premiere-libtorrent/src/choker.cpp

/*

Copyright (c) 2014, Arvid Norberg
All rights reserved.

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*/

#include "libtorrent/choker.hpp"
#include "libtorrent/peer_connection.hpp"
#include "libtorrent/aux_/session_settings.hpp"
#include "libtorrent/torrent.hpp"

#include <boost/bind.hpp>

namespace libtorrent
{

	// return true if 'lhs' peer should be preferred to be unchoke over 'rhs'

	// TODO: 3 split this funcion up into multiple functions, one for each
	// seed_choking_algorithm, and pick which one to use when calling
	// the sort function
	bool unchoke_compare(peer_connection const* lhs
		, peer_connection const* rhs, aux::session_settings const& sett)
	{
		// if one peer belongs to a higher priority torrent than the other one
		// that one should be unchoked.
		boost::shared_ptr<torrent> t1 = lhs->associated_torrent().lock();
		TORRENT_ASSERT(t1);
		boost::shared_ptr<torrent> t2 = rhs->associated_torrent().lock();
		TORRENT_ASSERT(t2);

		int prio1 = lhs->get_priority(peer_connection::upload_channel);
		int prio2 = rhs->get_priority(peer_connection::upload_channel);

		if (prio1 != prio2)
			return prio1 > prio2;

		// compare how many bytes they've sent us
		size_type c1;
		size_type c2;
		c1 = lhs->downloaded_in_last_round();
		c2 = rhs->downloaded_in_last_round();

		if (c1 != c2) return c1 > c2;

		if (sett.get_int(settings_pack::seed_choking_algorithm)
			== settings_pack::round_robin)
		{
			// the amount uploaded since unchoked (not just in the last round)
			c1 = lhs->uploaded_since_unchoked();
			c2 = rhs->uploaded_since_unchoked();

			// the way the round-robin unchoker works is that it,
			// by default, prioritizes any peer that is already unchoked.
			// this maintain the status quo across unchoke rounds. However,
			// peers that are unchoked, but have sent more than one quota
			// since they were unchoked, they get de-prioritized.

			int pieces = sett.get_int(settings_pack::seeding_piece_quota);
			// if a peer is already unchoked, and the number of bytes sent since it was unchoked
			// is greater than the send quanta, then it's done with it' upload slot, and we
			// can de-prioritize it
			bool c1_quota_complete = !lhs->is_choked() && c1
				> (std::max)(t1->torrent_file().piece_length() * pieces, 256 * 1024);
			bool c2_quota_complete = !rhs->is_choked() && c2
				> (std::max)(t2->torrent_file().piece_length() * pieces, 256 * 1024);

			// if c2 has completed a quanta, it shuold be de-prioritized
			// and vice versa
			if (c1_quota_complete < c2_quota_complete) return true;
			if (c1_quota_complete > c2_quota_complete) return false;

			// if both peers have either completed a quanta, or not.
			// keep unchoked peers prioritized over choked ones, to let
			// peers keep working on uploading a full quanta
			if (lhs->is_choked() < rhs->is_choked()) return true;
			if (lhs->is_choked() > rhs->is_choked()) return false;

			// if the peers are still identical (say, they're both waiting to be unchoked)
			// fall through and rely on the logic to prioritize peers who have waited
			// the longest to be unchoked
		}	
		else if (sett.get_int(settings_pack::seed_choking_algorithm)
			== settings_pack::fastest_upload)
		{
			c1 = lhs->uploaded_in_last_round();
			c2 = rhs->uploaded_in_last_round();
		
			// take torrent priority into account
			c1 *= prio1;
			c2 *= prio2;

			if (c1 > c2) return true;
			if (c2 > c1) return false;
		}
		else if (sett.get_int(settings_pack::seed_choking_algorithm)
			== settings_pack::anti_leech)
		{
			// the anti-leech seeding algorithm is based on the paper "Improving
			// BitTorrent: A Simple Approach" from Chow et. al. and ranks peers based
			// on how many pieces they have, prefering to unchoke peers that just
			// started and peers that are close to completing. Like this:
			//   ^
			//   | \                       / |
			//   |  \                     /  |
			//   |   \                   /   |
			// s |    \                 /    |
			// c |     \               /     |
			// o |      \             /      |
			// r |       \           /       |
			// e |        \         /        |
			//   |         \       /         |
			//   |          \     /          |
			//   |           \   /           |
			//   |            \ /            |
			//   |             V             |
			//   +---------------------------+
			//   0%    num have pieces     100%
			int t1_total = t1->torrent_file().num_pieces();
			int t2_total = t2->torrent_file().num_pieces();
			int score1 = (lhs->num_have_pieces() < t1_total / 2
				? t1_total - lhs->num_have_pieces() : lhs->num_have_pieces()) * 1000 / t1_total;
			int score2 = (rhs->num_have_pieces() < t2_total / 2
				? t2_total - rhs->num_have_pieces() : rhs->num_have_pieces()) * 1000 / t2_total;
			if (score1 > score2) return true;
			if (score2 > score1) return false;
		}
		
		// prioritize the one that has waited the longest to be unchoked
		// the round-robin unchoker relies on this logic. Don't change it
		// without moving this into that unchoker logic
		return lhs->time_of_last_unchoke() < rhs->time_of_last_unchoke();
	}

	bool upload_rate_compare(peer_connection const* lhs
		, peer_connection const* rhs)
	{
		size_type c1;
		size_type c2;

		c1 = lhs->uploaded_in_last_round();
		c2 = rhs->uploaded_in_last_round();
		
		// take torrent priority into account
		c1 *= lhs->get_priority(peer_connection::upload_channel);
		c2 *= rhs->get_priority(peer_connection::upload_channel);

		return c1 > c2;
	}

	bool bittyrant_unchoke_compare(peer_connection const* lhs
		, peer_connection const* rhs)
	{
		size_type d1, d2, u1, u2;

		// first compare how many bytes they've sent us
		d1 = lhs->downloaded_in_last_round();
		d2 = rhs->downloaded_in_last_round();
		// divided by the number of bytes we've sent them
		u1 = lhs->uploaded_in_last_round();
		u2 = rhs->uploaded_in_last_round();

		// take torrent priority into account
		d1 *= lhs->get_priority(peer_connection::upload_channel);
		d2 *= rhs->get_priority(peer_connection::upload_channel);

		d1 = d1 * 1000 / (std::max)(size_type(1), u1);
		d2 = d2 * 1000 / (std::max)(size_type(1), u2);
		if (d1 > d2) return true;
		if (d1 < d2) return false;

		// if both peers are still in their send quota or not in their send quota
		// prioritize the one that has waited the longest to be unchoked
		return lhs->time_of_last_unchoke() < rhs->time_of_last_unchoke();
	}

	int unchoke_sort(std::vector<peer_connection*>& peers
		, int max_upload_rate
		, time_duration unchoke_interval
		, aux::session_settings const& sett)
	{
		int upload_slots = sett.get_int(settings_pack::unchoke_slots_limit);

		// ==== BitTyrant ====
		//
		// if we're using the bittyrant unchoker, go through all peers that
		// we have unchoked already, and adjust our estimated reciprocation
		// rate. If the peer has reciprocated, lower the estimate, if it hasn't,
		// increase the estimate (this attempts to optimize "ROI" of upload
		// capacity, by sending just enough to be reciprocated).
		// For more information, see: http://bittyrant.cs.washington.edu/
		if (sett.get_int(settings_pack::choking_algorithm)
			== settings_pack::bittyrant_choker)
		{
			for (std::vector<peer_connection*>::const_iterator i = peers.begin()
				, end(peers.end()); i != end; ++i)
			{
				peer_connection* p = *i;
				if (p->is_choked() || !p->is_interesting()) continue;

				if (!p->has_peer_choked())
				{
					// we're unchoked, we may want to lower our estimated
					// reciprocation rate
					p->decrease_est_reciprocation_rate();
				}
				else
				{
					// we've unchoked this peer, and it hasn't reciprocated
					// we may want to increase our estimated reciprocation rate
					p->increase_est_reciprocation_rate();
				}
			}

			// if we're using the bittyrant choker, sort peers by their return
			// on investment. i.e. download rate / upload rate
			std::sort(peers.begin(), peers.end()
				, boost::bind(&bittyrant_unchoke_compare, _1, _2));

			int upload_capacity_left = max_upload_rate;

			// now, figure out how many peers should be unchoked. We deduct the
			// estimated reciprocation rate from our upload_capacity estimate
			// until there none left
			upload_slots = 0;

			for (std::vector<peer_connection*>::iterator i = peers.begin()
				, end(peers.end()); i != end; ++i)
			{
				peer_connection* p = *i;
				TORRENT_ASSERT(p);

				if (p->est_reciprocation_rate() > upload_capacity_left) break;

				++upload_slots;
				upload_capacity_left -= p->est_reciprocation_rate();
			}

			return upload_slots;
		}

		// ==== rate-based ====
		//
		// The rate based unchoker looks at our upload rate to peers, and find
		// a balance between number of upload slots and the rate we achieve. The
		// intention is to not spread upload bandwidth too thin, but also to not
		// unchoke few enough peers to not be able to saturate the up-link.
		// this is done by traversing the peers sorted by our upload rate to
		// them in decreasing rates. For each peer we increase our threshold
		// by 1 kB/s. The first peer we get to to whom we upload slower than
		// the threshold, we stop and that's the number of unchoke slots we have.
		if (sett.get_int(settings_pack::choking_algorithm)
			== settings_pack::rate_based_choker)
		{
			// first reset the number of unchoke slots, because we'll calculate
			// it purely based on the current state of our peers.
			upload_slots = 0;

			// TODO: optimize this using partial_sort or something. We don't need
			// to sort the entire list
			
			// TODO: make the comparison function a free function and move it
			// into this cpp file
			std::sort(peers.begin(), peers.end()
				, boost::bind(&upload_rate_compare, _1, _2));

			// TODO: make configurable
			int rate_threshold = 1024;

			for (std::vector<peer_connection*>::const_iterator i = peers.begin()
				, end(peers.end()); i != end; ++i)
			{
				peer_connection const& p = **i;
				int rate = int(p.uploaded_in_last_round()
					* 1000 / total_milliseconds(unchoke_interval));

				if (rate < rate_threshold) break;

				++upload_slots;

				// TODO: make configurable
				rate_threshold += 1024;
			}
			++upload_slots;
		}

		// sorts the peers that are eligible for unchoke by download rate and
		// secondary by total upload. The reason for this is, if all torrents are
		// being seeded, the download rate will be 0, and the peers we have sent
		// the least to should be unchoked
		
		// we use partial sort here, because we only care about the top
		// upload_slots peers.

		std::partial_sort(peers.begin(), peers.begin()
			+ (std::min)(upload_slots, int(peers.size())), peers.end()
			, boost::bind(&unchoke_compare, _1, _2, boost::cref(sett)));

		return upload_slots;
	}

}