An excellent explanation, I never knew that is how it worked. -----Original Message----- From: fedora-list-bounces@xxxxxxxxxx [mailto:fedora-list-bounces@xxxxxxxxxx]On Behalf Of John DeDourek Sent: Friday, October 15, 2004 8:48 PM To: Linn Kubler; For users of Fedora Core releases Subject: Re: TSC cannot be used as a time source? Linn Kubler wrote: >Hi, > >Frist time posting here, if this is the wrong list for this type of >question please forgive me. > >When I boot my server I get this message after logging in: >TSC cannot be used as a timesource. >Possible reasons for this are: > You're running Speedstep, > You don't have DMA enabled for you're hard disk (see hdparm), > Incorrect TSC synchronization on an SMP system (see dmesg). >Falling back to a sane timesource now. > >Can someone explain what's happening here and how to correct it? When >I installed the server I used the same timesource I use on other >non-Linux systems. > >BTW, this is an installation on VPC. > >Thanks in advance, >Linn > > I believe that I understand the meaning of the message, but unfortunately don't know enough about your system to suggest a fix. Perhaps though if I explain the message it will help you tracking down the problem. Time keeping in Linux is done by an interrupt which occurs regularly (configured on most systems to happen every 10ms, that's 0.01 sec). This means that every 0.01 second the time stored in the operating system is incremented by 0.01 second. If you were to create a time stamped log, the times would be like: 25.03 seconds or 25.04 seconds, but never 25.035 second. Some applications need a higher resolution. They want times like 25.036928 seconds. Modern Intel Pentium chips have a counter that just counts up by one at a high speed. But it just starts at zero when you boot and goes from there, so it doesn't really specify the time. Someone came up with the idea that the system could check how much gets added to the counter between two of the regular clock interrupts, that is in 0.01 second. As the system runs, Linux records the value of the counter at each of the interrupts. Then when you need a highly accurate time, Linux looks at the counter and uses this as an estimate of how far you are between interrupts. (Technically it does a linear interpolation; look at a math reference for explanation.) This idea works ONLY if the counter goes up about the same amount between each interrupt, that is in each 0.01 second. If it doesn't, obviously this scheme won't work. If you look at your error message, linux is saying just that. The counter (named: TSC) is not giving the same result for each 0.01 second interval. Therefore, linux has decided to cease trying to use the TSC. Result, you will get times that have a resolution of 0.01 second. If such times are satisfactory, just ignore the message; you don't need the TSC. If you DO NEED more precision in your times, then you will need to research why linux is finding that the TSC counter is not consistent. The error message gives the first things to check. For example, "speedstep". This is a feature that makes the processor (and therefore the TSC) run faster and slower. Faster means programs get done quicker, but it also means CPU runs hotter and bettery (in a laptop) runs down faster. And vice versa for slower. So if you are running in speedstep mode, turn it off. How do you know? How do you turn it off? Unfortunately, this is where my expertise ends and you'll need to search further. Good luck. -- fedora-list mailing list fedora-list@xxxxxxxxxx To unsubscribe: http://www.redhat.com/mailman/listinfo/fedora-list
