Lecture Notes for Operating Systems Concepts

23 September 2004 - Case Study: System Call Times

System vs. Procedure Calls

System calls only look like procedure calls.

std::cout << "my pid is " 
          << getpid() << "\n"

Unlike procedure calls, system calls have to cross the kernel-user protection boundry.
How much does it cost to cross that boundry?

A System Call

long getpid(void) returns the calling processes id.
On Linux, getpid() is implemented as
```
asmlinkage long sys_getpid(void)
  return current->tgid
```
- current is a pointer to the running process's descriptor.
- tgid is the process-descriptor field containing the process id.

A Fake System Call

For usefull results, the fake system call should approximate the same work done by the real system call.
```
typedef struct Pd
  long tpid

Pd pd
Pd * current = &pd

int fake_getpid()
  return current->tpid
```

Timing Code

sw.start()
for (unsigned i = 0; i < iters; i++) { }
sw.stop()
const unsigned overhead = sw.elapsed()

sw.start()
for (unsigned i = 0; i < iters; i++)
  getpid() or fake_getpid()
sw.stop()

The timing value is
```
(sw.elapsed() - overhead)/iters
```
See the code.

Test Environment

Solaris 2.9, SPARC 333 MHz CPU in an office.
Redhat Linux 2.4.22, Intel 728 MHz CPU
- Rockhopper with five users doing almost nothing.
iters = 10,000.

The Results

	Solaris	Linux
`getpid()`	1.54	0.45
`fake_getpid()`	0.04	0.01

All times in microseconds.

This page last modified on 14 November 2004.