Date: Fri, 11 Jan 2013 10:39:17 -0500 From: John Baldwin <jhb@freebsd.org> To: freebsd-net@freebsd.org Cc: Barney Cordoba <barney_cordoba@yahoo.com>, Peter Jeremy <peter@rulingia.com> Subject: Re: To SMP or not to SMP Message-ID: <201301111039.17673.jhb@freebsd.org> In-Reply-To: <20130110193659.GA27156@server.rulingia.com> References: <1357611958.66651.YahooMailClassic@web121603.mail.ne1.yahoo.com> <20130110193659.GA27156@server.rulingia.com>
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On Thursday, January 10, 2013 02:36:59 PM Peter Jeremy wrote: > On 2013-Jan-07 18:25:58 -0800, Barney Cordoba <barney_cordoba@yahoo.com> wrote: > >I have a situation where I have to run 9.1 on an old single core > >box. Does anyone have a handle on whether it's better to build a non > >SMP kernel or to just use a standard SMP build with just the one > >core? > > Another input for this decision is kern/173322. Currently on x86, > atomic operations within kernel modules are implemented using calls > to code in the kernel, which do or don't use lock prefixes depending > on whethur the kernel was built as SMP. My proposed change changes > kernel modules to inline atomic operations but always include lock > prefixes (effectively reverting r49999). I'm appreciate anyone who > feels like testing the impact of this change. Presumably a locked atomic op is cheaper than a function call then? The current setup assumes the opposite. I think we should actually do this for atomics in modules on x86: 1) If a module is built standalone, it should do whichever is cheaper: a function call or always use "LOCK". 2) If a module is built as part of the kernel build, it should use inlined atomics that match what the kernel does. Thus, modules built with a non-SMP kernel would use inlined atomic ops that do not use LOCK. We have a way to detect this now (some HAVE_FOO #define added in the past few years) that we didn't back when this bit of atomic.h was written. -- John Baldwin
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