Messages in this thread | | | From | Andi Kleen <> | Subject | Re: [PATCH] Use uname not sysctl to get the kernel revision | Date | Fri, 14 Jul 2006 21:09:34 +0200 |
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On Friday 14 July 2006 20:49, Benjamin Herrenschmidt wrote: > > We will need an implementation that will fall back to sys_sysctl for older > > kernels. This is already common practice in glibc. I don't really > > understand the performance concern because it seems to me that > > _is_smp_system() is only called once per process. > > > > But isn't this the kind of thing that the Aux Vector is for? I like vDSO > > too, but I think it is best deployed for information of a more dynamic > > nature and performance sensitive. > > For a simple "is_smp" kind of flag, I would tend to agree with the > above... for more complex NUMA topology and/or cache characteristics, > which is quite a bigger amount of information, I'm not sure it's worth > copying all of that data on every process exec (and making the initial > AT_ parsing slower). Especially since very few processes actually care > about those.
I've actually spent some thought on that recently. The motivation came from someone who wanted the number of CPUs in a fast way to tune AMD64 memcpy etc. better.
My proposal was to supply four new count: number of cores, number of siblings, number of sockets, number of nodes
These all fit easily in 16bit so it would be 2 new entries in the aux vector (128 bit total). Shouldn't be much overhead to write this.
If you need more exact topology you can probably eat the overhead of parsing /proc/cpuinfo or read it from sysfs (or just use libnuma which supplies most of this in an easy way)
Doing it in a vDSO would be in theory ok for me too, except that x86-64 doesn't have one so far. Even in vDSO I wouldn't add much more than this (like bitmaps and similar) because otherwise cpu/node hotplug could be racy. Also I'm reluctant to redo /proc/cpuinfo and /sys for this.
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