[LITMUS^RT] Questions about ft_trace

[Glenn Elliott]

CONFIG_NR_CPUS CPUs controls the maximum number of CPUs your compiled Linux kernel can support.  There is no harm in setting this value to be greater than the number of actual CPUs in your system-- it may just lead to minor waste of memory (the Linux kernel statically allocates memory based upon NR_CPUS).

-Glenn


On Feb 14, 2012, at 12:18 AM, Hang Su wrote:

> Hi Glenn:
> 
> Thanks for your quick reply. 
> 
> First I want to confirm the .config you mentioned is supposed to be located under /litmus-rt, in which arch, litmus and other main linux directory reside.  Is that right ?  If so, CONFIG_SCHED_OVERHEAD_TRACE is not set in .config. It means I have to recompile Litmus kernel by marking CONFIG_SCHED_OVERHEAD_TRACE item after issuing make menuconfig.
> 
> The result of cat /proc/cpuinfo under virtual machine shows only one processor in my computer, which is different from the one under my real machine. Probably, I do not allow virtual machine to use two cpus.
> CONFIG_NR_CPUS is 8 in my .config. It does not make sense. I do not have so many cpus. What does    it exactly stand for ?
> 
> Thanks
> 
> On Mon, Feb 13, 2012 at 11:46 PM, Glenn Elliott <gelliott at cs.unc.edu> wrote:
> Hi Hang Su,
> 
> WIth regards to ft_trace, please confirm that CONFIG_SCHED_OVERHEAD_TRACE=y in your .config.
> 
> With respect to the number of CPUs, is your virtual machine configured to run with two CPUs?  If so, what CONFIG_NR_CPUS set to in your .config?
> 
> Please note that we recommend using Litmus in a virtual machine only for development/debug purposes.  We've found that the time accounting mechanisms in virtual machines are not accurate enough for real-time performance.
> 
> Glad that you're trying Litmus out!
> 
> -Glenn
> 
> 
> On Feb 13, 2012, at 11:20 PM, Hang Su wrote:
> 
> > Hi all:
> >
> > LITMUS is new to me. After I installed LITMUS under Oracle Virtual Box on my Thinkpad x200, it seems that I can not find ft_trace device under /dev/litmus/. Only three devices can be seen under /dev/litmus:
> > ctrl log sched_trace0
> > How could I load a module to create ft_trace device ?
> >
> > Another question is about sched_trace buffer. As said in Tracing and Logging tutorial, sched_trace is a per-"processor" buffer. The upbound number after sched_trace is  given by num_online_cpus(). My CPU is Intel P8600 duo core processors. It is supposed to be two sched_trace buffer. But why do i have only one sched_trace buffer under /dev/litmus ?  it is because i install LITMUS on virtual machine instead of directly on real machine?
> >
> > Thanks
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> 
> 
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