Date: Sun, 4 Jul 1999 00:00:42 -0500 From: Jonathan Lemon <jlemon@americantv.com> To: hackers@freebsd.org Cc: grog@freebie.lemis.com, peter@netplex.com.au Subject: poll() scalability Message-ID: <19990704000042.59954@right.PCS>
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This is an earlier posting that I attempted to make, perhaps
it can provide a starting point for discussion. While this
is already implemented, I'm not adverse to tossing it all for
something better.
--
Jonathan
----- Forwarded message from owner-freebsd-arch@FreeBSD.ORG -----
Date: Mon, 5 Apr 1999 17:42:02 -0500
From: Jonathan Lemon <jlemon@cs.wisc.edu>
To: freebsd-arch@freebsd.org
I'd like to open discussion on adding a new interface to FreeBSD,
specifically, a variant of poll().
The problem is that poll() (and select(), as well) do not scale
well as the number of open file descriptors increases. When there
are a large number of descriptors under consideration, poll() takes
an inordinate amount of time. For the purposes of my particular
application, "large" translates into roughly 40K descriptors.
As having to walk this descriptor list (and pass it between user and
kernel space) is unpalatable, I would like to have the interface
simply take a "change" list instead. The kernel would keep the
state of the descriptors being examined, and would in turn, return
a short list of descriptors that actually had any activity.
In essence, I want to move the large "struct pollfd" array that I
have into the kernel, and then instruct the kernel to add/remove
entries from this array, and only return the array subset which
has activity.
A possible (actually, my current) implementation looks like this:
struct fd_change {
short fd;
short events;
};
int
new_poll(
int nchanges; // entries in new changelist
struct fd_change *changelist; // changes to be made
int n_events; // max size of output list
struct fd_change *event; // returned list of events
int timeout; // timeout (same as poll)
)
Where the returned value is either an error, or the number of events
stored in the returned changelist.
Some pseudo-code that would exercise the interface:
struct fd_change fc[ MAXCHANGE ];
fc[0].fd = 20;
fc[0].events = ADD | READ ; // add, mark read "interest"
fc[1].fd = -1; // ignore this one
fc[2].fd = 32;
fc[2].events = DELETE ; // delete previous fd
fc[3].fd = 46;
fc[3].events = WRITE ; // ask for writable events
n_changes = new_poll(4, fc, MAXCHANGE, fc, -1);
Comments? Note that I haven't discussed the implementation details;
the implementation is done, and can probably be altered/improved,
but I would like to solicit feedback on the feasability of the interface.
--
Jonathan
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