ZFS committed to the FreeBSD base.

Oliver Fromme olli at lurza.secnetix.de
Fri Apr 13 14:53:04 UTC 2007

Bruce Evans wrote:
 > Not for cmpxchg8b, at least.  It is a remarkably slow instruction.  On
 > AthlonXP's it has an execution latency of 39 cycles.  cmpxchg only has an
 > cmpxchg only has an execution latency of 6 cycles (both without a lock
 > prefix).  I don't know how to avoid using cmpxchg8b short of using a
 > mutex lock/unlock pair and slightly different semantics, or a generation
 > count and very different semantics, but without lock prefixes the
 > mutex pair would be much faster than the cmpxchg8b.

Using cmpxchg8b with a lock prefix wouldn't be a good idea
anyway.  If I remember correctly, the lock cmpxchg8b
combination was the cause of the infamous "F00F" bug of
old Pentium processors.  It causes them to freeze.
(FreeBSD has a hack to work around the problem, as you
certainly know ...  I don't know exactly how it works.)

Best regards

Oliver Fromme, secnetix GmbH & Co. KG, Marktplatz 29, 85567 Grafing b. M.
Handelsregister: Registergericht Muenchen, HRA 74606,  Geschäftsfuehrung:
secnetix Verwaltungsgesellsch. mbH, Handelsregister: Registergericht Mün-
chen, HRB 125758,  Geschäftsführer: Maik Bachmann, Olaf Erb, Ralf Gebhart

FreeBSD-Dienstleistungen, -Produkte und mehr:  http://www.secnetix.de/bsd

It's trivial to make fun of Microsoft products,
but it takes a real man to make them work,
and a God to make them do anything useful.

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