CMPXCHG - all or nothing

By: Henrik S (hstahl.delete@this.nospam.bea.nothanks.com), March 21, 2008 12:13 am
Room: Moderated Discussions
Linus Torvalds (torvalds@linux-foundation.org) on 3/20/08 wrote:

>However, considering that memory ordering is hard to think
>about, and a largish part of testing is likely done on
>platforms with pretty strong memory ordering (x86 and sparc),
>it does sound like a recipe for disaster.

I don't doubt that you're right, but given the potential for disaster it's perhaps surprising we very rarely hear of such issues with JRockit on Itanium (used by far more organisations than BEA). Maybe Intel and HP does a good job education their customers on the pitfalls? Or maybe it isn't such a large problem in practice because few apps use cross-thread communication to any real extent?

>But you're almost certainly right that the only apps that
>run on other architectures are likely fairly rare and will
>be validated exhaustively on that platform anyway due to
>it often being very much a big-iron issue. So maybe it's
>not too bad, but it's definitely a case of JVM's not
>all being functionally equal.

My experience is that other platform-specific issues cause larger problems than the memory model. File system or TCP/IP intricacies, or (heaven forbid!) native code used through JNI. And on Itanium specifically, we have had more problems with bugs in the OS (both Windows and Linux) than with user apps making incorrect assumptions about the JMM.

-- Henrik
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