Jiffies or a Jiffy (what are they?)

BonezTheGoon

Ok I have seen many strange posts regarding the range or value of Jiffies or a Jiffy. Near as I can tell from many sources I found while googling a Jiffy is supposed to represent one one-hundredth of a second (1/100). OK I like that definition OK, I can live with that. What I haven't been able to find out for sure is what the relationship (and there may not be one, I certainly don't know there is one, I suppose there is one though--which could be my folly) is between a Jiffy and a CPU Cycle or Clock. Given there are many threads indicating to change the Jiffies from 1000 to 100 (for everything from fixing frame rates in games to correcting drifting time on your machine) I think that maybe this change has to do with processor speed (Mhz). I did find one interesting find in my googling that show you how to display the jiffies on the second every second until you abort the command, once my new Gentoo 1.4 beta system is able I am going to run this command to see if a Jiffy on my system does indeed last 1/100 of a second. If not I intend to "fine tune" my jiffies value so that (one Jiffy == 1/100 of a second). Mostly I am posting this so that Rac or someone else with some actual working knowledge of this stuff will correct me--because I seem to be really good at coming up with creative theories that are never correct. Here is the link to the article that says how you can display your current jiffies. I'm not sure if the command they reference 'repeat' is part of some special debugging suite, I hope not! So if anyone is better at googling that I and can actually find a REALLY good definition of what a jiffy is and what the setting does (going from 1000 to 100) I would really appreciate any help in figuring this out.

Is it life or death? naw, I just want to know what I am doing--which I could NEVER do on M$ Windows! Now that everything is 'open' on my system I want to understand this stuff so that I can make it best do what I want it too!

Thanks in advance, and sorry for the long boring post!

Regards,
BonezTheGoon

rac · Posted: Sun Aug 18, 2002 12:34 am Post subject:

Here's the jargon file entry for "jiffy". According to the kernel sources, 100 jiffies per second on x86, 1024 on alpha. You can see every mention of jiffies in the kernel source with this:

BonezTheGoon · Posted: Sun Aug 18, 2002 12:44 am Post subject:

Well there is something I hadn't thought of, the setting is how many per second--DUH! So setting the jiffies to 100 makes one jiffy==1/100 of a second? Man I really do a good job of complicating these things for myself!! Thanks rac! Atleast now if someone actually searches the forums for an answer you already gave them one here! Thanks a bunch!

Regards,
BonezTheGoon

rommel · Posted: Sun Aug 18, 2002 6:04 am Post subject:

that definition refers to the swapper in the virtual memory management system....when ever i used reiserfs and LSR i get an md swapper pid (1) ioctl warning telling me to upgrade my software (posted during the boot)...but i have never figured out what its talking about.
would it be refering to something under the domain or el'jiffie?

delta407 · Posted: Sun Aug 18, 2002 6:29 am Post subject:

Evangelion · Posted: Sun Aug 18, 2002 9:38 am Post subject:

Correct me if I'm wrong: Increasing the jiffies will improve the responsivness of the system, right?

Is there any downsides in icreasing the jiffies? They are doing that in the 2.5.x kernels, but what downsides would there be if I did that with 2.4.x kernel?

delta407 · Posted: Sun Aug 18, 2002 5:20 pm Post subject:

It may or may not improve responsiveness. Some have said it does, but many have noticed no different.

The downside is that context switches take time and produce cache misses; if you're constantly switching processes, none of them will get anything done, and the processor will constantly be talking to RAM rather than it's on-die cache.
_________________
I don't believe in witty sigs.