Messages in this thread | | | From | (Eric W. Biederman) | Subject | Re: [PATCH] Use uname not sysctl to get the kernel revision | Date | Thu, 13 Jul 2006 08:39:20 -0600 |
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Ulrich Drepper <drepper@redhat.com> writes:
> Eric W. Biederman wrote: >> Ulrich what would be interesting besides the possibility of having >> multiple cpus? > > What is needed for various things like memory handling etc is all > topology information. Somebody might remember the numa library proposal > I had in April 2004 which was cast aside because people were only > looking for a "quick fix". Well, the problem still isn't solved. > > IMO the vdso should export information about: > > - processors and their relationship (hyperthreads, cores) > > - the CPU caches and how they relate to the cores (e.g., dual core > with shared L2) > > - local main memory for each processor > > - relative costs of the memory access of the various memory regions > (for numa local memory to a node, intra-node costs) > > - ideally, relative costs main memory and CPU caches > > > All this information can be steadily updated by the kernel as new > CPUs/memory get added/removed. The vdso should have functions to access > this information. It's easy enough to make this access race free. > > I guess I should try to come up with a representation for this > knowledge. Collecting the information (except the costs) should be > easy. Determining the costs also shouldn't be that hard but it can be > very useful. Some of this information could be determined at userlevel > but you really don't want every process to compute all this from > scratch. And stored data in a file is stale if the system changes.
The history of Linux shows that auto-tuning while not always perfect is much more effective than manual tuning. How are you envisioning using this information?
I find it really easy to see how topology information can be used to manually tune a system. I don't currently see how it can be used to automatically tune a system.
My fear is that we will end up making things brittle with user space over specifying things.
I have another related concern about what rules need to be in place so I can upgrade the kernel vdso while a user space program is running. That make me seriously wonder how sane the vdso concept is, but that is a completely different issue.
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