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SubjectRe: [patch] SMP alternatives
Bill Davidsen wrote:

> Linus Torvalds wrote:
>> On Wed, 23 Nov 2005, Daniel Jacobowitz wrote:
>>> Why should we use a silicon based solution for this, when I posit that
>>> there are simpler and equally effective userspace solutions?
>> Name them.
>> In user space, doing things like clever run-time linking things is
>> actually horribly bad. It causes COW faults at startup, and/or makes
>> the compiler have to do indirections unnecessarily. Both of which
>> actually make caches less effective, because now processes that
>> really effectively do have exactly the same contents have them in
>> different pages.
>> The other alternative (which apparently glibc actually does use) is
>> to dynamically branch over the lock prefixes, which actually works
>> better: it's more work dynamically, but it's much cheaper from a
>> startup standpoint and there's no memory duplication, so while it is
>> the "stupid" approach, it's actually better than the clever one.
>> The third alternative is to know at link-time that the process never
>> does anything threaded, but that needs more developer attention and
>> non-standard setups, and you _will_ get it wrong (some library will
>> create some thread without the developer even realizing). It also has
>> the duplicated library overhead (but at least now the duplication is
>> just twice, not "each process duplicates its own private pointer")
>> In short, there simply isn't any good alternatives. The end result is
>> that thread-safe libraries are always in practice thread-safe even on
>> UP, even though that serializes the CPU altogether unnecessarily.
>> I'm sure you can make up alternatives every time you hit one
>> _particular_ library, but that just doesn't scale in the real world.
>> In contrast, the simple silicon support scales wonderfully well.
>> Suddenly libraries can be thread-safe _and_ efficient on UP too. You
>> get to eat your cake and have it too.
> I believe that a hardware solution would also accomodate the case
> where a program runs unthreaded for most of the processing, and only
> starts threads to do the final stage "report generation" tasks, where
> that makes sense. I don't believe that it helps in the case where init
> uses threads and then reverts to a single thread for the balance of
> the task. I can't think of anything which does that, so it's probably
> a non-critical corner case, or something the thread library could
> correct.
In 2-3 years we might actually see the hardware solution, maybee .... I
am skeptical Intel will move quickly on it. A software solution will get
out faster.

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