Date: Wed, 2 Jan 2002 15:23:45 -0800 (PST) From: Matthew Dillon <dillon@apollo.backplane.com> To: Peter Jeremy <peter.jeremy@alcatel.com.au> Cc: Michal Mertl <mime@traveller.cz>, Bruce Evans <bde@zeta.org.au>, Mike Smith <msmith@FreeBSD.ORG>, Bernd Walter <ticso@cicely8.cicely.de>, arch@FreeBSD.ORG Subject: Re: When to use atomic_ functions? (was: 64 bit counters) Message-ID: <200201022323.g02NNjt60197@apollo.backplane.com> References: <200201012349.g01NnKA40071@apollo.backplane.com> <Pine.BSF.4.41.0201021003580.18429-100000@prg.traveller.cz> <20020103095701.B561@gsmx07.alcatel.com.au>
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:This depends on how it is implemented. Obviously
: int counter[NCPUS];
:will be just as expensive as performing atomic operations, but no-one
:in their right mind would do that. One approach is to aggregate all
:the per-CPU counters into a single region of KVM and arrange for that
:KVM to be mapped to different physical memory for each CPU. (Solaris
:does or did this). This means that the code to update the counter
:doesn't need to know whether a counter is per-CPU or not.
:
:The code to read the counters _does_ need to know that the counters
:are per-CPU and have to sum all the individual counters - which is
:more expensive than a straight read, but is normally far less frequent.
:
:Peter
Something like galloc()/gfree().
offset = galloc(bytes); /* allocate space in all cpu's per-cpu struct*/
gfree(offset, bytes); /* return previously reserved space */
And then macros to read and write it.
global_int(offset) /* returns address of global int @ offset */
global_quad(offset) /* returns address of global int @ offset */
e.g.
++*global_quad(ifc->counter_off);
Which GCC ought to be able to optimize fairly easily.
This isn't a recommendation, just one way we could do it.
-Matt
Matthew Dillon
<dillon@backplane.com>
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