25 Nov 2001 apm   » (Journeyer)

Something that's been on my todo list for Suneido for a long time is cleaning up a few places in the client-server network code where there might be bad interaction with the TCP/IP Nagle algorithm. However, I'd never actually done any testing to see if it was a real problem.

I won't try to explain the Nagle algorithm in detail. I'd recommend "Effective TCP/IP Programmer" by Snader if you're interested. But in short, it's a standard technique used to improve the efficiency of TCP/IP. However, it assumes you alternate between send's and receive's, e.g. send a request, receive a response. If you don't follow this pattern, for example, you do two send's e.g. a header and then data, then the Nagle algorithm can slow you down to 200ms per send/receive, or only 5 "messages" per second - pretty slow. One "fix" is to simply disable Nagle, but then you lose the improvements it brings.

So, last night I fixed the code to combine multiple send's and this morning I did some quick benchmarks. On the client side, the places affected were output, update, and delete. Sure enough, with the old code I was only getting 5 outputs per second (ouch!) With the new code I'm getting 2500 outputs per second, or 500 times faster! Not bad for a few hours work! (NOTE: This only affects client-server operation across the network, not standalone use. Standalone I get about 8000 outputs per second.)

It just goes to show that knowledge is a powerful tool. If I hadn't read about this problem it could have been a long time before I figured it out. Of course, if I had any brains, I'd have made this change a long time ago!

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