What is "CPU usage"?

Our servers run programs for many Web sites, so your PHP scripts and other programs you install can’t use the full resources of a server all the time. That’s okay, though, because your Web site won’t need to run scripts all the time (it will be idle between visitors, using no resources while it waits for the next page request).

When you run a script, it uses the resources of one of the CPU “cores” of our servers for a certain amount of time.

Because the CPU resources are limited, we measure how much you use, and our various plans include different amounts of CPU power, just like they include different amounts of disk space and e-mail mailboxes.

All our hosting plans include plenty of CPU power — in fact, your site can average 10% of a powerful server computer core on even our least expensive plan. In our experience, only very busy sites, or sites with very resource-intensive scripts, use more than that. Most of our customers use much less.

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So what’s a “CPU unit”?

CPU power is more abstract than disk space or e-mail mailboxes, so it’s difficult to concisely describe how much you use. We use the phrase “CPU units” as a simple description.

We define a “CPU unit” as being equivalent to 1% of the available resources of one core of a 2.33 GHz Intel Xeon L5410 processor. (You can see why we shortened the name!)

If you’re familiar with the concept of a Unix “load average”, 10 CPU units would be roughly equivalent to adding 0.1 to a server’s load all month long.

Do you measure CPU usage as an average, or by peak usage?

We measure CPU units as a monthly average, allowing you to use more during busy periods without any penalty.

So if your site uses 1 CPU unit for 23 hours a day, then “bursts” up to 50 CPU units for one busy hour every day, your monthly average would still only be 3 CPU units, and you wouldn’t need to upgrade to a larger plan.

We’ll notify you if your average CPU units are going to exceed the amount included in your plan (although that’s very rare) so you can avoid extra charges.

How can I see how many CPU units I’m using?

The usage page of our “My Account” control panel shows CPU usage graphs that are updated hourly.

If you make a technical change and you want to see how that affects your site, you should wait two hours and then review the graph. That way you can be sure that it shows a full hour of the changed usage.

How do CPU units compare to a “dedicated server”?

A modern single-core dedicated server that other companies might sell you can probably handle about 80 CPU units. If your site ever uses more than that — even for a short period of time — it would stop working properly if it was on that company’s dedicated server.

One of the advantages of using our “shared hosting” instead of a “dedicated server” is that we only charge you for the average usage, not the peak usage.

If your site uses 3 CPU units most of the time, but gets a huge number of visitors once every few months from a listing on a popular news site, that can be very expensive on a dedicated server. If your site needs, say, 280 CPU units to handle a traffic burst for three hours a month, then your dedicated server would need to be able to handle that worst case — you’d need a dedicated quad-core server that might cost a hundred times what we charge you. But of course, you won’t be using that power most of the time.

With our shared hosting, you don’t need to pay for the worst case. We can handle some pretty big “spikes”, and as long as your average usage remains low, the average is what you’ll pay for.

(On the other hand, if your average baseline usage is consistently more than 25 CPU units, you should use a dedicated server instead. Very few sites use that much, though.)

Can you give me an example of exactly how you’d calculate CPU units?

Sure thing. It’s a little geeky, but we want to make sure it’s understandable. The way it works is that we:

  • Add up the exact amount of CPU time used by each program you run in a month;
  • If your site isn’t on a 2.33 GHz Intel Xeon L5410 processor, adjust the number to match what it would have been (this makes sure the numbers are the same for all sites, even across servers of slightly different speeds);
  • Divide the total by the number of seconds in the month;
  • Convert that to a percent and round down to the next whole number. That’s how many CPU units you used.

So imagine that you install WordPress, which is a script that uses the full power of a CPU for about 0.125 seconds each time it runs.

For the first 7 days of February, your site is very busy and gets 50,000 page views per day. That’s 43,750 seconds of CPU time. Then for the next 21 days, your site gets 10,000 page views per day. That’s 26,250 more seconds of CPU time. The total usage for the month was 70,000 CPU seconds.

However, in this hypothetical example, let’s say your site is on a 2.2 GHz server instead of a 2.33 GHz server. To compensate, we lower the number to 66,037 (if your site had actually been on a 2.33 GHz server, it would have taken only that many CPU seconds to handle the page views).

There are 2,419,200 seconds in a 28 day month, so we divide 66,037 by 2,419,200. That’s about 0.0273, or 2.73%. We round that number down and calculate that you used 2 CPU units in February.

As you can see, even a busy site doesn’t use much CPU power compared to the amount we include. And in most cases, a small amount of effort to improve the efficiency of your scripts can make things much better; for example, it’s possible to tweak WordPress to handle hundreds of thousands of page views every day and still use less than 5 CPU units.