In *nix, what causes "sleeping" in top command?

Question

What causes these sleeping processes that I see in top? If I were to call PHP's sleep() function, would that add to the sleeping count I see in top? Are there any disadvantages to having a high number in sleeping?

Answer 1

A process is sleeping when it is blocked, waiting for something. For example, it might have called read() and is waiting on data to arrive from a network stream.

sleep() is indeed one way to have your process sleep for a while. Sleeping is, however, the normal state of all but heavily compute-bound processes - sleeping is essentially what a process does when it isn't doing anything else. It's the normal state of affairs for most of your processes to be sleeping - if that's not the case, it tends to indicate that you need more CPU horsepower.

Answer 2

They are processes which aren't running on the CPU right now. This is not necessarily a bad thing.

If you have huge numbers (10,000 on a server system, for example) of processes sleeping, the amount of memory etc used to keep track of them may make the system less efficient for non-sleeping processes.

Otherwise, it's fine.

Most normal server systems have 100 to 1000 much of the time; this is not a big deal.

Just because they're not doing anything just now doesn't mean they won't, very soon. Keeping them in memory, ready, reduces latency when they are required.

Answer 3

sleeping process is aka suspended process. a process can sleeps when:

1- It doing an I/O operation (blocking for I/O)
2- When you order it to sleep by sleep()

status of any process can be: *ready *sleeping *running ready: when it ready for execution and it's in the queue waiting the processor call with specific priority
running: when the processor execute a process it become running.
sleeping: when it was running and it blocked for I/O operation or when executing sleep()

Status Meaning

R Runnable

T Stopped

P Waiting on Pagein

D Waiting on I/O

S Sleeping < 20 secs

I Idle - sleeping >20 secs

Z Zombie or defunct