Re: [Boost-users] progress_timer, incorrect timing

From: Will Bryant (will_at_[hidden])
Date: 2005-11-23 04:04:24

__PPS__ wrote:
> So, my measurements (from the original post) show time taken by my 
> routine to execute, minus everything that's done by the OS?
>   
In general, for a single-threaded program there's:
1. Time spent executing the program itself
2. Time spent in library calls, in user mode (note that this includes 
libc on *nix and some of the Windows API implementations, so the 
distinction between OS and library is often not clear)
3. Time spent in the OS in kernel mode doing things for the program
4. Time when the process is blocked waiting on IO (from other processes 
or devices), or other kernel IPC events (semaphores, condition 
variables, message queues etc.)
5. Time when the process is runnable but some other process is running 
instead (ie. it has been preempted).

Which of these are included in the measure of "time" depends on the 
timing API, but in the case of "clock" which gives CPU time I'd expect 
it to include 1, 2, and possibly 3 (depends on the OS implementation), 
but not 4 or 5, during which time the process is just waiting to be 
resumed.  If you're using "sleep" or similar APIs that may or may not be 
counted, depending on the implementation.  "Wall clock", the elapsed 
real time, will include all 5.

So you need to distinguish between wall clock time and the process CPU 
time, and choose which is appropriate for your timing application.  For 
example, if benchmarking an application that uses an out-of-process 
database server, you would most likely want to include the time spent 
waiting for the SQL server to do its thing and so would want wall clock 
time, but if all you're interested in is the time taken to execute the 
actual computations in the process (the traditional example being 
billing on multiuser time-sharing systems, not a very common situation 
these days) then you might consider CPU time more appropriate.

-- 
Will Bryant
http://carcino.gen.nz/
will_at_[hidden]
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