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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