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What Exactly is Swappiness?

This is an issue that keeps rearing its ugly head over and over again, and since it greatly affects performance, it is most important that DBAs of any DMBS running on Linux come to grips with it. So I decided to do some research and try different settings on my notebook. Here are my findings.

What can you find on the web?

A Wikipedia search for the word swappiness will come up empty (any volunteers out there want to write an article?). A Google search will show some pretty old material—the best article I found is from 2004: Linux: Tuning Swappiness. This article includes a detailed discussion with some interesting remarks by Andrew Morton, a Linux kernel maintainer.

So, what is swappiness?

Towards the end of the email thread quoted in the article, you’ll find this definition (sort of):

> I’ve read the source for where swappiness comes into play. Yet I
> cannot make a statement about what it means. Can you?

It controls the level of page reclaim distress at which we decide to start reclaiming mapped pages.

We prefer to reclaim pagecache, but we have to start swapping at *some* level of reclaim failure. swappiness sets that level, in rather vague units.

“Rather vague units” . . .  So, for practical purposes, it’s easier to define it by its effects:

  • Set swappiness to 0 and the kernel is going to start swapping only when absolutely needed.
  • Set swappiness to 100 and the kernel will preemtivelyswap out memory you are not using.

To set it use the following command with sudo (it’s not a good practice to manipulate files under /proc directly as suggested in the article):

sudo sysctl -w vm.swappiness=0

For my workstation, the applications I usually have open take slightly more than the physical 2GB installed in my notebook. I also keep jumping from one window to the next. Setting a high swappiness value means I will have to wait for the applications to reclaim memory pages with each jump, creating a noticeable delay while changing windows.

On the other hand, if swappiness is set to 0, swapping will occur when I start editing a big document or navigate to a big web page. So, while I will be able to jump from window to window pretty quickly, I’ll experience delays when using one of the memory-hungry applications intensively. Try yourself values of 0, 100 and a few values in between to see which best fits your particular use profile and understand how each one of them will affect swapping.

How can I objectively measure swapping?

In the previous section, determining the best swappiness value is based entirely on personal perception. If swappiness is too low, you’ll notice your application might take a while to reclaim memory from a heavy loaded system. If swappiness is too high, the application will get the memory fast, but you’ll have to wait when switching between apps: for example going from OpenOffice to Firefox (both are memory-hungry).

For an objective measure, you can use sar, which is included in the sysstat package. sar can be used in two different ways:

  1. Install it in the crontab to gather samples in the background.
  2. Run it interactively.

Running sar from cron

Each Linux distribution behaves differently when you install the sysstat package. For Debian-based systems, installing the package doesn’t add it to the crontab. You will have to check your system’s documentation for the details. I installed it in the crontab to gather samples every ten minutes (the most frequent setting).

Here are an example of the two options I use the most to determine swappiness health:

sar -r -s 22:00:00
Linux 2.6.24-23-rt (maggie)     04/09/2009

10:05:01 PM kbmemfree kbmemused  %memused kbbuffers  kbcached kbswpfree kbswpused  %swpused  kbswpcad
10:15:01 PM     14716   1770256     99.18      5428    252664    946424     49596      4.98      1796
10:25:01 PM     14176   1770796     99.21      3292    229788    946424     49596      4.98      1796
Average:        14446   1770526     99.19      4360    241226    946424     49596      4.98      1796

sar -B -s 22:00:00
Linux 2.6.24-23-rt (maggie)     04/09/2009

10:05:01 PM  pgpgin/s pgpgout/s   fault/s  majflt/s  pgfree/s pgscank/s pgscand/s pgsteal/s    %vmeff
10:15:01 PM      2.48     96.11    202.55      0.03    158.34      0.00      0.00      0.00      0.00
10:25:01 PM     27.13     70.24    249.75      0.35    169.96      0.00      0.00      0.00      0.00
10:35:01 PM      4.70     39.58    213.53      0.04    149.19      0.00      0.00      0.00      0.00
Average:        11.44     68.64    221.94      0.14    159.16      0.00      0.00      0.00      0.00

Without the -s parameter it will show a day’s worth of statistics. For clarity, I’m only getting a few lines.

Option -r will show the overall memory usage, the last columns showing swapping information. With higher swappiness values you will notice greater numbers in the %swpused and kbswpcad columns.

Option -B specifically presents swapping statistics. The most significant columns here are majflt/s, which represents the pages that had to be loaded from disk (synonymous with I/O overhead), and %vmeff where the healthy values are 100 (every inactive page swapped out is being reused) or 0 (no pages were scanned during the interval).

To run it interactively, instead of using -s, you can specify an interval and a number of samples:

sar -B 2 5
Linux 2.6.24-23-rt (maggie)     04/09/2009

10:44:29 PM  pgpgin/s pgpgout/s   fault/s  majflt/s  pgfree/s pgscank/s pgscand/s pgsteal/s    %vmeff
10:44:31 PM      0.00      0.00    871.50      0.00    575.00      0.00      0.00      0.00      0.00
10:44:33 PM      0.00     62.00    110.00      0.00   1694.00      0.00      0.00      0.00      0.00
10:44:35 PM      0.00      0.00    481.09      0.00    179.10      0.00      0.00      0.00      0.00
10:44:37 PM      0.00     52.00     63.50      0.00    160.50      0.00      0.00      0.00      0.00
10:44:39 PM     20.00     96.00     52.00      0.00    116.00      0.00      0.00      0.00      0.00
Average:         4.00     41.96    315.78      0.00    544.56      0.00      0.00      0.00      0.00

In this case I’m taking 5 samples at 2 second intervals.

Which swappiness value should you use?

For workstations, it’s up to your particular usage profile. If you jump from application to application, 0 should be best. If you stay on the same memory-hungry application for a long time, 100 should be best.
When it comes to a database server, however, you have to keep in mind that it is a memory-craving application, and if get you have a sudden activity increase, having to wait to obtain additional memory from the system is really bad for performance. For database servers I recommend to keep in mind the following criteria:

  1. Use dedicated database servers and configure the most memory you can without exceeding the physical memory available. Running another memory-intensive application (e.g. an application server) on the same server as the database is a bad idea, as both will compete for memory, creating high swapping situations.
  2. For database servers, set swappiness to 0. Use sysctl to set it interactively as quoted above, or better still, set it in the /etc/sysctl.conf file to persist when the server is restarted.
  3. Constantly monitor the memory usage and swapping efficiency with a utility like sar.

Paying attention to these items you will avoid the I/O overhead that high swapping might create.

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