Bruce Allen wrote: > Hi Jesse, > >>> It's good to be talking directly to one of the e1000 developers and >>> maintainers. Although at this point I am starting to think that the >>> issue may be TCP stack related and nothing to do with the NIC. Am I >>> correct that these are quite distinct parts of the kernel? >> >> Yes, quite. > > OK. I hope that there is also someone knowledgable about the TCP stack > who is following this thread. (Perhaps you also know this part of the > kernel, but I am assuming that your expertise is on the e1000/NIC bits.) > >>> Important note: we ARE able to get full duplex wire speed (over 900 >>> Mb/s simulaneously in both directions) using UDP. The problems occur >>> only with TCP connections. >> >> That eliminates bus bandwidth issues, probably, but small packets take >> up a lot of extra descriptors, bus bandwidth, CPU, and cache resources. > > I see. Your concern is the extra ACK packets associated with TCP. Even > those these represent a small volume of data (around 5% with MTU=1500, > and less at larger MTU) they double the number of packets that must be > handled by the system compared to UDP transmission at the same data > rate. Is that correct?
A lot of people tend to forget that the pci-express bus has enough bandwidth on first glance - 2.5gbit/sec for 1gbit of traffix, but apart from data going over it there is significant overhead going on: each packet requires transmit, cleanup and buffer transactions, and there are many irq register clears per second (slow ioread/writes). The transactions double for TCP ack processing, and this all accumulates and starts to introduce latency, higher cpu utilization etc... Auke -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html
