Unix sockets vs. localhost: We posted a lot of tuning advice on this blog already. Now we thought we would take it a step further and share some more advice and common “mistakes” that we see people making when configuring web applications. Improving web application performance is more critical than ever, because most applications in use are browser-based.

Small step for mankind – giant leap for performance

In most cases, web applications run a large number of very small statements. In some cases, the database is even on the same server as the application itself.

To run small statements on the same host, PostgreSQL provides two means:

  • UNIX sockets
  • “localhost” (= loopback device)

Most people don’t really care. “localhost” is as good as a UNIX socket …

Well – it is not!

We have compiled an example showing what can happen:

a.sql is a simple script, which just creates a random number and SELECTs it. So it is the most simplistic statement possible. There is nothing simpler than just fetching a number.

So, let us run this benchmark on a normal laptop:

  • pgbench will run 5 threads
  • 10 concurrent users
  • 10 seconds

UNIX sockets provide us with 79.000 transactions per second:


[hs@zenbook postgresql-9.5.0]$ pgbench -f /tmp/a.sql -j 5 -c 10 -T 10 test
starting vacuum...end.
transaction type: Custom query
scaling factor: 1
query mode: simple
number of clients: 10
number of threads: 5
duration: 10 s
number of transactions actually processed: 791171
latency average: 0.126 ms
tps = 79096.447917 (including connections establishing)
tps = 79136.229601 (excluding connections establishing)

The interesting thing is that the average latency is 0.126 milliseconds.

Let us repeat the very same test. This time “localhost” is added to the example:


[hs@zenbook postgresql-9.5.0]$ pgbench -f /tmp/a.sql -j 5 -c 10 -T 10 -h localhost test
starting vacuum...end.
transaction type: Custom query
scaling factor: 1
query mode: simple
number of clients: 10
number of threads: 5
duration: 10 s
number of transactions actually processed: 473210
latency average: 0.211 ms
tps = 47297.885523 (including connections establishing)
tps = 47331.482977 (excluding connections establishing)

The latency skyrockets from 0.126 ms to 0.211 milliseconds. At the same time TPS drop from 79.000 to 47.300.

Just by adding “localhost” performance dropped to 60% of the original speed.

NOTE: In real life, the drop won’t be that large because we expect users to run slightly more complicated SQL – however, the difference is real and there.

Why is that? UNIX sockets are actually a pretty simple thing. The loopback device is much more complicated and therefore the overhead relative to those simple queries is huge.

In case you need any assistance, please feel free to contact us.
 


In order to receive regular updates on important changes in PostgreSQL, subscribe to our newsletter, or follow us on Twitter, Facebook, or LinkedIn.