Messages in this thread | | | From | Jim Nance <> | Date | Tue, 10 May 2005 02:23:01 +0000 | Subject | Re: /proc/cpuinfo format - arch dependent! |
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Good Afternoon Bill,
Thanks for the input. Let me make a couple of comments.
On Mon, May 09, 2005 at 02:14:14PM -0400, Bill Davidsen wrote:
> Might I suggest that if you like the "we know best just trust us" > approach, there is another OS to use. Making information available to > good applications will improve system performance, or at least allow > better limitation of requests for resources, and bad applications will > be bad regardless of what you hide. You don't hide the CPU hardware any > more than the memory size.
You could use a similar argument for cooperative rather than preemptive multitasking. It might even be a valid argument, assuming you controlled all the processes running on the system. But it didn't work very well in practice.
I see two problems with encouraging applications to get involved with processor selection.
The first is they don't have enough information to get it right. There are going to be other processes running on the machine. The optimal set of processors to run on is going to depend on what else is running and what it is doing at that instant. This isn't information a usermode process has good access to. Say I have an application that wants to bind its 2 threads to the two processors on a single SMT chip. Now say I run two of these applications on a machine with 2 SMT chips on it. What keeps both of them from binding themselves to the same chip? Should it be the applications responsibility to look through the process table and see what other applicatioins are bound to what processors? What prevents races if they do?
The second is that once you give userland an interface, it becomes very difficult to remove it once it no longer makes sense. See the thread on this mailing list concerning the C/H/S values returned for disk drives as an example. Having to support a particular interface may make it impossible to add improvements we want to add in the future. For example, if at some point in the future we come up with a really great scheduling algorithm, it won't help if the programs have already bound themselvs to particular processors.
Now I know there are exceptions to rules. But in general I would say that if an application needs to know about the configuration of the processors, then its compensating for shortcommings in the kernel.
Thanks,
Jim
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