A few weeks ago I wrote about “MySQL Performance at High Concurrency” and why it is important, which was followed up by Vadim’s post on ThreadPool in Percona Server providing some great illustration on the topic. This time I want to target an opposite question: why MySQL performance at low concurrency is important for you.
I decided to write about this topic as a number of recent blog posts and articles look at MySQL performance starting with certain concurrency as the low point. For example, MySQL 5.6 DBA and Developer Guide has a number of graphs starting with 32 connections at the low point. Mark Callaghan also starts with a concurrency of 8 in some of the results he publishes. All of this may make it look as though single-thread performance is no more a concern. I believe it still is!
First, performance at concurrency of 1 serves as an important baseline that offers the best response times (MySQL server does not use multiple threads to execute query in parallel at this point). Response times at the variety of concurrency levels can be compared with this baseline to see how they are affected and how the system scales with increasing concurrency in effect.
Second, there are many cases in which single-thread execution is what is going to happen — many batch jobs are written to be single-threaded. MySQL replication is single-thread too, and MySQL 5.6 brings some abilities of parallel replication but these apply only to some use cases. Many database maintenance operations such as “alter table” are run in single-thread too.
In fact, chances are your system actually runs at low concurrency most of the time. The majority of systems I see in the wild have Threads_running on average of 5 or less, at least the system is performing well, pointing to rather low concurrency in the normal system operation. Much higher concurrency in most cases is seen when the system is overloaded and suffering.
So what is the problem with looking at performance at just high-concurrency values? This might mislead you on how your production will be impacted. Think about, for example, a system that is slower at low concurrency but a lot faster at high concurrency (which is the typical situation). If you just look at performance at high concurrency you might not understand why the system is showing worse response times when you are actually running it in real life with low concurrency.
When it comes to benchmarks I would love to see results published starting from concurrency of 1 if possible so we can see the full picture. In terms of benchmarks, Mark Callaghan published an example where 8 sysbench tables are used which means the lowest concurrency you can run with to compare apples to apples is 8. Running a comparable benchmark with concurrency of 1 and the same tool is not possible.
Want to talk more about performance? Come to Percona Live MySQL Conference and Expo April 22-25 in Santa Clara, California, where “performance” is going to be a big topic