In this course, you will learn to design the computer architecture of complex modern microprocessors.
Deadlock
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Do curso por Princeton University
Computer Architecture
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Large Multiprocessors (Directory Protocols)
This lecture covers the motivation and implementation of directory protocol used for coherence on large multiproccesors.
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David WentzlaffAssociate Professor
Electrical Engineering
Okay, so this brings us to the end of interconnection networks.
And what I wanted to talk about was Deadlock.
So let's assume that you have three nodes labeled one, two, and three here.
It's on a 1D, or excuse me, yeah, oh 1D tourist, that is unidirectional.
So links only flow in the clockwise direction here.
Now, all of a sudden, you have node one wants to send to node three, node two
wants to send to node one, and node three wants to send to, to two.
And we're going to look at a wormhole routing network.
So the wormhole routing network you start to send the header words and [INAUDIBLE]
has not, potentially not even been injected into the network at this point.
Let's say they all start at exactly the same time.
So one is going to start sending to three.
Two's going to start sending to one. Three's going to start sending to two.
And if you'll notice, is each set of these has one overlapping link with the
previous one. So there's going to be some contention on
that link. So very quickly, you can see that.
If we were to all, if we were to launch these three messages exactly at same
time, and the message length was long. None of them would actually get to the
receivers. One would use this link, or one sending
to three would use this link, two sending to one would use this link, three sending
to two would use this wraparound link. But then all of a sudden, they would try
to acquire the next link but it's already been reserved.
But they can't give up the link because they've already acquired the link, so all
of a sudden you have a deadlock. So these numbers get pretty tricky.
and one way that we go about trying to see if our routing protocol is deadlock
free or actually can deadlock is we start to do something called Waits-for-analysis
or waits-for and Holds analysis. And waits for, and Waits-for and Holds
analysis, the basic idea is, so this is looking at
our example that we saw before where we have three routers.
What's happening here is we're injecting into A and this A is acquiring this link
or this [INAUDIBLE] here, and then it needs to acquire the next
one. So let's draw this as a diagram.
So A holds TQ2, so A holds TQ2.
This one here, this inbound Q here.
And when we draw Waits-for and Hold analysis diagrams.
When we are waiting on something we draw a edge from the thing that is the, the
actor to the resource that we're waiting on,
and the holds actually goes in the opposite direction.
So if a holds TQ2 here we actually draw the edge the other direction.
all of a sudden, A is holding TQ2 two, and it's waiting to acquire TQ3, this one
here. because it needs to transit this link to deliver it to TQ3.
But at the same time, B has acquired TQ3, so it's holding that and it's waiting for
TQ1. But C currently holding TQ1 and its
waiting on TQ2. This is how we do deadlock analysis from
a proof perspective. We actually write these dependency graphs or Waits-for and
Hods graphs and if you find a cycle in your protocol, your protocol can have a
deadlock. It need a couple of some way to color
link your, color, color edge in this graph or you need to really design your
protocol. But this is the sort of canonical way
that you can do this and if you start to look at different routing protocols some
of them could possibly deadlock and some of'em can, you can prove statically to
all possible weights for of whole graphs you can come up with will never have
cycles. So I, I think I mentioned last time, if
you look at dimension order routing with one more routing.
All possible inputs and outputs on that you never actually going to have a
deadlock, you never call for cycle in the graph.
And the reason for this is the only ever quire X routers and then Y routers, will
say if you have the dimension that you acquire X and quire Y.
But you never going into have effectively something that is holding Y trying to
acquire something on the X axis. So, you never going to have a, a cycle in
that Waits-for and Holds whole analysis graph.
I just briefly wanted to some up here that Deadlock is actually not an enemy.
You can try to use deadlock, to your benefit or, or just live with
deadlock. Just because your protocol has deadlock
does not mean you can't try to recover from it.
So what I mean by this is deadlock avoidance can be very expensive.
So trying to draw all possible graphs here, you should think about, you know,
where your deadlocks happen in your systems, and plan them out and make sure
they don't happen often. But some of the things you need to do, to
try and cut edges here, might be too restrictive.
So for instance, dimensional routing may only route in the X dimension and then in
the Y dimension, could be very expensive relative to an adaptive routing scheme,
or you try to route around congestion in your network.
So this deadlock avoidance, you know, you'll, you'll design the protocol to
never deadlock, this could be quite expensive.
So an alternative, and this is something you should not necessarily be afraid of,
but you have to be careful of this is that you find the deadlock and then you
try to recover from it. So what I mean by this is on-chip network
or a, a, multi-chip network you can actually have a deadlock occur,
notice that a deadlock occurred. And either try to roll back the state and
somehow jitter the state so the deadlock won't happen a second time.
Or many times these deadlocks happen because your dependent on one particular
buffer in the system and two people trying to acquire that buffer.
So you can try to virtualize that buffer. Effectively saving the state of that
buffer in memory and adding more, more states.
So you, a lot of these protocols, if you were to add more FIFO entries into the
system, it would actually break the deadlock.
So, sort of going back into this picture here,
if all of a sudden if I added another, let's say, FIFO here that the inbound
traffic went into and then this other traffic tried to bypass it, it would
actually cut one of these edges, and all of a sudden you would not be having a
deadlock. So you can think about trying to have a
deadlock recovery mechanism that's an example of it.
So a good example this is actually in the raw micro processor from an IT where we
I, you can actually have that much on the on-chip network.
Its dimensional routed, but you can still have message depended deadlocks.
So what I mean by that is while the network itself is not going to deadlock,
the traffic flows you have on top of it that from since your memory coherence
protocol on top of that could be potentially deadlock.
So how do you recover from that? Well you can actually have a counter,
a timer which goes off when you determine that the network has not moved for a 1000
cycles. So all the sudden you're running along
and no traffic moves on your network. Well it's a pretty good indicator that
you have a deadlock condition, because something should be flowing.
There should be some forward progress guarantees.
[COUGH] And if you notice that, an interrupt will go off.
So this timer will go off saying nothing has moved on my network for 1,000 cycles,
this is probably a deadlock. So that, you could take an interrupt and
then software can go look at all this data in the network and introduce more
buffering into the network. So, effectively through software try to
virtualize a particular FIFO entry and that can break the deadlock.
So there are ways to recover from deadlocks.
If your network is for instance used for message passing network, you can just use
memory to go do that. So actually on the raw processor, our
memory coherence protocol, we used deadlock avoidance and then our message
passing protocol, our message passing networks use deadlock recovery.
later on in, in Tylera actually we have something depending on which memory
network we have mixtures of these two. And you might say that sounds scary,
but if you walk through the proofs and are very careful about it and you are
guaranteed that you don't need anymore resources to go break the deadlock you
may be okay. But yeah it's, its just some say it's
playing with fire. You've got to be a little careful of
these, these sorts of solutions when trying to play with deadlock recovery.
But if you're guaranteed that you're not going to need any more resources you can
just resolve it right then. And oh,
so the reason you would want to, to use the lock recovery is it does not restrict
your protocol. And by doing that, you can have the
common case of your protocol be very, very fast.
And, you could make it so that the deadlock almost never happens.
Or never, in practical concerns ever happens.
Then, when the deadlock does happen oh, you take a little bit of performance
bump, or a performance hit there. Because you have to virtualize it in
software. But the probability of it happening is so
low.
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