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    <p>The main cost (for typical TCP/IP stack implementations) is in transmit and receive buffers living in kernel memory -- the recommended size in <a href="http://datatag.web.cern.ch/datatag/howto/tcp.html" rel="nofollow noreferrer">this</a> good (if maybe dated) short essay is <code>socket buffer size = 2 * bandwidth * delay</code>. So, if the delay is 100 ms (ping time, a round trip of two delays, would then be 200 ms), and the bandwidth is about a gigabit/second (call it 100 MB per second for ease of computation;-), to get top performance you need a socket buffer size of about 20 MB per socket (I've heard of no kernel configures so generously by default, but you can control the buffer size per-socket just before you open the socket...).</p> <p>A typical socket buffer size might be, say, 256 KB; with the same hypothetical 100 ms delay, that should be fine for a bandwidth of up to 5 megabit/second -- i.e., with that buffer size and delay, you're not going to be fully exploiting the bandwidth of even a moderate quality broadband connection ("long narrow pipes" -- large bandwidth <em>and</em> large delay, so their product is <em>quite</em> large -- are notoriously difficult to exploit fully). Anyway, with that buffer size, you'd be eating up 256 MB of kernel memory by keeping 1000 TCP sockets open and connected (albeit inactive).</p> <p>I don't think (at least on a 64-bit kernel) there are other intrinsic limits besides the amount of RAM allocated to the kernel (where the buffers live). Of course you can configure your kernel to limit that number (to avoid all of kernel memory going to socket buffers;-).</p>
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    1. POkeeping 1000 tcp connections open inspite of very few(10/20) actual communications
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    1. USAlex Martelli
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    1. POkeeping 1000 tcp connections open inspite of very few(10/20) actual communications
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    1. COthx for the detail. so its memory.. eh? is kernel memory swappable? coz in my case it'll help a lot if the socket buffers of unused conxns are swapped out until they get a signal or something from the other end. i am not sure if the kernel is designed so.
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    2. CO@Seeker, AFAIK, kernel memory is not swappable (but maybe there's some experimental kernel you can try that changes that?) since when it's needed it's needed in near-real-time -- and a hard page fault can easily mean that an unlucky memory access could take 40,000 times longer than usual.
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