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    SubjectRe: Linux 2.4.1 network (socket) performance
    Hi!

    > Hello, I am trying to find the reason for very, very poor network
    > performance with sustained data transfers on Linux 2.4.1. I found
    > a work-around, but don't think user-mode code should have to provide
    > such work-arounds.
    >
    > In the following, with Linux 2.4.1, on a dedicated 100/Base
    > link:
    >
    > s = socket connected to DISCARD (null-sink) server.
    >
    > while(len)
    > {
    > stat = write(s, buf, min(len, MTU));
    > /* Yes, I do check for an error */
    > buf += stat;
    > len -= stat;
    > }
    >
    > Data length is 0x00010000 bytes.
    >
    > MTU Average trans rate Fastest trans rate
    > ---- ----------------- -----------------
    > 65536 0.468 Mb/s 0.902 Mb/s
    > 32768 0.684 Mb/s 0.813 Mb/s
    > 16384 2.989 Mb/s 3.121 Mb/s
    > 8192 5.211 Mb/s 6.160 Mb/s
    > 4094 8.212 Mb/s 9.101 Mb/s
    > 2048 8.561 Mb/s 9.280 Mb/s
    > 1024 7.250 Mb/s 7.500 Mb/s
    > 512 4.818 Mb/s 5.107 Mb/s
    >
    > As you can see, there is a maximum data length that can be
    > handled with reasonable speed from a socket. Trying to find
    > out what that was, I discovered that the best MTU was 3924.
    > I don't know why. It shows:

    Looks like that's page_size - epsilon.

    > MTU Average trans rate Fastest trans rate
    > ---- ----------------- -----------------
    > 3924 8.920 Mb/s 9.31 Mb/s

    But even this is *not* reasonable speed for 100MBit ethernet!

    > If the user's data length is higher than this, there is a 1/100th
    > of a second wait between packets. The larger the user's data length,
    > the more the data gets chopped up into 1/100th of a second intervals.
    >
    > It looks as though user data that can't fit into two Ethernet packets
    > is queued until the next time-slice on a 100 Hz system. This severely
    > hurts sustained data performance. The performance with a single
    > 64k data buffer is abysmal. If it gets chopped up into 2048 byte
    > blocks in user-space, it's reasonable.
    >
    > Both machines are Dual Pentium 600 MHz machines with identical eepro100
    > Ethernet boards. I substituted, LANCE (Hewlett Packard), and 3COM boards
    > (3c59x) with essentially no change.

    Strange. Do you have interrupts working okay? [I'm able to get 4Mbit with
    ne2000 hooked on timer IRQ, so this is not totally stupid Question.]

    > Does this point out a problem? Or should user-mode code be required

    Definitely problem.

    > to chop up data lengths to something more "reasonable" for the kernel?
    > If so, how does the user know what "reasonable" is?

    --
    Philips Velo 1: 1"x4"x8", 300gram, 60, 12MB, 40bogomips, linux, mutt,
    details at http://atrey.karlin.mff.cuni.cz/~pavel/velo/index.html.

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