Re: [RFC 1/4] kevent: core files.

[Evgeniy Polyakov <johnpol@2ka.mipt.ru>]

On Mon, Jul 31, 2006 at 03:00:28PM -0700, David Miller (davem@davemloft.net) wrote:
> From: Evgeniy Polyakov <johnpol@2ka.mipt.ru>
> Date: Mon, 31 Jul 2006 23:41:43 +0400
> 
> > Since kevents are never generated by kernel, but only marked as ready,
> > length of the main queue performs as flow control, so we can create a
> > mapped buffer which will have space equal to the main queue length
> > multiplied by size of the copied to userspace structure plus 16 bits for
> > the start index of the kernel writing side, i.e. it will store offset
> > where the oldest event was placed.
> >
> > Since queue length is a limited factor and thus no new events can be added
> > when queue is full, that means that buffer is full too and userspace
> > must read events. When syscall is called to add new kevent and provided 
> > there offset differs from what kernel stored, that means that all events 
> > from kernel to provided index have been read and new events can be added.
> > Thus we can even allow read-only mapping. Kernel's index is incremented
> > modulo queue length. If kevent was removed after it was marked as
> > ready, it's copy stays in the mapped buffer, but special flag can be
> > assigned to show that kevent is no longer valid.
> 
> This sounds reasonable.
> 
> However we must be mindful that the thread of control trying to
> add a new event might not be in a position to drain the queue
> of pending events when the queue is full.  Usually he will be
> trying to add an event in response to handling another event.
> 
> So we'd have cases like this, assume we start with a full event
> queue:
> 
> 	thread A		thread B
> 
> 	dequeue event
> 	aha, new connection
> 	accept()
> 				register new kevent
> 				queue is now full again
> 	add kevent on new
> 	connection
> 
> At this point thread A doesn't have very many options when the kevent
> add fails.  You cannot force this thread to read more events, since he
> may not be in a state where he is easily able to do so.

By default all kevents are not removed from the queue, so accept events
will be in the queue and thread B will fail to register new kevent.
To remove kevent from the queue user should either set one-shot flag or
do it by special command.
So if we are in position when queue is full and all events are not
one-shot, control thread must think about what does it do, and remove
some of them (and next time add them with one-shot flag).

-- 
	Evgeniy Polyakov
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