[Date Prev][Date Next]
Re: >1024 connections in slapd / select->poll
On Mon, Nov 15, 2004 at 02:47:34PM -0800, Howard Chu wrote:OK. But most servers where performance limits are being experienced have
a higher proportion of active connections to idle connections. The most
widely used protocols - http, smtp, pop, etc. have extremely short
lists around. But on a heavily loaded server with thousands of active
sessions, the fact is that your list of interesting descriptors is not
static. Ultimately the server must iterate across all the thousands of
descriptors, because the majority of them are probably active, and the
server must insert and delete descriptors from the list continuously,
because client sessions tend to come and go continually.
All true, but the argument I think is that epoll has no inherent limits, and is
optimal for many idle connections.
With thousands of active connections I think
a central select/poll/epoll/whatever is broken anyway.
Only a SMP machine withDescriptor-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.
a descriptor-passing architecture might help here.
But yes, to your main point, a software design that handles tens of
thousands of descriptors in a single process is broken. I think the fact
that epoll/kqueue can scale to tens of thousands of descriptors is
somewhat academic, since you probably can't do anything useful on those
connections in a single process. A typical thread subsystem will let you
use a couple thousand threads, nowhere near enough to service every
descriptor with a low latency.
(At this point I'm just carrying on this conversation out of theoretical
interest. I've already decided to look into migrating to libevent
http://www.monkey.org/~provos/libevent/ to take advantage of these newer
-- Howard Chu
Chief Architect, Symas Corp. Director, Highland Sun
Symas: Premier OpenSource Development and Support