Charles-François Natali added the comment:
> The program does *not* demonstrate starvation because you are servicing the
> resource represented by the "starved" duplicate fds before calling poll()
> again.
No.
What the program does is the following:
while all the write FDs have not been returned by epoll.wait() at least once:
1) make all writer FDs ready by draining the pipe
2) fetch the list of event reported by epoll(), and add them to
the list of seen FDs
3) make all the writers FDs not ready by filling the PIPE
4) fetch the list of event reported by epoll(), and add them to
the list of seen FDs
With the default 'maxevents' parameters, it never completes.
This shows that the FDs returned at step 2 are actually a strict
subset of all the ready FDs, and this forever: some FDs which are
actually ready (they are all ready at step 2) are *never* reported.
This is starvation.
By increasing epoll.wait() maxevents, it completes immediately.
> You are creating thousands of duplicate handles for the same resource and
> then complaining that they do not behave independently!
The fact that that the FDs are duped shouldn't change anything to the
events reported: it works while the number of FDs is less than
FD_SETSIZE (epoll_wait() maxevents argument).
See also epoll() documentation:
"""
Q0 What is the key used to distinguish the file descriptors
registered in an
epoll set?
A0 The key is the combination of the file descriptor number
and the open file
description (also known as an "open file handle", the
kernel's internal
representation of an open file).
Q1 What happens if you register the same file descriptor on an
epoll instance
twice?
A1 You will probably get EEXIST. However, it is possible to
add a duplicate
(dup(2), dup2(2), fcntl(2) F_DUPFD) descriptor to the same
epoll instance.
This can be a useful technique for filtering events, if the
duplicate file
descriptors are registered with different events masks.
"""
I just used dup() to make it easier to test, but you'll probably get
the same thing it your FDs were sockets connected to different
endpoints.
> I tried modifing your program by running poll() in a loop, exiting when no
> more unseen fds are reported as ready. This makes the program exit
> immediately.
>
> So
>
> ready_writers = set(fd for fd, evt in
> ep.poll(-1, MAXEVENTS) if fd != r)
> seen_writers |= ready_writers
>
> becomes
>
> while True:
> ready_writers = set(fd for fd, evt in
> ep.poll(-1, MAXEVENTS) if fd != r)
> if ready_writers.issubset(seen_writers):
> break
> seen_writers |= ready_writers
Of course it does, since the returned FDs are a subset of all the
ready file descriptors.
The point is precisely that, when there are more FDs ready than
maxevents, some FDs will never be reported.
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Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue16873>
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