Grisha Trubetskoy

Notes to self.

Load Testing Tgres

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So I finally got around to some load testing of Tgres. Load testing is mysterious, it never goes the way you think it would, and what you learn is completely unexpcted.

Given that I presently don’t have any spare big iron at my disposal and my “servers” are my macbook and an old thinkpad, all I really was after is making sure that Tgres is “good enough” whatever that means. And I think it is.

I was hoping to gather some concrete numbers and may be even make a chart or two, but in the end it all turned out to be so tedious and time consuming, running the tests with various setting for hours on, that I just gave up for now - after all, “premature optimization is the root of all evil”.

I also wanted to see how it stacks up against Graphite carbon-cache.py. As in, is it on par, or much better or much worse. My expectation was that Graphite could outperform it, because what it does is so much simpler (and I was right). First thing I tried to do is overwhelm Graphite. I never succeeded in that - I probably could have tried harder, but I quickly learned that I don’t know what symptoms I’m looking for. I wronte a Go program that blasted UDP data points at 10K/sec across 10K different series, and taking it to over 20K/sec saturated my network before Graphite showed any signs of deterioration. There was also no reliable way for me to audit the data points - may be some of them got lost, but at 600K+ per minute, I don’t know of any practical way of doing it. Not without a lot of work, at least.

With Tgres things were much easier. The weakest link is, not surpisingly, PostgreSQL. What I learned was that there are two kinds of deterioration when it comes to PostgreSQL though. The first one is outright, and that one manifests in database requests getting progressively slower until Tgres gets stuck with all its channels full.

You can make PostgreSQL very significantly faster with a few simple tricks. For example the following settings can make it much faster:

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synchronous_commit = off
commit_delay = 100000

This post isn’t about PostgreSQL, and so I’m not going to get into the details of what this does, there is plenty of documentation and blog posts on the subject. If you plan on hosting a busy Tgres set up, you should probably have the above settings.

The second way PostgreSQL deteriorates is not immediately apparent - it is the infamous table bloat. Getting autovacuum to keep up with the ts table (which stores all the time series) is tricky, and once you’ve ran out of options to tweak, this is probably it - the maximum load the database can handle, even if it may seem relatively calm.

Autovacuum has a lot of knobs, but ultimately they all exist to take advantage of the variability of load in a database, i.e. you can let it get behind during the day and catch up at night when the database is not as busy. It doesn’t really work with time series, which are not variable by nature - if you’re receiving 5 thousand data points per second at noon, you can expect the same rate at 4am. I think the setting that worked best for me were:

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autovacuum_max_workers = 10
autovacuum_naptime = 1s
autovacuum_vacuum_threshold = 2000
autovacuum_vacuum_scale_factor = 0.0
autovacuum_vacuum_cost_delay = 0 # disable cost based

To the best of my undestanding the above setting disables cost-based autovacuum (meaning it doesn’t pause periodically to yield resources to the normal db tasks), makes autovacuum kick in after 2K updates (which happens in no time), and sleeps 1s in between runs, which means it’s running pretty much continuosly.

I was able to sustain a load of ~6K datapoints per second across 6K series - anything higher caused my “database server” (which is a 2010 i7 Thinkpad) autovacuum to get behind.

I also did some testing of how TOAST affects performance. There is no setting for turning TOAST on or off, but it can easily be done in Tgres by changing the number of data points per row. The default is 768 which is about 75% of a page. If you for example double it, then each row becomes larger than a page and TOAST kicks in. TOAST is compressed, which is an advantage, but it is a separate table, which is a disadvantage. In the end it seemed like the database detirorated quicker with TOAST, but it was rather inconclusive.

In the end the key factor, or the weakest link, was the rate of queries per second. I now added a special rate limiting setting feature to Tgres (max-flushes-per-second) which trumps all other settings and will keep your database happy at the expense of Tgres possibly caching a little more points in memory than expected.

I will probably get back to some more load testing in a while, but for now this is it.

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