Date: Mon, 14 Jan 2013 23:07:56 +0200 From: Konstantin Belousov <kostikbel@gmail.com> To: Bryan Venteicher <bryanv@daemoninthecloset.org> Cc: freebsd-net@freebsd.org, Peter Jeremy <peter@rulingia.com>, John Baldwin <jhb@freebsd.org> Subject: Re: To SMP or not to SMP Message-ID: <20130114210756.GM2561@kib.kiev.ua> In-Reply-To: <201301141507.50250.jhb@freebsd.org> References: <330287752.17.1358057713463.JavaMail.root@daemoninthecloset.org> <201301141507.50250.jhb@freebsd.org>
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--5dzPFofyJ6dbelGf Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Mon, Jan 14, 2013 at 03:07:50PM -0500, John Baldwin wrote: > On Sunday, January 13, 2013 1:15:13 am Bryan Venteicher wrote: > >=20 > > ----- Original Message ----- > > > From: "John Baldwin" <jhb@freebsd.org> > > > To: freebsd-net@freebsd.org > > > Cc: "Barney Cordoba" <barney_cordoba@yahoo.com>, "Peter Jeremy" <pete= r@rulingia.com> > > > Sent: Friday, January 11, 2013 9:39:17 AM > > > Subject: Re: To SMP or not to SMP > > >=20 > > > 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? > > > >=20 > > > > 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. > > >=20 > > > Presumably a locked atomic op is cheaper than a function call then? > > > The > > > current setup assumes the opposite. > > >=20 > > > I think we should actually do this for atomics in modules on x86: > > >=20 > > > 1) If a module is built standalone, it should do whichever is cheaper: > > > a function call or always use "LOCK". > > >=20 > > > 2) If a module is built as part of the kernel build, it should use in= lined > > > 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. > > > > >=20 > > 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.=20 Could you please, clarify why does UP kernel needs it ? Shouldn't the hypervisor context switching provide neccessary serialization anyway ? > >=20 > > 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). > >=20 > > (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.) >=20 > Only x86 does this weirdness. The simplest workaround might be to require > guest kernels to be compiled with SMP for now. >=20 > --=20 > 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" --5dzPFofyJ6dbelGf Content-Type: application/pgp-signature -----BEGIN PGP SIGNATURE----- Version: GnuPG v2.0.19 (FreeBSD) iQIcBAEBAgAGBQJQ9HOsAAoJEJDCuSvBvK1B56gP/0iKE8Dz7UabdPih8xmRPu96 9tVVCaaM7XC4Jr+JnO53WRuajrbj+XUETvNp81SQXxB9+4XJtpVLDyoHFOXKlMTV kvCGYX2mgxuy/oyQaEvSEc0XpPEfxEV4DZ/7q3dtWFumQEhwYXElDMS9E/z+3zgx 5k5JeRX4U5BmratmzNmcEwRedmSuxxh5bFmbmSFkykiWyYU0jLMzMGJWwXVoNEFJ R2Tk1Gr8M/+oHko6IjMyyiwFRr+FgP+wMB3KJfPQaZl6hLfxLWlmwM1hTuyVOeQZ +jWHPwcS+DUMJQuD0oSSc7y3DOeSTCjQZACJI1syBWgi9SsysiF2Hf0KC+/F8FtT L/HCFRPrSlCOpQEtBQHGFy3Ywzz8abpam7cViaJYWPsnxgd6CssDde4waLUZKUa+ iJRwtjVpmTnvtac37illdqkfhAWMNucp7w/BD+2S0O4s4GFUZvhG2wziNwmjEY8Z u2CGcKoYNN7Xi9QWR3x6iZdFEr4dV7FPePT6cHq5PzXoz2AmZgl0aSiw0UZmuffI sR8vrYsy2xfqaYAjpriEymGaPw1UVLoFR9nGjb4IVPE39TC9ip5n1Sv5DvhmtGkB d2DUMzu1rURskZz0S/CC1aHc+rSyt7bwNmJZ0tlp+nW9GIg/WES7qZKNfhLTYqo+ rD2AmuUkRhWmhNbxuWoH =V+NN -----END PGP SIGNATURE----- --5dzPFofyJ6dbelGf--
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