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Re: >1024 connections in slapd / select->poll
On Wed, Nov 17, 2004 at 01:33:52AM -0800, Howard Chu wrote:You don't issue multiple accept()s at once. (Although Unix semantics
shouldn't present any problem there.)
Descriptor-passing per se isn't necessary as long as you have descriptor
inheritance and shared memory. In that case you only need to provide one
master process performing select()s on the listener sockets, and a pool
of child processes inheriting those listeners. The child processes
rotate through performing accept()s so that the live session sockets are
only present in the children. You can spawn as many of these child
processes as necessary to keep the total number of active descriptors in
each process down to a manageable limit, using shared memory to
coordinate the rest of the program.
Okay, I did not know that you can issue multiple accept()s on a single
How does the system guarantee at least close to even loadYou can leave it random, or you can explicitly manage the child
processes. There are any number of ways - the parent/master can just
cycle thru the children in round-robin fashion, that's probably the
easiest. This is standard IPC stuff, nothing exotic.
1) the master creates listener sockets for all the network ports, and
creates a pipe for communicating to its children.
2) It forks as many children as desired, at any time during the life
of the master. Each child thus has a copy of all of the listeners and
3) The master performs a select for read on the listeners, waiting
however long. When a new connection arrives,
4) (optional) the master performs a select for write on the pipe with
a zero timeout. If it indicates a block, all the current children are
busy, fork another child.
5) the master writes a byte into the pipe indicating which listener is
6) loop back to 3
In each child process:
1) select for read on the pipe and any active connections
2) do a non-blocking read on the pipe. if a byte is read from the
pipe, perform an accept on the corresponding listener descriptor
3) for any active connections - do whatever protocol processing is
4) loop back to 1.
A scheme like the above will scale to millions of connections if you
really want to, and if you have a big enough multiprocessor machine to
handle it. Any child process that has more active connections will
automatically lower its own probability of getting the next new
connection by virtue of being slower to get from step 3 back to step 1,
thus balancing the load.
-- Howard Chu
Chief Architect, Symas Corp. Director, Highland Sun
Symas: Premier OpenSource Development and Support