On Mon, Aug 19, 2013 at 2:13 AM, Peter Todd <p...@petertodd.org> wrote:
> In any case given that SPV peers don't contribute back to the network
> they should obviously be heavily deprioritized and served only with
> whatever resources a node has spare.
Well, I'm glad we're making progress towards this kind of model :)
If I had to write a scoring function for node importance, I'd start by
making nodes I connected to more important than nodes that connected to me.
That should prevent the kind of attacks you're talking about. You can then
score within those subsets with greater subtlety, like using how long the
connection has been active (or extending that with signed timestamps).
This doesn't have any in-built bias against SPV nodes, which is probably
very hard to technically implement anyway. But it encodes the intuitive
notion that nodes I selected myself are less likely to be DoS attackers
than nodes which connected to me.
But the trick is to implement the prioritisation code. The usual way to do
this is to have a thread pool that pops requests off a queue. You can
either have multiple queues for different priority bands, or code that
locks the queue and re-orders it when something new is added. I tend to
find the multiple queues approach simpler, especially, it's simpler to
export statistics about that via RPC that make it easy to understand what's
going on underneath the hood.
So IMHO a patch to address I/O exhaustion should look something like this:
1. Add a thread pool of 2-3 threads (to give the kernel room to overlap
IO) which take in CBlock load requests and then do the load/parse/filter in
the background.
2. Each thread starts by blocking on a counting semaphore which
represents the total number of requests.
3. The network thread message loop is adjusted so it can receive some
kind of futures/callbacks/closure object (I guess Boost provides this,
alternatively we could switch to using C++11). The closures should also
have the score of the node they were created for (note: score not a CNode*
as that complicates memory management).
4. At the start of the network loop a thread-local (or global) variable
is set that contains the nodes current score, which is just an n-of-m score
where M is the total number of connected nodes and N is the ranked
importance. At that point any code that needs to prioritise nodes off
against each other can just check that variable whilst doing work. The
network loop looks at which file descriptors are select()able and their
scores, which closures are pending execution and their scores, then decides
whether to handle new network data or run a closure. If there is a draw
between the scores, closures take priority to reduce memory pressure and
lower latency.
5. Handling of "getdata" then ends up calling a function that requests a
load of a block from disk, and runs a closure when it's finished. The
closure inherits the nodes current score, of course, so when the block load
is completed execution of the rest of the getdata handling takes priority
over handling new traffic from network nodes. When the closure executes, it
writes the loaded/filtered data out over the network socket and deletes
The function that takes a CBlockIndex and yields a future<CBlock> or
closure or whatever would internally lock the job queue(s), add the new
task and then do a stable sort of the queue using the scoring function,
which in this case would simply use the node score as the job score.
It's a fair amount of work, but should ensure that "good" nodes outcompete
"bad" nodes for disk IO. Any other disk IO operations can be done in the
same way. Note that the bulk of LevelDB write work is already handled on a
background thread. The foreground thread only writes a log entry to disk
and updates some in-memory data structures.
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