Date: Fri, 21 Oct 2005 10:40:28 +0400 From: Gleb Smirnoff <glebius@FreeBSD.org> To: Yar Tikhiy <yar@comp.chem.msu.su> Cc: freebsd-net@FreeBSD.org, ru@FreeBSD.org, Andrew Thompson <thompsa@FreeBSD.org> Subject: Re: vlan patch Message-ID: <20051021064028.GZ59364@cell.sick.ru> In-Reply-To: <20051021060654.GC69814@comp.chem.msu.su> References: <20051019102559.GA45909@heff.fud.org.nz> <20051020070054.GZ59364@cell.sick.ru> <20051020085721.GC27114@comp.chem.msu.su> <20051021053033.GW59364@cell.sick.ru> <20051021060654.GC69814@comp.chem.msu.su>
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On Fri, Oct 21, 2005 at 10:06:55AM +0400, Yar Tikhiy wrote: Y> On Fri, Oct 21, 2005 at 09:30:33AM +0400, Gleb Smirnoff wrote: Y> > On Thu, Oct 20, 2005 at 12:57:21PM +0400, Yar Tikhiy wrote: Y> > Y> The hash code consists of literally a couple of #define's. And the Y> > Y> difference between ng_vlan(4) and vlan(4) is that each ng_vlan node Y> > Y> gets its own instance of the hash table. OTOH, in vlan(4) we need Y> > Y> to decide if the hash table will be per parent interface or a single Y> > Y> global instance. In the latter case we could hash by a combination Y> > Y> of the VLAN tag and parent's ifindex. Perhaps this approach will Y> > Y> yield more CPU cache hits during hash table lookups. In addition, Y> > Y> it will be thriftier in using memory. Locking the global hash table Y> > Y> should not be an issue as we can use an sx lock in this case for Y> > Y> optimal read access. Y> > Y> > The sx lock is slow. We'd better use per interface hash, and thus Y> > get locking instantly, with per-vlan lock. In other case, we will Y> > acquire per-vlan lock + the sx lock on every packet. The sx lock Y> > actually means mtx_lock+mtx_unlock, thus we will make 3 mutex Y> > operations instead of one. Y> Y> OK, let's forget about sx locks. However, a per-interface hash is Y> associated with a _physical_ interface, hence we must find the vlan Y> to lock using the hash first. If there were a physical interface Y> lock held by its driver in each case, it could protect the hash as Y> well. Can we rely on this? Oops. When speaking about per-vlan lock, I really meant per-trunk lock. Or are you going to implement per-vlan lock? Is this going to be a benefit? Since all packets on trunk are serialized by NIC driver, can there be any benefit in creating a mutex per vlan interface, not per vlan trunk? -- Totus tuus, Glebius. GLEBIUS-RIPN GLEB-RIPE
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