Fine grain select locking.

[Jeff Roberson]

Here is an update that avoids the malloc per fd when there are no 
collisions.  This unfortunately adds 64bytes to every socket in the 
system.  This is less than 10% of the size of the socket.  Vnodes only 
allocate their selinfo structures on demand so this does not cause a 
per-file overhead.  This was suggested by Peter.   This patch also uses a 
vm zone for the selfd structures.  I can shrink them slightly by using a 
SLIST in one case vs TAILQ as well.

http://people.freebsd.org/~jeff/select2.diff

Thanks,
Jeff

On Mon, 2 Jul 2007, Jeff Roberson wrote:

> I have a diff which makes the following improvements to select:
>
> 1) Per-thread wait channel rather than global select wait channel.
> 2) Per-thread select lock.
> 3) Rescan after sleep scans only descriptors which have come active.
> 4) No exposed select internals.
> 5) selwakeuppri() works again.
> 6) No thread_lock()ing in select, no TDF_SELECT required.
> 7) No more collisions.
>
> This is based on an approach from Alfred with some locking and rescan 
> improvements by me.  It only required changing select users in cases where 
> they assumed only one thread could select at a time.
>
> The unfortunate cost of this patch is that a descriptor per select fd must be 
> allocated to track individual threads.  This is what allows us to know which 
> descriptor has fired an event and allows us to use per-thread locking etc.
>
> The one thing I haven't fixed is netsmb and netncp which both have some wonky 
> select implementation that could be replaced with kern_select(). That could 
> be done seperately from this patch but is required for this to go in.
>
> http://people.freebsd.org/~jeff/select.diff
>
> Comments and suggestions welcome.
>
> Thanks,
> Jeff
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