| From: | Andres Freund <andres(at)anarazel(dot)de> |
|---|---|
| To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
| Cc: | Markus Wanner <markus(at)bluegap(dot)ch>, Aidan Van Dyk <aidan(at)highrise(dot)ca>, Robert Haas <robertmhaas(at)gmail(dot)com>, pgsql-hackers(at)postgresql(dot)org, Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>, Bruce Momjian <bruce(at)momjian(dot)us>, Josh Berkus <josh(at)agliodbs(dot)com> |
| Subject: | Re: Latches with weak memory ordering (Re: max_wal_senders must die) |
| Date: | 2010-11-19 16:18:58 |
| Message-ID: | 201011191718.59085.andres@anarazel.de |
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| Lists: | pgsql-hackers |
On Friday 19 November 2010 16:51:00 Tom Lane wrote:
> Markus Wanner <markus(at)bluegap(dot)ch> writes:
> > Well, that certainly doesn't apply to full fences, that are not specific
> > to a particular piece of memory. I'm thinking of 'mfence' on x86_64 or
> > 'mf' on ia64.
> Hm, what do those do exactly? We've never had any such thing in the
> Intel-ish spinlock asm, but if out-of-order writes are possible I should
> think we'd need 'em. Or does "lock xchgb" imply an mfence?
Out of order writes are definitely possible if you consider multiple
processors.
Locked statments like 'lock xaddl;' guarantee that the specific operands (or
their cachelines) are visible on all processors and are done atomically - but
its not influencing the whole cache like mfence would.
Andres
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