Re: Premptible Kernels and Timer Frequencies

[Steven Rostedt <rostedt@goodmis.org>]

On Thu, 2005-11-03 at 18:50 -0500, AndyLiebman@aol.com wrote:
> Hello Steve, 

Hello Andy

>  
> I'm going to add my message at the top of this post, because it's not
> a direct response to your post. 
>  
> Thanks for the good start at explaining some of the new kernel
> compiling options. I really appreciate the time and effort you made at
> explaining some of these new choices. And I took your suggestion and
> tried out the 2.6.14-rt1 kernel with total success (at least getting
> it to run). 

The -rt branch is a fast moving target, and is always getting better.
It's still in development, but is getting pretty stable. As I write
this, the current version is at 2.6.14-rt5.  I'd recommend downloading
that one. Although for what you are doing, vanilla linux should be good
enough (see below).

>  
> However, ultimately you don't really answer my fundamental question
> about what options are best for what kind of application. Maybe
> someone like Linus or Andrew Morton could chime in here. Put a little
> write up on the kernel.org site??? 

Actually Alan Cox might be a good person to chime in, since he does a
lot in networking.

>  
> You guys must have a good idea about WHY one would choose a timer
> frequenzy of 100 versus 1000 -- YOU put these options in the kernel.
> Please give us some examples about where the different sets of options
> are appropriate -- what options should be used together and which one
> could contradict each other. 
>  
> To recap MY particular case, I have a file server that is sending out
> -- and taking in -- video and audio files from 5 to 30 workstations.
> The data rates typically range form 3.8 MB/sec per workstation up
> to 60 MB/sec per workstation. All of the transactions are handled by
> TCP/IP. In some cases, we are using a Chelsio 10 Gigabit Ethernet card
> with TCP/IP offload. But generally, we have a bunch of Intel Pro 1000
> MT server adapters feeding one or more workstations each. Or we are
> bonding a group of NICS and sending all the data to a managed switch. 
>  
> So, we have:
>  
> a lot of TCP/IP transactions
> NIC drivers running
> 3ware Hardware RAID -5 drivers running
> Sometimes Linux Software RAID -0 striping two 3ware cards together
> Samba
> Netatalk
> Occasional use of a Twisted Web Server
> Occasional use of TightVNC
> A mostly inactive KDE session
> A UPS program
>  
> That's pretty much what's running. Only rarely do any applications
> "run" on the server (once in a while, an administrator will do
> something to "manage" the system -- but that's maybe 1 or 2 minutes
> out of every 24 hours. We are running on both 32-bit and 64-bit
> systems. 

Your system looks pretty much interrupt driven, and I would assume that
all these are of same importance.  So I would actually recommend the
vanilla kernel, with HZ=100 and preemption turned off.

Why?

Well, every time you get preempted, or happen to do a task switch, there
is overhead.  So if all processes or actions are of the same importance,
whatever is running now, doesn't need to be preempted by something that
just woke up.  And since this is mainly work that is in the kernel, with
only the user apps applying services.  I wouldn't think you really need
the preemption.

The HZ value is what makes up time slices for applications, and since
what you explained, is interrupt driven, you want a low value for HZ.
This will keep the timer interrupt down from preempting whatever is
going on.


>  
> So, my question was, what kernel compiling options are appropriate for
> the best performance?  (for ME, best performance = lowest latency in
> delivering random data to -- and taking data from -- a group of video
> editing workstations, AND best throughput -- goals which may be
> mutually exclusive). For MY application, how should I set: 
>  
> Preemptible Kernel Model
>     No Preemption
>     Voluntary Preemption
>     Premptible Kernel
>  
> Prempt The Big Kernel Lock (Y/N)
>  
> Timer Frequency
>     100 hz
>     250 hz
>     1000 hz
>  
> While my case might be different from sombody else's case, I'm sure it
> would be useful to give three or four examples of different server
> uses that at least "in theory" would benefit from a particular group
> of settings. I'm not sure that "desktop" versus "server" is the most
> helpful distinction. Are ALL servers (including mine) best off with: 
>     No premption
>     Saying No to Preempt The Big Kernel Lock
>     Timer Frequence of 100hz
>  
> Somehow, I doubt it. 

As I explained up above. Yes!

>  
> Yeah, I'm ready to experiment. I've been doing that with Linux for
> three years and getting great results. But I'd love to hear the theory
> of what SHOULD be best. It's about seeing the forest through the
> trees, getting the "big picture" after looking at all the minute
> details. 
>  
> Thanks in advance for more information... Linux rocks! 
>  
> 

-- Steve


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