[REVIEW/TEST] polling(4) changes

[Gleb Smirnoff]

  dima,

On Fri, Oct 07, 2005 at 01:28:58PM +0400, dima wrote:
d> > d> Seems to be a first considerable step regarding the ideas discussed in March :)
d> > d> But, my idea about the separate locking of each interface dissappeared from this implementation. mtx_poll is good to protect the pollrec array and other sensitive variables. But we could get advantage of SMP machines writing polling loops like this:
d> > d> 
d> > d> for( i = 0; i < poll_handlers; ++i ) {
d> > d>   mtx_lock( &iface_lock[i] );
d> > d>   pr[i].handler(pr[i].ifp, POLL_ONLY, count);
d> > d>   mtx_unlock( &iface_lock[i] );
d> > d> }
d> > 
d> > What is the benefit here? The driver must have its own lock.
d> 
d> Well, consider the absense of the mtx_poll lock:
d> 
d> - mtx_lock( &mtx_poll );
d>   for( i = 0; i < poll_handlers; ++i ) {
d> +   mtx_lock( &iface_lock[i] );
d>     pr[i].handler( pr[i].ifp, POLL_ONLY, count );
d> +   mtx_unlock( &iface_lock[i] );
d>   }
d> - mtx_unlock( &mtx_poll );
d> 
d> So, several kernel threads in an SMP machine can poll different interfaces simultaneously. And mtx_lock should only be used in ether_poll_[de]register().

Imagining that we will have several polling threads in future, the above design
has some disadvantages, I think:

First, we still need to protect the array pr[], with some mutex while traversing
it, and while editing it in ether_poll_[de]register. May be like it was done in
kern_poll.c, rev 1.21.

Second, the approach above won't give a nice parallelization. Imagine two threads,
both working in a cycle shown above. They will contest on the lock of each interface:

- t1 starts
- t1 locks iface_lock[1]	- t2 starts
- t1 polls pr[1]...		- t2 blocks on iface_lock[1]
- t1 polls pr[1]...
- t1 polls pr[1]...
- t1 polls pr[1]...
- t1 polls pr[1]...
- t1 unlocks iface_lock[1]	- t2 locks iface_lock[1]
- t1 locks iface_lock[2]	- t2 polls empty pr[1], quickly returns
- t1 polls pr[2]...		- t2 unlocks iface_lock[1]
- t1 polls pr[2]...		- t2 blocks on iface_lock[2]
- t1 polls pr[2]...
- t1 polls pr[2]...
- t1 polls pr[2]...
- t1 polls pr[2]...
- t1 unlocks iface_lock[2]	- t2 locks iface_lock[2]
- t1 locks iface_lock[3]	- t2 polls empty pr[2], quickly returns
- t1 polls pr[3]...		- t2 unlocks iface_lock[2]

So, one thread works, and other just goes after the first one, and picks
only a small number of packets, or even just wastes CPU cycles.

Really we do not have several kernel threads in polling. netisr_poll() is always
run by one thread - swi1:net. Well, we have also idle_poll thread, but it is
very special case. Frankly speaking, it can't work without help from netisr_poll().
The current polling is designed for a single threaded kernel, for RELENG_4. We
can't achieve parallelization with strong redesign. The future plans are to create
per-interface CPU bound threads. The plans can change. You are welcome to help.

-- 
Totus tuus, Glebius.
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