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From Brian Pane <bri...@apache.org>
Subject Re: [design] work around new apr_poll leakage?
Date Fri, 19 Jul 2002 06:28:15 GMT
William A. Rowe, Jr. wrote:

> At 11:21 PM 7/18/2002, Brian Pane wrote:
>
>> I'm almost in complete agreement with this proposal.  We
>> need a wrapper object to hold the temp pollset so that we
>> can re-use it for all subsequent poll calls on the same
>> fd set.
>
>
> Notice that apr_pollfd_internal_t *int; below _IS_ entirely
> opaque...  [and using a reserved keyword, the member name
> REALLY sucked... how about simply *private.] 


Right, but the pointer to the vector of apr_pollfd_t
objects is transparent.  And that's the one that will
cause problems if we try to use something like /dev/poll,
because the set of descriptors in that vector can get
out of synch with the descriptors that have been registered
with /dev/poll.

(Yes, we could avoid that problem by including within
the apr_pollfd_internal_t a list of the descriptors that
we used on the last poll call for this apr_pollfd_set_t,
which would make it possible to detect added/removed
descriptors since the last call.  But that would require
an O(n)-time scan through the entire vector on each request,
which defeats the scalability advantages of using a
faster poll interface.)

Speaking of linear-time code, I just checked the current
apr_poll(), and it's doing an O(n)-time loop to copy from
the apr_pollfd_t structs to the pollfd structs.  That's
a major scalability problem for larger values of n.

Fortunately, your new design offers a way to fix the O(n)
problem: build the pollfd vector once, and re-use it on
subsequent calls if the apr_pollfd_t vector hasn't changed.
(The transparency of the apr_pollfd_t vector and apr_pollfd_set_t
makes this a little tricky, though.)

--Brian



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