Re: IP over FireWire Performance
Re: IP over FireWire Performance
- Subject: Re: IP over FireWire Performance
- From: Wade Tregaskis <email@hidden>
- Date: Fri, 6 Jun 2003 15:15:31 +1000
Any ideas as to why the FireWire throughput isn't higher? I've read
that it "can reserve up to 80% ... for one or more isochronous
channels." Does this mean I should never expect better than 640Mbps?
Any ideas why the FireWire receiver would consume so much more cpu in
the kernel_task as compared to Ethernet? May it be caused by
differences in interrupt handling or DMA support? I would expect
Ethernet and FireWire to perform similarly in hardware considering
both PHYs connect to the U2 controller.
I'm not a driver or Firewire expert, so this is only my [slightly]
educated guess, but I'd say there are two primary problems here:
a) The firewire controller doesn't have all the extra capabilities of
the ethernet controller, so there's probably quite a few things being
done in software 'emulation', if you like. Plus, it's probably tuned
for moving a hand full of very large blocks, not thousands of tiny ones*
b) The drivers simply aren't as tuned as those for the ethernet device.
Remember that most of Darwin's ethernet code is probably derived
almost entirely from well-tested, well-optimised code in the various
BSD's. Apple have, from what I can gather, written the majority of
their IP over Firewire driver from scratch. Probably borrowing from
the ethernet interface code, certainly, but there will undoubtedly be a
lot more the IP over Firewire driver has to do (like emulating certain
things normally done in hardware, as mentioned, for example).
* = Why the heck does TCP still use 1500 MTU's? Shouldn't the packet
size have scaled along with the bandwidth? Surely the original packet
size was chosen to provide best performance on the first networks...
why should it not be changed as appropriate for newer networks?
Wade Tregaskis
-- Sed quis custodiet ipsos custodes?
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