Messages in this thread | | | Date | Tue, 25 Feb 2003 13:19:33 -0800 | From | Chris Wedgwood <> | Subject | Re: Minutes from Feb 21 LSE Call |
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On Tue, Feb 25, 2003 at 02:59:05PM -0500, Scott Robert Ladd wrote:
> HT is not the same thing as SMP; while the chip may appear to be two > processors, it is actually equivalent 1.1 to 1.3 processors, > depending on the application.
You can't have non-integer numbers of processors. HT is a hack that makes what appears to be two processors using common silicon.
The fact it's slower than a really dual CPU box is irrelevant in some sense, you still need SMP smart to deal with it; it's only important when you want to know why performance increases aren't apparent or you loose performance in some cases... (ie. other virtual CPU thrashing the cache).
> Multicore processors and true SMP systems are unlikely to become > mainstream consumer items, given the premium price charged for such > systems.
I overstated things thinking SMP/HT would be in low-end hardware given two years.
As Alan pointed out, since the 'Walmart' class hardware is 'whatever is cheapest' then perhaps HT/SMT/whatever won't be common place for super-low end boxes in two years --- but I would be surprised if it didn't gain considerable market share elsewhere.
> That given, I see some value in a stripped-down, low-overhead, > consumer-focused Linux that targets uniprocessor and HT systems, to > be used in the typical business or gaming PC.
UP != HT
HT is SMP with magic requirements. For multiple physical CPUs the requirements become even more complex; you want to try to group tasks to physical CPUs, not logical ones lest you thrash the cache.
Presumably there are other tweaks possible two, cache-line's don't bounce between logic CPUs on a physical CPU for example, so some locks and other data structures will be much faster to access than those which actually do need cache-lines to migrate between different physical CPUs. I'm not sure if these specific property cane be exploited in the general case though.
> I'm not sure such is achievable with the current config options; > perhaps I should try to see how small a kernel I can build for a > simple ia32 system...
Present 2.5.x looks like it will have smarts for HT as a subset of NUMA.
If HT does become more common and similar things abound, I'm not sure if it even makes sense to have a UP kernel for certain platforms and/or CPUs --- since a mere BIOS change will affect what is 'virtually' apparent to the OS.
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