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SubjectRe: [patch] SMP alternatives
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.


--
-bill davidsen (davidsen@tmr.com)
"The secret to procrastination is to put things off until the
last possible moment - but no longer" -me
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