lkml.org 
[lkml]   [2005]   [Dec]   [5]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH] CPU frequency display in /proc/cpuinfo
> JACK needs to know the CPU speed in order to be able to use RDTSC for
> timing. Yes that might be "broken" but gettimeofday() is simply not

It is broken then. Was always broken, will be broken etc.

> fast enough for our use, we can't afford the overhead of thousands of
> system calls per second. And until recently 99.999% of desktop machines
> had a monotonic TSC so this worked very well.

You're wrong. First if you say "monotonic" you don't understand the problem.
Monotonicity is only a small part of it. The bigger one is just
getting the current frequency and figuring out of if the information
is safe to use.

Chips where it doesn't work:

- Intel Prescott and derivatives (newer Pentium 4, newer Xeon, on
Celeron you are still lucky because they normally don't have speedstep):
TSC runs at a different frequency than the current P state and P state often
varies with speedstep [These are the most common desktop chips in the world,
although not all have speedstep enabled. Many newer ones have though.]
- Intel P-M and earlier P4,P3 mobile chips, Athlon 64, Athlon XP-M,
Opteron etc:
TSC varies with P state and user space has no way to get timely
updates on P-state changes.
- VIA C3: I believe TSC stops in idle (at least it used to on
older versions, don't know if they fixed it)

-Andi

-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2005-12-05 19:40    [W:0.068 / U:0.408 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site