[mnet-devel] patch: better blockwrangling strategy

[Arno Waschk]

okay, let's try it... the former was not optimal anyway, and this one is 
for sure simple enough... Arno

On 17 Feb 2004 12:02:14 -0500, Zooko O'Whielacronx <zooko at zooko.com> wrote:

>
> This backs off from stalled downloads.  It also improves downloads in 
> another
> way: how do you decide which peer to query next in the hunt for blocks?  
> Before
> this patch, there was a very complicated sorting algorithm which, among 
> other
> things, forced peers that you had already queried to be at the end of 
> the list.
> Then there was a random selection which was biased towards the front of 
> the
> list.  The only thing this patch does is get rid of the random selection 
> and
> just take the front peer from the list.  Since he goes to the back of 
> the list
> once he has been queried, this guarantees that we march through all peers
> instead of randomly trying different ones each time, possibly 
> redundantly.
>
> The theoretical improvement is significant.  (With the old way, it could 
> be an
> awful long time before you ever got around to trying the peer who had 
> the block
> you needed, and this awful long time would increase greatly with the 
> number of
> peers on the network.)
>
> On the other hand, it isn't absolutely required for correct operation.  
> The
> download would eventually work anyway without this patch.  (Unless of 
> course the
> relevant peer went off-line before you got around to it.)
>
> I've tested this patch.
>
> --- common/BlockWrangler.py	7 Dec 2003 17:35:50 -0000	1.94
> +++ common/BlockWrangler.py	17 Feb 2004 16:06:37 -0000
> @@ -536,8 +537,7 @@ class BlockWrangler(nummedobj.NummedObj)
>          """
>          @return: a list of peerIds;  This omits peers which currently 
> have an
>              outstanding "do you have blocks" transaction, and it sorts 
> the peers in
> -            order of preference for the "do you have blocks" 
> transaction.
>          """
>          assert self.data._assert_consistency()
>          peerIds = self.mtm._peerman.get_servers("block server")
> --- common/BlockWranglingStrategy.py	7 Dec 2003 17:35:50 -0000	1.33
> +++ common/BlockWranglingStrategy.py	17 Feb 2004 16:06:37 -0000
> @@ -119,7 +119,7 @@ class GSR(BlockWranglingStrategy):
>          return false
>
>      def _try_to_find_more_blocks(self):
> -        # To which peer?  Well, we'll pick a random peer, but weighted 
> towards our favorites (exponential distribution, in fact).
> +        # To which peer?  The peers are sorted increasing order of how 
> many times we have already asked them for blocks, so just taking the 
> first one will make sure we try all of them in order.
>          peerIds = 
> self.blockwrangler.list_sorted_peers_without_outstanding_searches()
>          origpeerIds = copy.copy(peerIds) # for consistency check
>          if len(peerIds) == 0:
> @@ -128,17 +128,13 @@ class GSR(BlockWranglingStrategy):
>              self.schedule_event(delay=15*60) # Now iterate -- this 
> schedules another call to `event()' to happen ASAP after 15 minutes from 
> now on the DoQ.
>              return
>
> -        while len(peerIds) > 0: # Note: there is a "return" in here 
> that usually exits this loop.
> -            index = int(randutil.random()**2 * len(peerIds)) # This 
> picks a random peer, but more likely one towards front of the list.
> -            peerId = peerIds[index]
> -            del peerIds[index]
> +        while peerIds: # Note: there is a "return" in here that usually 
> exits this loop.
> +            peerId = peerIds.pop(0)
>              (blockIds, minimalalreadytried,) = 
> choose_closest_wanted_blocks(self.blockwrangler, peerId)
>              if len(blockIds) > 0:
> -                debugprint("%s: searching on peer %s (delay %s) for 
> blocks %s\n", args=(self, peerId, minimalalreadytried, blockIds,), v=4, 
> vs="BlockWranglingStrategy")
> -                
> DoQ.doq.add_task(self.blockwrangler.look_for_blocks_on_peer, 
> args=(peerId, blockIds,), delay=minimalalreadytried)
> -                debugprint("%s scheduling next event, delay: %s, 
> because: %s\n", args=(self, 3.0, "issued search",), v=7, 
> vs="BlockWranglingStrategy")
> -                self.schedule_event(delay=minimalalreadytried) # Now 
> iterate -- this schedules another call to event() to happen later on the 
> DoQ.
> +                debugprint("%s: searching on peer %s (and then delay 
> %s) for blocks %s\n", args=(self, peerId, minimalalreadytried, 
> blockIds,), v=4, vs="BlockWranglingStrategy")
> +                
> DoQ.doq.add_task(self.blockwrangler.look_for_blocks_on_peer, 
> args=(peerId, blockIds, minimalalreadytried))
>                  return # We initiated a look_for_blocks_on_peer() -- no 
> need to iterate this while loop more.
>
>          # consistency check: if we reach this code then there must not 
> be any peers that we are not querying (or downloading from) right now!  
> Or else there must not be any blocks that we want.
>
>
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