many get_parent_map calls during walk to common revisions

While pulling from lp:bzr-svn into my copy, with the remote in 2a and my branch in a pack format, I see it make very slow progress with many small get_parent_map calls. I eventually interrupted it in

175.807 Traceback (most recent call last):
  File "/home/mbp/bzr/trunk/bzrlib/commands.py", line 842, in exception_to_return_code
    return the_callable(*args, **kwargs)
  File "/home/mbp/bzr/trunk/bzrlib/commands.py", line 1037, in run_bzr
    ret = run(*run_argv)
  File "/home/mbp/bzr/trunk/bzrlib/commands.py", line 654, in run_argv_aliases
    return self.run(**all_cmd_args)
  File "/home/mbp/bzr/trunk/bzrlib/builtins.py", line 1017, in run
    possible_transports=possible_transports, local=local)
  File "/home/mbp/bzr/trunk/bzrlib/decorators.py", line 192, in write_locked
    result = unbound(self, *args, **kwargs)
  File "/home/mbp/bzr/trunk/bzrlib/workingtree.py", line 1611, in pull
    local=local)
  File "/home/mbp/bzr/trunk/bzrlib/decorators.py", line 192, in write_locked
    result = unbound(self, *args, **kwargs)
  File "/home/mbp/bzr/trunk/bzrlib/branch.py", line 948, in pull
    possible_transports=possible_transports, *args, **kwargs)
  File "/home/mbp/bzr/trunk/bzrlib/branch.py", line 3194, in pull
    _override_hook_target=_override_hook_target)
  File "/home/mbp/bzr/trunk/bzrlib/branch.py", line 3071, in pull
    overwrite=overwrite, graph=graph)
  File "/home/mbp/bzr/trunk/bzrlib/decorators.py", line 192, in write_locked
    result = unbound(self, *args, **kwargs)
  File "/home/mbp/bzr/trunk/bzrlib/branch.py", line 896, in update_revisions
    overwrite, graph)
  File "/home/mbp/bzr/trunk/bzrlib/branch.py", line 3014, in update_revisions
    self.target.fetch(self.source, stop_revision)
  File "/home/mbp/bzr/trunk/bzrlib/decorators.py", line 192, in write_locked
    result = unbound(self, *args, **kwargs)
  File "/home/mbp/bzr/trunk/bzrlib/branch.py", line 579, in fetch
    pb=pb)
  File "/home/mbp/bzr/trunk/bzrlib/repository.py", line 1695, in fetch
    find_ghosts=find_ghosts, fetch_spec=fetch_spec)
  File "/home/mbp/bzr/trunk/bzrlib/decorators.py", line 192, in write_locked
    result = unbound(self, *args, **kwargs)
  File "/home/mbp/bzr/trunk/bzrlib/repository.py", line 3413, in fetch
    pb=pb, find_ghosts=find_ghosts)
  File "/home/mbp/bzr/trunk/bzrlib/fetch.py", line 81, in __init__
    self.__fetch()
  File "/home/mbp/bzr/trunk/bzrlib/fetch.py", line 103, in __fetch
    search = self._revids_to_fetch()
  File "/home/mbp/bzr/trunk/bzrlib/fetch.py", line 172, in _revids_to_fetch
    find_ghosts=self.find_ghosts)
  File "/home/mbp/bzr/trunk/bzrlib/decorators.py", line 138, in read_locked
    result = unbound(self, *args, **kwargs)
  File "/home/mbp/bzr/trunk/bzrlib/repository.py", line 1522, in search_missing_revision_ids
    revision_id, find_ghosts)
  File "/home/mbp/bzr/trunk/bzrlib/decorators.py", line 138, in read_locked
    result = unbound(self, *args, **kwargs)
  File "/home/mbp/bzr/trunk/bzrlib/repository.py", line 3479, in search_missing_revision_ids
    return self._walk_to_common_revisions([revision_id])
  File "/home/mbp/bzr/trunk/bzrlib/repository.py", line 3435, in _walk_to_common_revisions
    next_revs_part, ghosts_part = searcher.next_with_ghosts()
  File "/home/mbp/bzr/trunk/bzrlib/graph.py", line 1303, in next_with_ghosts
    self._advance()
  File "/home/mbp/bzr/trunk/bzrlib/graph.py", line 1313, in _advance
    found, ghosts, next, parents = self._do_query(self._next_query)
  File "/home/mbp/bzr/trunk/bzrlib/graph.py", line 1336, in _do_query
    parent_map = self._parents_provider.get_parent_map(revisions)
  File "/home/mbp/bzr/trunk/bzrlib/graph.py", line 99, in get_parent_map
    new_found = parents_provider.get_parent_map(remaining)
  File "/home/mbp/bzr/trunk/bzrlib/graph.py", line 167, in get_parent_map
    parent_map = self._get_parent_map(needed_revisions)
  File "/home/mbp/bzr/trunk/bzrlib/remote.py", line 1390, in _get_parent_map_rpc
    verb, args, body)
  File "/home/mbp/bzr/trunk/bzrlib/remote.py", line 72, in _call_with_body_bytes_expecting_body
    method, args, body_bytes)
  File "/home/mbp/bzr/trunk/bzrlib/smart/client.py", line 167, in call_with_body_bytes_expecting_body
    method, args, body=body, expect_response_body=True)
  File "/home/mbp/bzr/trunk/bzrlib/smart/client.py", line 78, in _call_and_read_response
    expect_body=expect_response_body),
  File "/home/mbp/bzr/trunk/bzrlib/smart/message.py", line 299, in read_response_tuple
    self._wait_for_response_args()
  File "/home/mbp/bzr/trunk/bzrlib/smart/message.py", line 264, in _wait_for_response_args
    self._read_more()
  File "/home/mbp/bzr/trunk/bzrlib/smart/message.py", line 277, in _read_more
    bytes = self._medium_request.read_bytes(next_read_size)
  File "/home/mbp/bzr/trunk/bzrlib/smart/medium.py", line 456, in read_bytes
    return self._read_bytes(count)
  File "/home/mbp/bzr/trunk/bzrlib/smart/medium.py", line 468, in _read_bytes
    return self._medium.read_bytes(count)
  File "/home/mbp/bzr/trunk/bzrlib/smart/medium.py", line 149, in read_bytes
    return self._read_bytes(bytes_to_read)
  File "/home/mbp/bzr/trunk/bzrlib/smart/medium.py", line 805, in _read_bytes
    bytes = osutils.until_no_eintr(self._read_from.read, bytes_to_read)
  File "/home/mbp/bzr/trunk/bzrlib/osutils.py", line 1855, in until_no_eintr
    return f(*a, **kw)
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