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Re: iMAC I7 Jumbo Frames Finaly Enabled

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Steve ModicaRe: iMAC I7 Jumbo Frames Finaly Enabled
by on May 18, 2010 at 7:19:29 pm

Sorry to be late in responding. I think there's a lot of confusion over the issue of jumbo frames, segmentation offload, and the new i7 performance.

The new imac (and the new Mac Book Pros) use the Broadcom 5764 chipset and it does not support jumbo frames. It supports a feature called TSO (transmit segmentation offload) that is meant to provide jumbo frame performance without the requirement of switch and partner support. Large packets get handed to the card, and it chops them up and applies headers. This saves the OS the trouble. That's all good and true enough.

On the receive side, you still receive 1500 byte packets. The receiver has to strip the headers and reassemble. This takes a lot of CPU. It generates more interrupts and more context switches. It causes more work for the server. One solution to this is receive side coalescing, but no gigabit chips today support that. (The Intel 82599 10Gb chip on our newest 10Gb card does). So for the present, when an imac is reading in video, there's 0 jumbo frame "help" so to speak. It's full on tiny packets.

As for performance, I don't have a good quantitative answer for what you lose using an i7 vs an older core 2 duo. Walter made a qualitative judgment that the older system does a better job and doesn't drop. He has some fairly complex timelines. In house, I find my older core 2 duo laptop was about on par with the new imac. I would not recommend either for serious editing. (When I say serious, I mean at least 2 active Pro Res HQ streams coming over the wire at 60MB/sec. If you are doing DV editing or something lower bandwidth than pro res, I'm sure the imac is fine)

The USB dongle sited here was tested by Bob Z and it didn't support jumbo frames. Since USB is limited to 480MBits, it's also clear it won't get us to 2 Pro Res HQ streams. So it was a non-starter for me. Even if it did jumbo frames, I would not expect to see more than 60MB/sec over the wire and with overhead, that won't guarantee two Pro Res HQ streams (although in practical use, it might be OK for short things).

So all this being said, I think if you are a pro res editor, and each stream is a reasonably large fraction of your gigabit bandwidth, you need a mac pro. If you are doing something else, imac, mac book pro, mac book, mac mini are all potential candidates for you. Just be fore-warned that they won't compare to the capability of the larger system.

One last note, I prefer it when we can get one of our gigabit cards in the client. We have tools that let us dump stats and see the state of the chips. Apple doesn't have anything like that, so when something hangs, it's very hard to debug. We've had problems like this recently. With our PCIE cards, I can dump all sorts of good stuff to see why it hung.



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