

  3. Apache DBCP: commons-dbcp:commons-dbcp:1.4 – class DbcpDataSourceBenchmark

(In the project there is another benchmark of my own experimental async pool. However, for the purpose of this post, I am ignoring it.)

To run the benchmark yourself, you should set up MySQL with the following table:

Then, update the Credentials object to point at this MySQL installation.

When running the benchmarks, a sample result would look like this:

##Reading the charts The first three charts (acquire, release, overhead) are bucket charts based on performance. The Y-axis indicates the number of operations that completed within a certain time range (shown on the X-axis). The default rule of thumb here is that the higher the bars to the left, the better. The 4th chart is a waterfall chart, where each horizontal line indicates one DB operation. Brown indicates time waiting to acquire a connection, green is time to execute the DB operation, and blue indicates time to return a connection to the connection pool.

Looking at the charts, we see that, generally, C3P0 acquires a connection within 3.2-10 microseconds, and releases connections within 3.2-10 microseconds. That is definitely some impressive performance. However, C3P0 has another peak at about 3.2-32 milliseconds, as well as a long tail getting as high as 320–1000 milliseconds. It is this second peak that causes our exceptions.

What’s going on with C3P0? What causes this small but significant percentage of extremely long operations, while most of the time performance is pretty amazing? Looking at the 4th chart can point us toward the answer. The 4th chart has a clear diagonal line from top left to bottom right, indicating that, overall, connection acquisition is starting in sequence. But we can identify something strange—we can see brown triangles, indicating cases when multiple threads try to acquire a connection, the first thread waits more time than subsequent threads. This translates to two performance ‘groups’ for acquiring a connection. Some threads get a connection extremely quickly, whereas others reach starvation waiting for a connection while latecomer threads’ requests are answered earlier.

Such behavior, where an early thread waits longer than a subsequent thread, means unfair synchronization. Indeed, when digging into C3P0 code, we saw that during the acquisition of a connection, C3P0 uses the ‘synchronized’ keyword three times. In Java, the ‘synchronized’ keyword creates an unfair lock, which can cause thread starvation. We may try patching C3P0 with fair locks later on. If we do so, we will share our findings.

Here’s the C3P0 configuration for this benchmark:

Minimum pool size: 20 Initial pool size: 20 Maximum pool size: 20 Acquire increment: 10 Number of helper threads: 6

##BoneCP We tried BoneCP at Wix with mixed results. At the moment, we are not sure if we like it. Here we include the results of the BoneCP benchmarks, though the analysis is not as comprehensive.

Looking at the charts, we can see that BoneCP’s connection acquisition performance is outstanding—most of the operations are completed within 3.2 microseconds, much faster than C3P0. However, we also observe that the connection release time is significant, about 1-10 milliseconds, which is much too high. We also observe that BoneCP has a long tail of operations, with overhead getting as high as 320 milliseconds.

Looking at the data, it appears BoneCP is better compared to C3P0—both in the normal and ‘extreme’ cases. However, the difference is not large, as evidenced by the charts. Looking at the 4th chart, we see we have less brown compared to C3P0 (since connection acquisition is better), but trailing blue lines have appeared, indicating the periods of time that threads wait for a connection to be released.

As mentioned above, since we are ambivalent at best about using BoneCP, we have not invested significant resources in analyzing this connection pool’s performance issues.

##Apache DBCP Apache DBCP is known as the Old Faithful of datasources. Let’s see how it fares compared to the other two.

One thing is evident: DBCP performance is superior to both C3P0 and BoneCP. It outperforms the others in terms of connection checkout times, connection release time, and in the form of the waterfall chart.

So, What Datasource Should You Be Using?

Well, that is a non-trivial question. It is clear that with regards to connection pool performance, we have a clear winner: DBCP. It also seems that C3P0 should be easy to fix, and we may just try that. However, it is important to remember that the scope of this investigation was limited only to the performance of the actual connection acquisition/release. Actual datasource selection is a more complex issue.

This benchmark, for example, ignores some important aspects, such as growth and shrinkage of the pool, handling of network errors, handling failover in case of DB failure, and more.

This post was written by Yoav Abrahami.


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