Bruce Allen wrote: > Hi Auke, > >>>>> 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... > > Based on the discussion in this thread, I am inclined to believe that > lack of PCI-e bus bandwidth is NOT the issue. The theory is that the > extra packet handling associated with TCP acknowledgements are pushing > the PCI-e x1 bus past its limits. However the evidence seems to show > otherwise: > > (1) Bill Fink has reported the same problem on a NIC with a 133 MHz > 64-bit PCI connection. That connection can transfer data at 8Gb/s.
That was even a PCI-X connection, which is known to have extremely good latency numbers, IIRC better than PCI-e? (?) which could account for a lot of the latency-induced lower performance... also, 82573's are _not_ a serverpart and were not designed for this usage. 82546's are and that really does make a difference. 82573's are full of power savings features and all that does make a difference even with some of them turned off. It's not for nothing that these 82573's are used in a ton of laptops like from toshiba, lenovo etc.... A lot of this has to do with the cards internal clock timings as usual. So, you'd really have to compare the 82546 to a 82571 card to be fair. You get what you pay for so to speak. > (2) If the theory is right, then doubling the MTU from 1500 to 3000 > should have significantly reduce the problem, since it drops the number > of ACK's by two. Similarly, going from MTU 1500 to MTU 9000 should > reduce the number of ACK's by a factor of six, practically eliminating > the problem. But changing the MTU size does not help. > > (3) The interrupt counts are quite reasonable. Broadcom NICs without > interrupt aggregation generate an order of magnitude more irq/s and this > doesn't prevent wire speed performance there. > > (4) The CPUs on the system are largely idle. There are plenty of > computing resources available. > > (5) I don't think that the overhead will increase the bandwidth needed > by more than a factor of two. Of course you and the other e1000 > developers are the experts, but the dominant bus cost should be copying > data buffers across the bus. Everything else in minimal in comparison. > > Intel insiders: isn't there some simple instrumentation available (which > read registers or statistics counters on the PCI-e interface chip) to > tell us statistics such as how many bits have moved over the link in > each direction? This plus some accurate timing would make it easy to see > if the TCP case is saturating the PCI-e bus. Then the theory addressed > with data rather than with opinions. the only tools we have are expensive bus analyzers. As said in the thread with Rick Jones, I think there might be some tools avaialable from Intel for this but I have never seen these. 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
