Date: Tue, 29 Jun 1999 10:50:31 -0700 From: Erich Boleyn <erich@uruk.org> To: Kevin Van maren <vanmaren@cs.utah.edu> Cc: freebsd-smp@FreeBSD.ORG Subject: Re: high-efficiency SMP locks - submission for review Message-ID: <E10z21f-0002AZ-00@uruk.org> In-Reply-To: Your message of "Tue, 29 Jun 1999 11:05:02 MDT." <199906291705.LAA20627@zane.cs.utah.edu>
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Kevin Van maren <vanmaren@cs.utah.edu> wrote:
> I'm really glad to see that there is so much activity on the list!
Yeah... I'm tempted to de-lurk. ;)
...
> Terry Lambert said:
> > No one is currently bothering with anything but the Intel MESI
> > coherency model for SMP, anyway, so I don't understand the
> > relevence of bus coherency to the argument.
>
> This is mostly true. Even on the IA64. Section 4.4.6.2 of the manual
> says that I-caches are not coherent with other I-caches or D-caches.
> But at least the D-caches are coherent (on my first glance, I thought
> they weren't either, which really worried me!)
I- and D-caches are coherent on IA32. The instruction stream itself
is guaranteed to be synchronized after jumps.
Most other processor architectures (including IA64) require the use of
special instructions to synchronize the I- and D-sides.
> On the x86, you do need to lock the bus to guarantee operations
> are atomic, with the exception of xchg (but not the variants),
> which is guaranteed to be atomic. They also must be naturally-aligned.
This is true for read/modify/write operations, but individual reads and
writes are guaranteed to be atomic on IA32 as long as they are naturally
aligned. (at least, for existing implementations)
> To clarify: there DO exist some dual-processor 486 systems. They
> use APICs, and in *theory* can run FreeBSD without too much difficulty
> (there is no mptable, the processor APICs are at different addresses,
> so you have to know which processor you are on to access the APIC,
> and the AP initialization is a little different). I don't think
> anyone cares enough to implement the support code, and Intel discontinued
> the parts necessary to build them, so it probably won't be too painful
> to break possible support for them.
I'd be really surprised if anyone wanted to support MP 486 systems in
FreeBSD as anything but a hobby. They were going out of style 3-4 years
ago, and were expensive systems with special OS support.
--
Erich Stefan Boleyn \_ <erich@uruk.org>
Mad but Happy Scientist \__ http://www.uruk.org/
Motto: "I'll live forever or die trying" ---------------------------
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