网站上的第一页请求非常慢

The first page I load from my site after not visiting it for 20+ mins is very slow. Subsequent page loads are 10-20x faster. What are the common causes of this symptom? Could my server be sleeping or something when it's not receiving http requests?

It's probably not sleeping. It's just not visited for a while and releases it's resources. It takes time to get it started again.

If the site is visited frequently by many users it should response quickly every time.

It sounds like it could be caching. Is the server running on the same machine as your browser? If not, what's the network configuration (same LAN, etc...)?

I will answer this question generally because I'm sure it's something that confuses a lot of newcomers.

The really short answer is: caching.

Just about every program in your computer uses some form of caching to remember data that has already been loaded/processed recently, so it doesn't have to do the work again.

The size of the cache is invariably limited, so stuff has to be thrown out. And 99% of the time the main criteria for expiring cache entries is, how long ago was this last used?

  • Your operating system caches file data that is read from disk
  • PHP caches pages and keeps them compiled in memory
  • The CPU caches memory in its own special faster memory (although this may be less obvious to most users)

And some things that are not actually a cache, work in the same way as cache:

  • virtual memory aka swap. When there not enough memory available for certain programs, the operating system has to make room for them by moving chunks of memory onto disk. On more recent operating systems the OS will do this just so it can make the disk cache bigger.
  • Some web servers like to run multiple copies of themselves, and share the workload of requests between them. The copies individually cache stuff too, depending on the setup. When the workload is low enough the server can terminate some of these processes to free up memory and be nice to the rest of the computer. Later on if the workload increases, new processes have to be started, and their memory loaded with various data.

(Note, the wikipedia links above go into a LOT of detail. I'm not expecting everyone to read them, but they're there if you really want to know more)