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MySQL and Linux Context Switches

 | November 9, 2017 |  Posted In: MySQL

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In this blog post, I’ll look at MySQL and Linux context switches and what is the normal number per second for a database environment.

You might have heard many times about the importance of looking at the number of context switches to indicate if MySQL is suffering from the internal contention issues. I often get the question of what is a “normal” or “acceptable” number, and at what point should you worry about the number of context switches per second?

First, let’s talk about what context switches are in Linux. This StackOverflow Thread provides a good discussion, with a lot of details, but basically it works like this:  

The process (or thread in MySQL’s case) is running its computations. Sooner or later, it has to do some blocking operation: disk IO, network IO, block waiting on a mutex or yield. The execution switches to the other process, and this is called voluntary context switch.On the other hand, the process/thread may need to be preempted by the scheduler because it used an allotted amount of CPU time (and now other tasks need to run) or because it is required to run high priority task. This is called involuntary context switches. When all the process in the system are added together and totaled, this is the system-wide number of context switches reported (using, for example, vmstat):

This is a global number. In many cases, however, it is better to look at it as context switches per CPU logical core. This is because cores execute tasks independently. As such, they have mostly independent causes for context switches. If you have a large number of cores, there can be quite a difference:

MySQL Context Switches

The number of context switches per second on this system looks high (at more than 1,000,000). Considering it has 56 logical cores, however, it is only about 30,000 per second per logical core (which is not too bad).

So how do we judge if the number of context switches is too high in your system? One answer is that it is too high if you’re wasting too much CPU on context switches. This brings up the question: how many context switches can the system handle if it is only doing context switches?

It is easy to find this out!  

Sysbench has a “threads” test designed specifically to measure this. For example:

Check the vmstat output or the Context Switches PMM graph:

MySQL Context Switches 1

We can see this system can handle up to 35 million context switches per second in total (or some 500K per logical CPU core on average).

I don’t recommend using more than 10% of CPU resources on context switching, so I would try to keep the number of the context switches at no more than 50K per logical CPU core.

Now let’s think about context switches from the other side: how many context switches do we expect to have at the very minimum for given load? Even if all the stars align and your query to MySQL doesn’t need any disk IO or context switches due to waiting for mutexes, you should expect at least two context switches: one to the client thread which processes the query and one for the query response sent to the client.    

Using this logic, if we have 100,000 queries/sec we should expect 200,000 context switches at the very minimum.

In the real world, though, I would not worry about contention being a big issue if you have less than ten context switches per query.

It is worth noting that in MySQL not every contention results in a context switch. InnoDB implements its own mutexes and RW-locks, which often try to “spin” to wait for a resource to become available. This wastes CPU time directly rather than doing a context switch.

Summary:

  • Look at the number of context switches per logical core rather than the total for easier-to-compare numbers
  • Find out how many context switches your system can handle per second, and don’t get too concerned if your context switches are no more than 10% of that number
  • Think about the number of context switches per query: the minimum possible is two, and values less than 10 make contention an unlikely issue
  • Not every MySQL contention results in a high number of context switches
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Peter Zaitsev

Peter managed the High Performance Group within MySQL until 2006, when he founded Percona. Peter has a Master’s Degree in Computer Science and is an expert in database kernels, computer hardware, and application scaling.

4 Comments

  • And what do you recommend in case of too many context swtiches per logical CPU? Get another server and split load or something else?

  • It depends on the cause. If the contention comes from too much load on the server when workload needs to be optimized or split to multiple servers. If it is mainly contention related when there are many things you can do – restrict innodb_thread_concurrency, work with some innodb mutex tunables, use Percona Server with thread pool plugin etc.

  • Very interesting. Couple of questions:
    – Why is the least possible amount of context switches per query is 2? Is it because results are generated by one thread and SQL is parsed in another in MySQL? Are there databases where it can be lower?
    – Why logical cores and not physical cores?

    Thanks!

    • In theory there might be less than 2 context switched per query. It is possible for the context switch FROM one query to be same as context switch TO some other query. I use it here more as a ballpark here. No context switches is not possible for MySQL with thread per connection as once all the data is sent back from server to the client server has to wait for new command to come… which will come only after client has consumed and processed the query results. The new MySQLX Protocol might make things better here.

      Though at MySQL scale if you have just 1-2 context switches per query it is extremely unlikely to be your main bottleneck so you do not get to focus on it.

      Re Physical vs Logical cores it is for simplicity purpose and also because from kernel standpoint even logical cores operate as separate CPUs… the fact they do not have complete CPU Core logic available for each is much lower level question.

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