Category: Databases

Benchmarking with Postgres PT2

In this example we run pgbench with a scale factor of 1000 which equates to a database size of around 15GB. The linux VM has 32G RAM, so we don’t expect to see many reads.

Using prometheus with the Linux node exporter we can see the disk IO pattern from pgbench. As expected the write pattern to the log disk (sda) is quite constant, while the write pattern to the database files (sdb) is bursty.

pgbench with DB size 50% of Linux buffer cache.

I had to tune the parameter checkpoint_completion_target from 0.5 to 0.9 otherwise the SCSI stack became overwhelmed during checkpoints, and caused log-writes to stall.

default pgbench – notice the sharp drop in log-writes before tuning.


Benchmarking with Postgres PT1

Image By Daniel Lundin

In this example, we use Postgres and the pgbench workload generator to drive some load in a virtual machine.  Assume a Linux virtual machine that has Postgres installed. Specifically using a Bitnami virtual appliance.

  • Once the VM has been started, connect to the console
  • Allow access to postgres port 5432 – which is the postgres DB port or allow ssh
  • Note the postgres user password (cat ./bitnami_credentials)
  •  Login to psql from the console or ssh
  • Optionally change password (the password prompted is the one from bitnami_credentials for the postgres database user).
  • Create a DB to run the pgbench workload.  In this case I name the db pgbench-sf10 for “Scale Factor 10”.  Scale Factors are how the size of the database is determined.
  • Initialise the DB with data ready to run the benchmark.  The “createdb” step just creates an empty schema.
    • -i means “initialize”
    • -s means “scale factor” e.g. 10
    • pgbench-sf10 is the database schema to use.  We use the one just created pgbench-sf10
  • Noe run a workload against the DB schema called pgbench-sf10

The workload pattern, and load on the system will vary greatly depending on the scale factor.  

Scale-Factor        Working Set Size

1                                   23M
10                                157M
100                             1.7GB
1000                          15GB
2500                          37GB
5000                         74GB
10000                       147GB

 

 

Performance gains for postgres on Linux with hugepages

For this experiment I am using Postgres v11 on Linux 3.10 kernel. The goal was to see what gains can be made from using hugepages. I use the “built in” benchmark pgbench to run a simple set of queries.

Since I am interested in only the gains from hugepages I chose to use the “-S” parameter to pgbench which means perform only the “select” statements. Obviously this masks any costs that might be seen when dirtying hugepages – but it kept the experiment from having to be concerned with writing to the filesystem.

Experiment

The workstation has 32GB of memory
Postgres is given 16GB of memory using the parameter


pgbench creates a ~7.4gb database using a scale-factor of 500

Run the experiment like this

Result

Default : No hugepages :
tps = 62190.452850 (excluding connections establishing)

2MB Hugepages
tps = 66864.410968 (excluding connections establishing)
+7.5% over default

1GB Hugepages
tps = 69702.358303 (excluding connections establishing)
+12% over default

Enabling hugepages

Getting the default hugepages is as easy as entering a value into /etc/sysctl.conf. To allow for 16GB of hugepages I used the value of 8400, followed by “sysctl -p”

To get 1GB hugepages, the kernel has to have it configured during boot e.g.

Then reboot the kernel

I used these excellent resources
How to modify the kernel command line
How to enable hugepages
and this great video on Linux virtual memory