non-blocking I/O

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http://squid.nlanr.net/Squid/ says
 
Squid offers high performance proxy caching for Web clients, It
supports FTP, Gopher, and HTTP requests. The cache
software, available only in source, is relatively fast because it
never needs to fork, is implemented with non-blocking I/O,
keeps meta data and hot objects in VM, and caches DNS
lookups. Squid caches can be arranged hierarchically for an
improvement in response times and a reduction in bandwith
usage. Squid runs on all popular Unix platforms.
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From the above, What's non-blocking I/O mean?
Received on Wed Apr 07 1999 - 23:38:55 MDT