Re: [patch] SMP alternatives

From: Linus Torvalds
Date: Wed Nov 23 2005 - 18:09:09 EST

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.

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