The Digital Ocean 1-click LEMP install is already configured to take full advantage of as many cores and as much RAM as is available. During the course of these tests I never needed to tweak any server settings. That doesn’t mean that more performance couldn’t have been extracted through configuration tweaks, but even with these settings we were able to gain useful information about WordPress performance on the different Digital Ocean droplet levels.
With regards to WordPress configuration… there isn’t any. No caching, CDNs, or compression. Just raw WordPress. Its easy to get high performance out of WordPress if you cache everything, but seeing what performance looks like in the worst-case scenario is far more interesting.
For this series of tests I imported the contents of my personal blog (Re-cycledair) to the target and then used Kernl to run the load tests. Due to the nature of my blog this test is very read heavy. While this isn’t realistic for everyone, for my personal blog the test is representative of the actual traffic that it receives.
The test itself looked like:
Concurrent users – 500
Ramp up – 2 users per second
Duration – 30 minutes
Request Rate – Each user makes 1 request every second
Droplet location – New York City
Load test generator location – Amsterdam
As expected the amount of traffic that we could handle scaled linearly with the amount of hardware we were using (and cost).
One interesting thing you’ll see in the graph is that in two places the requests per second actually trend down.
Looking at the data above you can see that I tried Digital Ocean’s standard droplets as well as their CPU optimized droplets. The difference between them is cost and dedicated hyper-threads. From a cost perspective, you’re better off going with the standard droplet in the same price category. Personally I expected the CPU optimized droplet to perform better, but this might not be the best type of workload for it.
What About Caching?
Just for fun I tried out the one-click install of WordPress with LiteSpeed configured on a $5 / month droplet.
Things went really really really well.
Thats right. By the time the test was completed my little $5 / month droplet was receiving 1800 requests / second WITH NO ERRORS. For perspective, thats 4.6 billion requests per month.
The data does a good job speaking for itself here, but in general you should definitely stick to Digital Ocean’s standard droplets when running WordPress. Even without caching you can get really good performance out of the $40/month droplet.
There are a lot of different reasons to load test.
Infrastructure and Hosting – Kernl WordPress load testing gives you confidence that you are making the right decisions with your infrastructure and hosting. Looking to change hosts but aren’t sure how big or expensive of a plan you need? Run a load test.
Performance Testing – Load testing gives you confidence that the SQL query you just wrote isn’t going to collapse your website under load.
Confidence with your clients – Load testing lets you tell your clients with confidence that their new website can handle 100,000 visitors a day without any degradation in response time.
How Does It Work?
Kernl’s WordPress Load Testing solution makes load testing your WordPress site a breeze. You only need verify your ownership of the site with an easy to use WordPress plugin and then start testing. No coding or infrastructure management needed.
How Much Does it Cost?
Kernl’s WordPress Load Testing is included with your Kernl subscription. Usage per plan is as follows:
Enterprise & Above
If your needs don’t fit neatly into one of these categories feel free to reach out. We’d be happy to create a custom plan to suit your needs.
To horizontally scale any app is an exercise in breaking things apart as much as possible. In the case of WordPress there are a few shared components that I wanted to break up:
File System – The file system is the most problematic part of scaling WordPress. Unless you change how WordPress stores plugins, themes, media, and other things you need to have a shared file system that all nodes in your cluster can access. There are likely some other solutions here, but this one provides a lot of flexibility.
MySQL – In many WordPress installs MySQL lives on the same machine as WordPress. For a horizontally scaled cluster this doesn’t work so we need a MySQL that is external.
Memcached – It was brought to my attention that during part 1 of this series using WP Super Cache to generate static pages was sort of cheating. In the spirit of making this harder for myself I introduced W3 Total Cache instead and will be using an external Memcached instance as the shared cache.
Now that the basic why and what is out of the way lets talk about the how. I’m a huge fan of Digital Ocean. I use them for everything except file storage so I’m going to use them for this WordPress cluster as well. Here’s how its going down:
Create a droplet that will act as the file system for our cluster. Using NFS all droplets in the cluster will be able to mount it and use it for WordPress. I’m also going to use this for Memcached since NFS doesn’t take up many resources.
Create a base droplet that has Nginx and PHP7.2-FPM installed on it. There is a little bit of boilerplate configuration here, but in general the install is typical. The only change to the Nginx configuration where I set the root directory to be the NFS mount. Use this base droplet to configure WordPress database settings.
Use Compose.io create a MySQL database. I wanted something that was configured well that I didn’t have to think about. Totally worth the $27 / month.
Once the above are done take a snapshot of the base droplet and use it to create more droplets. If all goes well you shouldn’t need to do any configuration.
Using Digital Ocean’s load balancer service add your droplets to the load balancer.
Voila! Thats it.
No Cache Smoke Test
200 users, 10 minutes, 2 users/sec ramp up, from London
As with every load test that I do, the first test is always just to shake out any bugs in the load test itself. For this test I didn’t have any caching enabled and only a single app server behind the load balancer. It was effectively the same as the first load test I did during part 1 of this blog series.
As you can see from the graph below, performance was what we would expect from the setup that I used. We settled in to 21 requests / second with no errors.
The response time distribution wasn’t very great. 90% of requests finished in under 5 seconds, but thats still a very long time. Generally if I saw this response time distribution I would think that its time to add caching or scale up/out.
So. Many. Failures.
2000 users, 120 minutes, 2 users/sec ramp up, from London
The next test I decided to run was the sustained heavy load test. This is generally where I start to see failures from managed WordPress hosting providers. Given that I didn’t add any more app servers to the load balancer and had no caching things went as poorly as you would expect.
Everything was fine up until ~25 req/s and then the wheels fell off. The response time distribution was bad too. No surprises here.
Looks like its time to scale.
2000 users, 120 minutes, 2 users/sec ramp up, from London
Before adding Memcached to the setup I wanted to see how it scaled without it. That means adding more hardware. For this test I added four more application servers (Nginx + PHP) to the load balancer and ran the test again.
As you can see from the request/failure graph we experience roughly linear growth in our maximum requests/second. Given we originally maxed out at ~20 req/s on one machine, maxing out at ~100 req/s with five machines seems like exactly the sort of result that I would expect to see. The response time distribution also started to look better:
Obviously a 90% score of 4 seconds isn’t awesome, but it is a lot better than the previous test. I did make a tiny tweak to the load balancer configuration that may have helped though. I decided to use the ‘least connections’ options instead of ’round robin’. ‘Least connections’ tells the load balancer to send traffic to the app server with the least number of active connections. This should help with dog piling on a server with a few slower connections.
Given the results above we can safely assume linear growth tied to the number of app servers that we have for quite some time. Meaning for each app server that I add I can expect to handle an additional ~20 req/s. With that in mind, I wanted to see what would happen if I enabled some caching on this cluster.
Gotta Go Fast
In my previous test of vertical scaling I used WP Total Cache to make things go quick. WP Total Cache generates static HTML pages for your site and then serves those. The benefit being that static pages are extremely fast to serve. In this test I wanted to try a more dynamic approach using Memcached and W3 Total Cache. W3 Total Cache takes a very different approach to caching by storing pages, objects, and database queries in Memcached. In general this caching model is more flexible, but possibly a bit slower. I installed Memcached on the same server as the NFS mount because it was under utilized. In a real production scenario I wouldn’t violate this separation of concerns.
Once I enabled W3 Total Cache and re-ran the last test I got some pretty great results.
With W3 Total Cache enabled and 5 app servers we settled in at ~370 requests/second. More impressive is that we only saw 5 failures during the entire test. For perspective Kernl pushed 1,329,470 requests at the WordPress cluster I created. Thats a failure rate of 0.0003%.
My favorite part of this test was the response time distribution. Without having to wait on MySQL for queries the response times became crazy good.
99% of requests finished in 29ms. And the outlier at 100% was only 2.5 seconds. Not bad for WordPress.
Being the good software developer that I am I wanted to push this setup to it’s limits. So I decided to try a test that is an order of magnitude more difficult:
20,000 users, 10 users/sec ramp up, for 60 minutes, from London
Things didn’t go great but not because of WordPress. I won’t show any graphs of this test but I started to get limited by the network card on the NFS/Memcached machine. Digital Ocean says that I can expect around 30MB/sec out of a given droplet and with this test I was starting to bump in to that limit. If I wanted to test it further I would have had to load balance Memcached which felt a little bit outside of scope. In a real production scenario I would likely pay for a hosted Memcached service to deal with this for me.
With Kernl I’m always weighing the build versus buy question when it comes to infrastructure and services. Given how much effort I had to put in to making this setup horizontally scalable and how much effort it would take to make it reproducible and manageable, it hardly seems worth creating and managing my own infrastructure.
Aside from my time the cost of the hardware was also not cheap.
Load Balancer – $10 / month
MySQL Database – $27 / month
Memcached (if separate from NFS) – $5 / month
NFS Mount (if separate from Memcached) – $5 / month
At $72 / month I could easily have any of the managed WordPress hosting companies (GoDaddy, SiteGroup, WPEngine, etc) run my setup, handle updates, security, etc. The only potential hiccup is the traffic limits they place on your account. This setup can handle millions of requests per day and while their setups can too, they’ll charge you a hefty fee for it.
As with any decision about hardware and scaling the choice varies from person to person and organization to organization. If you have a dedicated Ops team and existing hardware, maybe scaling on your own hardware makes sense. If you’re a WordPress freelancer and don’t want to worry about it, maybe it doesn’t. IMHO I wouldn’t scale WordPress on my own. I’d rather leave it to the professionals.
As I went through the first round of tests I kept thinking: “I wonder how they achieve that level of performance with WordPress?”. This blog post and the post that will follow it are a chronicle of my attempts to scale WordPress to the levels that these managed cloud providers are achieving in an economical fashion.
Having done a handful of load tests against other cloud providers I figured that I should hold myself to the same tests. The scale I’m going to try and achieve is:
200 concurrent users for 10 minutes.
2000 concurrent users for 2 hours.
20000 concurrent users for 1 hour.
The first test is just to shake out bugs in the load test, but I have seen some providers start to throw errors at that level. The second test is testing for sustained load. And the third test is simulating a heavy traffic spike.
To get things started I created a super basic WordPress install on a $5/month Digital Ocean droplet. The droplet specs:
The first test went really well. I didn’t performance tune anything and didn’t have any sort of cache enabled. After 10 minutes we had settled into 35 requests / second and didn’t see any failures at all.
For 90% of people this is probably more performance than they would ever need. The response time distribution was even awesome. 100% of requests finished in ~500ms.
And Then The Wheels Fell Off
After my early success with the basic 200 user load test I thought it was time to throw some serious load at my WordPress install. This time I did the 2000 concurrent users for 2 hours test. At this point there still wasn’t any caching plugin installed.
As you can see things didn’t go well. We peaked at around 40 requests/s but then our failure rate started to increase is a really bad way. You can also see that we sorta stopped fielding requests after awhile. Looking at the system load information, you can see why things went poorly. The $5 droplet just couldn’t handle anymore.
As you would expect in this situation, the response time distribution was pretty dismal. In fact, this is the worst response time distribution that I’ve seen in all the load testing that I’ve performed 🙂
After reaching the max capacity of the $5 droplet with no tuning, it was time to try and scale.
WP Super Cache Me
WP Super Cache is a caching plugin that generates static HTML files of your WordPress site. For read-heavy sites its tough to beat in terms of performance. The blog that I’m load testing with definitely falls into this category so it was the right choice for this test.
This test was simply a repeat of the last test (2000 users, 2 hours, etc) but with caching enabled. The results were pretty great.
With WP Super Cache enabled on the $5 droplet we were able to field around 135 req/s, however you can see that our error rate was elevated during much of the test. If you expect to see this sort of traffic on a regular basis then this isn’t a great outcome but still pretty respectable for $5/month. The response time distribution tells a different story though:
Whats the point of serving 135 req/s if it takes more than 10s per request for 33% of your users? People are just going to close the tab after 1 second so we obviously have some more work to do.
Scale Me Up
When scaling any website you have 2 options (and they aren’t mutually exclusive):
Scale up (vertically)
Scale out (horizontally)
Scaling up is usually the easiest thing to do because you’re basically throwing more hardware at the problem. Digital Ocean makes scaling up really easy so I decided to give that a go first. This test was once again just a repeat of the 2000 users for 2 hours test but with better hardware. I upgraded from 1 CPU to 3 CPUs which seemed like the right choice given that it didn’t appear that memory was the problem in my previous tests.
So how did it go? Real good actually. Once all the load test users were sending requests we settled in at 344 request / second. If that rate continued all day that comes out to 29 million requests. Not bad for $15/month.
We’re still seeing some failures, but relative to the number of requests it is much lower than the previous test. We can do better but that will likely take some more vertical or horizontal scaling. But what about the response times? Turns out adding more CPUs helped out quite a bit.
100% of our requests finished in under 1.6s. While not SUPER fast it is still a respectable showing for the sort of load that this box was receiving. Even more impressive is that 90% of requests finished in under 100ms and some of that could be attributed to latency. The droplet was spun up in NYC3 and the load test generators were in Toronto, Canada.
The biggest selling point (for me) with WordPress is that it’s easy. With very little configuration or effort I was able to get a WordPress installation serving > 300 req/s. Sure it wasn’t perfect. I am still getting elevated error rates and vertical scaling can only take us so far. But this is likely good enough for almost anyone.
In part 2 of this series I’ll attempt to scale WordPress horizontally by using shared block storage to host the WordPress file system, a dedicated MySQL machine, and a bunch of application servers running behind a load balancer. The goal is serve 20,000 (or more!) concurrent users for 1 hour without any errors and response times below 1 second. Follow @kernl_ on Twitter to be notified when part 2 is published!
For this series of blog posts we judge the platform via 3 different load tests.
Baseline – This test is for 200 concurrent users, for 10 minutes, with a 2 user / second ramp up. We use this test to double-check our configuration before throwing heavier load at the provider.
Sustained Traffic – 2000 concurrent users, for 2 hours, ramping up at 2 users / second. This test represents what a high traffic WordPress site might see on a day to day basis.
Traffic Spike – The traffic spike test simulates what might happen if a Twitter or Instagram celebrity mentioned your site. 20,000 concurrent users, for 1 hour, ramping up at 10 users/s.
For all 3 tests traffic is generated out of Digital Ocean‘s San Francisco 2 (SFO2) data center.
What WordPress.com plan was used?
For this test we used the Free WordPress.com plan. We didn’t need any of the bells and whistles, plus (as you’ll see later) the performance didn’t suffer at all. If performance had been impacted we would have increased our plan to somewhere around the $10/month mark. As with all of our load tests we don’t do any configuration. We simply import the content of http://www.re-cycledair.com and then start testing.
The sustained traffic test is where WordPress.com’s hosting really started to shine. As of now it is hands down the best host that we’ve tested. The throughput settled in at around 258 requests / second. There also weren’t any surprises on our way up to that number of requests.
The most impressive part about this entire load test was the failure rate. Over a 2 hour test, under heavy load, for more than 1.4 million requests, not a single request failed. Thats some serious stability.
While not as impressive as the 0% failure rate, the response time distribution was still pretty amazing. 99% of all requests finished in well under 100ms. There was an outlier in the ~1500ms range, but that isn’t uncommon for load tests.
WordPress.com is blazing fast. It didn’t event flinch with 20000 concurrent users. The request rate settled in at 1717 requests / second (!). On the way up to that request rate there were no surprises or stutter steps.
The failure rate was exceptional as well. For an hour long test, with sustained heavy load, and a total of 4.3 million requests, there were 0 errors.
Finally, the most impressive graph in this entire test! For the traffic spike test, WordPress.com’s distribution chart is nothing short of fantastic. 99% of traffic had response times below 50ms, and even the 100% outlier was still only 1 second. Great work WordPress.com team!
If you need extremely robust performance and are OK with the restrictions of WordPress.com they seem like a great choice.
Want to be part of the Kernl WordPress Load Testing Beta? Sign up and then send an email to email@example.com
Cloudways will be tested using 3 different load tests:
The Baseline – This is a 200 concurrent user, 10 minutes, 2 user/s ramp up test from San Francisco. This test is used to verify the test configuration and to make sure that Cloudways doesn’t go belly-up before we get started 🙂
The Sustained Traffic Test – This test is for 2000 concurrent users, ramps up at 2 users/s, from San Francisco, for 2 hours. The sustained traffic test represents a realistic load for a high traffic site.
The Traffic Spike Test – This test is intentionally brutal. It simulates 20000 concurrent users, ramps up at 10 users/s, from San Francisco, for 1 hour. It represents the sort of traffic pattern you might see if a Twitter celebrity shared a link to your blog.
What CloudWays plan was used?
For this test we used the lowest tier plan available while hosting on Vultr. The cost of the plan is $11 / month and includes full SSH access to the box that CloudWays deploys your WordPress instance on.
Where does the traffic originate?
The traffic for this load test originates in Digital Ocean‘s SFO2 (San Francisco) data center. The Vultr server lives in their Seattle data center.
The baseline WordPress load test that we did with CloudWays is used to test configuration. CloudWays performed well on this test. You can see from the request graph that we settled in at around 25 requests / second.
The failure graph for the baseline load test was empty, which is generally expected for the baseline test.
Finally the request distribution graph for the baseline test. You can see that 99% of the requests finished in ~200ms. There was at least one outlier at the ~5000ms mark, but this isn’t uncommon for load tests.
The sustained traffic load test represents what a WordPress site with high readership might look like day over day. The CloudWays setup responded quite well for the hardware that it was on.
You can see that performance was great for the first 10% of the test. The CloudWays setup had no trouble handling the load thrown at it. However once we started getting to around 85 requests / second the hardware had trouble keeping up with the request volume. You can see from the choppy behavior of request graph that the Varnish server which sits in front of WordPress was starting to get overwhelmed by the request volume. Considering that this particular CloudWays plan was deployed to a low-level Vultr VM, this performance isn’t bad at all.
The failure graph was a little disappointing, but not unexpected knowing the hardware that we tested on. It is very likely that if we tested on a more robust underlying Vultr box we would have had much better results. You can see that failures increased in a fairly linear rate through the whole load test.
The final graph for this test is the response distribution graph. This graph shows you for a given percentage of requests how many milliseconds they took to complete. In this case CloudWays didn’t perform great, but once again I’ll point to the fact that the underlying Vultr hardware isn’t that robust.
From the graph you can see that 99% of requests completed in ~95 seconds. Yes, you read that correctly. You can interpret this graph as you like but taking the other graphs into consideration you can see that Varnish and the underlying Vultr hardware were completely overwhelmed. Knowing that makes this a little less terrible. We suspect that a smaller load test (maybe 750 concurrent users?) might yield a far better response time distribution. Once a server becomes overwhelmed the response time distribution tends to go in a bad direction.
Given what we know about the sustained traffic load test your expectations for how this test went are probably spot on. CloudWays did as good as can be expected with how the underlying hardware is allocated, but you would likely need to upgrade to a much larger plan to handle this level of traffic. We ended up stopping this load test after about 30 minutes due to the increased failure rate.
The requests per second never really leveled out. It isn’t clear what the underlying reason was for the uneven level at the top of the graph. Regardless, top-end performance was similar to the sustained traffic test.
The failure chart looks as we expected it to. After a certain point we start to see increased failure rates. They continue up and to the right in a mostly linear fashion.
The response time distribution is really bad for this test.
As you can see 80% of the requests finished in < 50s which means that 20% of the requests took longer than that. The 99% mark was only reached after > 200s, at which point the user is likely long gone.
For $11 / month the CloudWays managed WordPress installation did a great job, but there are better performers out there in the same price range (GoDaddy for instance). For the sake of this review which only looks at raw performance, CloudWays probably isn’t the best choice. But if you’re looking for good-enough performance with extreme flexibility then you would be hard pressed to find a better provider.
It has been a busy few months for Kernl. Lots of great work has gone into the WordPress load testing feature work as well as a few structural changes to increase reliability.
Cache moved to Redis – For as long as Kernl has existed our cache backend was powered by Memcached. We have now finished migrating to Redis hosted at Compose.io.
AngularJS Upgrade to 1.7.5 – Fairly straight-forward upgrade to Angular 1.7.5. We wanted to take advantage of performance improvements and few bug fixes.
WordPress Load Testing – Over the past few months we’ve been cooking up something new. Imagine if you could easily test performance changes to you or your client’s WordPress installation? Or be able to tell your client with confidence how many customers at a time their site can support (and what their experience will be like!). What if you could do all this without writing a single line of code or spinning up your own testing infrastructure? We’re ready to start beta testing so send an email to firstname.lastname@example.org if you would like to be a part of it.