Slashdot Mirror
Is There a Place for a $500 Ethernet Card?
prostoalex writes "ComputerWorld magazine runs a story on Level 5 Networks, which emerged from the stealth startup status with its own brand of network cards and software called EtherFabric. The company claims they are reducing the load on the servers CPUs and improve the communications between the servers. And it's not vaporware: 'The EtherFabric software shipping Monday runs on the Linux kernel 2.4 and 2.6, with support for Windows and Unix coming in the first half of next year. High volume pricing is $295 for a two-port, 1GB-per-port EtherFabric network interface card and software, while low volume quantities start from $495.'"
Plain old 32-bit PCI has a bandwidth of 32 bits * 33 MHz = 1.056 Gbps. A 64-bit bus would obviously be twice that. Unless there is a lot of other traffic on the PCI bus, I suspect the limitation is the driver, not the bus.
Peripherals like that built into the motherboard are generally on a PCI bus segment anyway. You can see by looking at the device manager in Windows or by using lspci in Linux. In both cases you will see a vendor ID, bus number, and slot number.
With newer Linux-kernels this is quite simply not the case.
To avoid the torrent of interupts from a fast nic the Linux-kernel detects that the card gets packets so often that essentially there's "always" one or more packets waiting in the cards buffers.
It responds to this condition by disabling interupts for the card in question and switch to polling it regularily.
Normally polling is inefficient, because it amounts to asking over and over again "got anything for me now?", where in most situations the answer is "no" 99.99% of the time, which makes it a waste of resources to ask in the first place.
This changes when the answer is "yes" basically all the time. There's no need for the network card to tell the cpu over and over and over "I got a packet for you", instead the cpu collects packets regularily.
It's sorta like having your lawyer receive legal letters for you.
If you get very few, it'd be a waste for you to drop by him every day and ask if he's gotten any for you (polling) most of the days you'd be making the trip for naught. In this situation interupts (i.e. having your lawyer call you and inform you on the rare occasions when a letter *does* arrive) makes more sense.
But if your letter-flow increases to the point where there's normally 3-5 letters every day and it's rare that no letter arrives, then it no longer makes sense that the lawyer calls you every day to tell you you got letters (interupts), you already assumed that. In this scenario it makes more sense for you to come by regularily and pick up letters without being prompted to do so. (polling)
Thus, the flow of interupts from a Gb-nic being flooded with 100byte small-packets (say a loaded dns-server) is not 1 million interupts every second -- it is zero interupts.
(allthough what you write is correct for kernels older than 2.6.10 and for less clever OSes.)