Date: Sun, 13 Jan 2013 00:15:13 -0600 (CST) From: Bryan Venteicher <bryanv@daemoninthecloset.org> To: John Baldwin <jhb@freebsd.org> Cc: freebsd-net@freebsd.org, Peter Jeremy <peter@rulingia.com> Subject: Re: To SMP or not to SMP Message-ID: <330287752.17.1358057713463.JavaMail.root@daemoninthecloset.org> In-Reply-To: <201301111039.17673.jhb@freebsd.org>
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----- Original Message ----- > From: "John Baldwin" <jhb@freebsd.org> > To: freebsd-net@freebsd.org > Cc: "Barney Cordoba" <barney_cordoba@yahoo.com>, "Peter Jeremy" <peter@rulingia.com> > Sent: Friday, January 11, 2013 9:39:17 AM > Subject: Re: To SMP or not to SMP > > 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. > It would be nice to have the LOCK variants available even on UP kernels in non-hackish way. For VirtIO, we need to handle an guest UP kernel running on an SMP host. Whether this is an #define that forces the SMP atomics to be inlined, or if they're exposed with an _smp suffix. VirtIO currently uses mb() to enforce ordering. I have a patch to change to use atomic(9), but can only do so when VirtIO is included in the an SMP kernel (among other constraints - must have 16-bit atomic operations too). (FreeBSD's VirtIO is x86 only for now - but that will be changing soon; I haven't looked if other arch's atomic(9) behave differently for UP/SMP.) > -- > John Baldwin > _______________________________________________ > freebsd-net@freebsd.org mailing list > http://lists.freebsd.org/mailman/listinfo/freebsd-net > To unsubscribe, send any mail to > "freebsd-net-unsubscribe@freebsd.org" >
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