Date: Mon, 14 Jan 2013 16:57:58 -0500 From: John Baldwin <jhb@freebsd.org> To: freebsd-net@freebsd.org Cc: Konstantin Belousov <kostikbel@gmail.com>, Peter Jeremy <peter@rulingia.com>, Bryan Venteicher <bryanv@daemoninthecloset.org> Subject: Re: To SMP or not to SMP Message-ID: <201301141657.58727.jhb@freebsd.org> In-Reply-To: <20130114210756.GM2561@kib.kiev.ua> References: <330287752.17.1358057713463.JavaMail.root@daemoninthecloset.org> <201301141507.50250.jhb@freebsd.org> <20130114210756.GM2561@kib.kiev.ua>
next in thread | previous in thread | raw e-mail | index | archive | help
On Monday, January 14, 2013 4:07:56 pm Konstantin Belousov wrote: > 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: > > > > > > ----- 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. > Could you please, clarify why does UP kernel needs it ? > Shouldn't the hypervisor context switching provide neccessary serialization > anyway ? I thought this, too, but in the case of virtio you are presumably sychronizing with other threads in the hypervisor itself which might be running concurrently on another physical CPU. -- John Baldwin
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?201301141657.58727.jhb>