Author :
Nick Mathewson
Date :
2010-10-24 11:51:14
Hash :2c66983a Message :Simplify the logic for choosing EPOLL_CTL_ADD vs EPOLL_CTL_MOD
Previously, we chose "ADD" whenever old_events==new_events, (since
we expected the add to fail with EEXIST), or whenever old_events
was==0, and MOD otherwise (i.e., when old_events was nonzero and not
equal to new_events).
But now that we retry failed MOD events as ADD *and* failed ADD
events as MOD, the important thing is now to try to guess right the
largest amount of the time, since guessing right means we do only
one syscall, but guessing wrong means we do two.
When old_events is 0, ADD is probably right (unless we're hitting
the dup bug, when we'll fall back).
And when old_events is set and != new_events, MOD is almost
certainly right for the same reasons as before.
But when old_events is equal to new events, then MOD will work fine
unless we closed and reopened the fd, in which case we'll have to
fall back to the ADD case. (Redundant del/add pairs are more common
than closes for most use cases.)
This change lets us avoid calculating new_events, which ought to
save a little time in epoll.c
Nick Mathewson