On Wed, 17 Jul 2002, Karthik M wrote:
> hi everyone,
>
> as a follow up to the previous posting regarding
> effects of measurment, I was testing gettimeofday().
> The results that i got out of my test program were not
> consistent and were varying. Wanted to get an opinion
> abt it.
>
> ----snip-----
>
> the values are varying a lot and did'nt have a clue
> why it was the case. Did a little search on google but
> could'nt find an answer.
IMHO "Scheduling granularity" is the key here. The linux system on x86
architectures has a heartbeat of 100Hz = 10ms. What this means is that
the system tries to complete its range of tasks such as scheduling
various subsystems, checking on blocked I/O, sending & receiving packets
etc etc. every 10ms. Since some tasks have more priority over others,
they might cause a delay for these low priority tasks which are
generally user processes.
Trying this on an Alpha which has HZ=1000 (IIRC) will give you better
results.
You can improve the granularity of the system by using real-time
versions which give some kind of guarantees. An example is a real-time
patch developed at our university called KURT (www.ittc.ku.edu/kurt/).
Just enable UTIME to get better granularity. But even then it might not
be perfect. To get a perfect schedule you would have to use the KURT
scheduler.
Or if you donot want to be bothered with it, just try changing HZ to
1000 in /include/asm/param.h. This *might* give you better results.
Warning: This value might not be suitable for a typical desktop, we
usually use it only on servers.
ciao,
Amit
--
I'm an angel!!! Honest!
The horns are just there to hold the halo up straight.
^-^-^-^-^-^-^-^-^-^-^-^-^-^-^-^-^-^-^-^-^-^-^-^-^-^-^
Amit Kucheria
EECS Grad. Research Assistant
University of Kansas @ Lawrence
(R): +1-785-830-8521 ||| (C): +1-785-760-2871
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