There are various things that can be done to make tests run faster. But let’s talk about databases.
When Django is configured to use SQLite, it does a nice little trick when running the test suite; it creates the database in memory. As a result, database access is very fast, since there is no I/O, which results in reduced execution time for the tests.
For example, we run the test suite for the projects app of Transifex:
time ./manage.py test projects
and the results are:
real 2m52.429s user 2m5.132s sys 0m2.953s
For the resources app (the test suite of which is much bigger) the results are:
real 6m29.072s user 4m35.209s sys 0m8.956s
Of course, the above numbers are not a benchmark, but just an indication of how long it takes to run those tests on a machine with 8GB of RAM.
However, Transifex.net runs on PostgreSQL and all testing should be done with the setup that is used in production. For instance, PostgreSQL is much more strict about transaction semantics than SQLite and that affects many tests. That means, tests should be run against PostgreSQL.
But, the default setup of PostgreSQL is not optimized at all. In fact, the default settings are chosen, so that PostgreSQL can run on servers with as little as 64MB of RAM (or something like that).
With the default setup of PostgreSQL, the projects test suite runs in:
real 4m40.891s user 2m16.898s sys 0m3.816s
and the resources app in
real 10m7.483s user 4m58.841s sys 0m9.566s
Both running times are mush worse than those achieved, when using SQLite as database backend.
There are a few settings, however, which could be optimized to make PostgreSQL faster for testing. In my machine, I have
shared_buffers = 512MB work_mem = 16MB fsync = off synchronous_commit = off wal_buffers = 64MB checkpoint_segments = 36 checkpoint_timeout = 10min random_page_cost = 2.0 effective_cache_size = 1024MB
The goal is to allow PostgreSQL to use much more memory and, as a result, to choose more efficient execution plans for the queries. For instance, we set the
work_mem to 16MB, a value large enough (for the tests of Transifex), so that all
SORT operations are executed in RAM.
At the same time, we try to reduce the I/O that PostgreSQL will perform. For example, we deactivate the
fsync option, which instructs PostgreSQL to do a
fsync() call, whenever it writes something to disk, and increase the
checkpoint_segments option, which instructs PostgreSQL to flush data in larger intervals.
You can see what each option is for in the manual of PostgreSQL.
With the above settings, the execution times are:
real 3m4.360s user 2m14.458s sys 0m3.360s
for the projects app and
real 6m49.579s user 4m49.101s sys 0m9.256s
for the resources app, which are comparable to the ones obtained, when using SQLite.
The values chosen depend, of course, on the CPU and available memory you have. Additionally, some of the options (like
fsync) should not be used in production.
Keep also in mind that you will probably need to increase the maximum size of a shared memory segment with the command
sysctl -w kernel.shmmax=8589934592
in order to use the above settings (or add the new value in