Date: Wed, 02 Jan 2002 15:29:20 -0800 (PST) From: John Baldwin <jhb@FreeBSD.org> To: Matthew Dillon <dillon@apollo.backplane.com> Cc: arch@FreeBSD.ORG, Bernd Walter <ticso@cicely8.cicely.de>, Mike Smith <msmith@FreeBSD.ORG>, Bruce Evans <bde@zeta.org.au>, Michal Mertl <mime@traveller.cz>, Peter Jeremy <peter.jeremy@alcatel.com.au> Subject: Re: When to use atomic_ functions? (was: 64 bit counters) Message-ID: <XFMail.020102152920.jhb@FreeBSD.org> In-Reply-To: <200201022323.g02NNjt60197@apollo.backplane.com>
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On 02-Jan-02 Matthew Dillon wrote: > >: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. Look at PCPU_GET/PCPU_SET. Note that since an interrupt can preempt you and push you off onto another CPU, you have to use a critical section while updating per-CPU variables. If desired, some kind of free area could be stuck in struct pcpu (or more likely, struct pcpu would hold a pointer to the area) that could be galloc/gfree'd or some such. -- John Baldwin <jhb@FreeBSD.org> <>< http://www.FreeBSD.org/~jhb/ "Power Users Use the Power to Serve!" - http://www.FreeBSD.org/ To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-arch" in the body of the message
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