Rev 1: Starting point. Interface tests hooked up and failing. in http://people.ubuntu.com/~robertc/baz2.0/plugins/groupcompress/trunk
Robert Collins
robertc at robertcollins.net
Sat Jul 5 19:16:14 BST 2008
At http://people.ubuntu.com/~robertc/baz2.0/plugins/groupcompress/trunk
------------------------------------------------------------
revno: 1
revision-id: robertc at robertcollins.net-20080705181540-mua3hkzj7c9qa5f6
committer: Robert Collins <robertc at robertcollins.net>
branch nick: trunk
timestamp: Sun 2008-07-06 04:15:40 +1000
message:
Starting point. Interface tests hooked up and failing.
added:
COPYING copying-20080705181503-ccbxd6xuy1bdnrpu-1
DESIGN design-20080705181503-ccbxd6xuy1bdnrpu-2
NEWS news-20080705181503-ccbxd6xuy1bdnrpu-3
README readme-20080705181503-ccbxd6xuy1bdnrpu-4
TODO todo-20080705181503-ccbxd6xuy1bdnrpu-5
__init__.py __init__.py-20080705181503-ccbxd6xuy1bdnrpu-6
errors.py errors.py-20080705181503-ccbxd6xuy1bdnrpu-7
groupcompress.py groupcompress.py-20080705181503-ccbxd6xuy1bdnrpu-8
setup.py setup.py-20080705181503-ccbxd6xuy1bdnrpu-9
tests/ tests-20080705181503-ccbxd6xuy1bdnrpu-10
tests/__init__.py __init__.py-20080705181503-ccbxd6xuy1bdnrpu-11
tests/test_errors.py test_errors.py-20080705181503-ccbxd6xuy1bdnrpu-12
tests/test_groupcompress.py test_groupcompress.p-20080705181503-ccbxd6xuy1bdnrpu-13
=== added file 'COPYING'
--- a/COPYING 1970-01-01 00:00:00 +0000
+++ b/COPYING 2008-07-05 18:15:40 +0000
@@ -0,0 +1,340 @@
+ GNU GENERAL PUBLIC LICENSE
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+
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+Public License instead of this License.
=== added file 'DESIGN'
--- a/DESIGN 1970-01-01 00:00:00 +0000
+++ b/DESIGN 2008-07-05 18:15:40 +0000
@@ -0,0 +1,115 @@
+This document contains notes about the design for groupcompress, replacement
+VersionedFiles store for use in pack based repositories. The goal is to provide
+fast, history bounded text extraction.
+
+Overview
+++++++++
+
+The goal: Much tighter compression, maintained automatically. Considerations
+to weigh: The minimum IO to reconstruct a text with no other repository
+involved; The number of index lookups to plan a reconstruction. The minimum
+IO to reconstruct a text with another repositories assistance (affects
+network IO for fetch, which impacts incremental pulls and shallow branch
+operations).
+
+Current approach
+================
+
+Each delta is individually compressed against another text, and then entropy
+compressed. We index the pointers between these deltas.
+
+Solo reconstruction: Plan a readv via the index, read the deltas in forward
+IO, apply each delta. Total IO: sum(deltas) + deltacount*index overhead.
+Fetch/stacked reconstruction: Plan a readv via the index, using local basis
+texts where possible. Then readv locally and remote and apply deltas. Total IO
+as for solo reconstruction.
+
+Things to keep
+==============
+
+Reasonable sizes 'amount read' from remote machines to reconstruct an arbitrary
+text: Reading 5MB for a 100K plain text is not a good trade off. Reading (say)
+500K is probably acceptable. Reading ~100K is ideal. However, its likely that
+some texts (e.g NEWS versions) can be stored for nearly-no space at all if we
+are willing to have unbounded IO. Profiling to set a good heuristic will be
+important. Also allowing users to choose to optimise for a server environment
+may make sense: paying more local IO for less compact storage may be useful.
+
+Things to remove
+================
+
+Index scatter gather IO. Doing hundreds or thousands of index lookups is very
+expensive, and doing that per file just adds insult to injury.
+
+Partioned compression amongst files.
+
+Scatter gather IO when reconstructing texts: linear forward IO is better.
+
+Thoughts
+========
+
+Merges combine texts from multiple versions to create a new version. Deltas
+add new text to existing files and remove some text from the same. Getting
+high compression means reading some base and then a chain of deltas (could
+be a tree) to gain access to the thing that the final delta was made against,
+and that delta. Rather than composing all these deltas, we can just just
+perform the final diff against the base text and the serialised invidual
+deltas. If the diff algorithm can reuse out of order lines from previous
+texts (e.g. storing AB -> BA as pointers rather than delete and add, then
+the presence of any previously stored line in a single chain can be reused.
+One such diff algorithm is xdelta, another reasonable one to consider is
+plain old zlib or lzma. We could also use bzip2. One advantage of using
+a generic compression engine is less python code. One advantage of
+preprocessing line based deltas is that we reduce the window size for the
+text repeated within lines, and that will help compression by a simple
+entropy compressor as a post processor.
+lzma appears fantastic at compression - 420MB of NEWS files down to 200KB.
+so window size appears to be a key determiner for efficiency.
+
+Delta strategy
+++++++++++++++
+
+Very big objects - no delta. I plan to kick this in at 5MB initially, but
+once the codebase is up and running, we can tweak this to
+
+Very small objects - no delta? If they are combined with a larger zlib object
+why not?
+
+Other objects - group by fileid (gives related texts a chance, though using a
+file name would be better long term as e.g. COPYING and COPYING from different
+projects could combine). Then by reverse topological graph(as this places more
+recent texts at the front of a chain). Alternatively, group by size, though
+that should not matter with a large enough window.
+Finally, delta the texts against the current output of the compressor. This is
+essentially a somewhat typed form of sliding window dictionary compression. An
+alternative implementation would be to just use zlib, or lzma, or bzip2
+directory.
+
+Unfortunately, just using entropy compression forces a lot of data to be output
+by the decompressor - e.g. 420MB in the NEWS sample corpus. When we only want
+a single 55K text thats inefficient. (An initial test took several seconds with
+lzma.)
+
+The fastest to implement approach is probably just 'diff output to date and add
+to entropy compressor'. This should produce reasonable results. As delta
+chain length is not a concern (only one delta to apply ever), we can simply
+cap the chain when the total read size becomes unreasonable. Given older texts
+are smaller we probably want some weighted factor of plaintext size.
+
+In this approach, a single entropy compressed region is read as a unit, giving
+the lower bound for IO (and how much to read is an open question - what byte
+offset of compressed data is sufficient to ensue that the delta-stream contents
+we need are reconstructable. Flushing, while possible, degrades compression(and
+adds overhead - we'd be paying 4 bytes per record guaranteed). Again - tests
+will be needed.
+
+A nice possibility is to output mpdiff compatible records, which might enable
+some code reuse. This is more work than just diff (current_out, new_text), so
+can wait for the concept to be proven.
+
+Implementation Strategy
++++++++++++++++++++++++
+
+Bring up a VersionedFiles object that implements this, then stuff it into a
+repository format. zlib as a starting compressor, though bzip2 will probably
+do a good job.
=== added file 'NEWS'
--- a/NEWS 1970-01-01 00:00:00 +0000
+++ b/NEWS 2008-07-05 18:15:40 +0000
@@ -0,0 +1,24 @@
+-------------------------------
+bzr-groupcompress Release Notes
+-------------------------------
+
+.. contents::
+
+IN DEVELOPMENT
+--------------
+
+ NOTES WHEN UPGRADING:
+
+ CHANGES:
+
+ FEATURES:
+
+ IMPROVEMENTS:
+
+ BUGFIXES:
+
+ API BREAKS:
+
+ TESTING:
+
+ INTERNALS:
=== added file 'README'
--- a/README 1970-01-01 00:00:00 +0000
+++ b/README 2008-07-05 18:15:40 +0000
@@ -0,0 +1,26 @@
+groupcompress, a bzr plugin providing new compression logic.
+Copyright (C) 2008 Canonical Limited.
+
+This program is free software; you can redistribute it and/or modify
+it under the terms of the GNU General Public License version 2 as published
+by the Free Software Foundation.
+
+This program is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with this program; if not, write to the Free Software
+Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
+
+groupcompress
++++++++++++++
+
+bzr repositories are larger than we want them to be; this tries to implement
+some of the things we have been considering.
+
+Documentation
+=============
+
+See DESIGN.
=== added file 'TODO'
--- a/TODO 1970-01-01 00:00:00 +0000
+++ b/TODO 2008-07-05 18:15:40 +0000
@@ -0,0 +1,5 @@
+Todo for groupcompress:
+
+ * layers - gc reader/writer and vf layers should be separate
+ * mpdiff usage
+ * other stuff from design
=== added file '__init__.py'
--- a/__init__.py 1970-01-01 00:00:00 +0000
+++ b/__init__.py 2008-07-05 18:15:40 +0000
@@ -0,0 +1,43 @@
+# groupcompress, a bzr plugin providing new compression logic.
+# Copyright (C) 2008 Canonical Limited.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License version 2 as published
+# by the Free Software Foundation.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
+#
+
+"""groupcompress will provide smaller bzr repositories.
+
+groupcompress
++++++++++++++
+
+bzr repositories are larger than we want them to be; this tries to implement
+some of the things we have been considering. The primary logic is deep in the
+VersionedFiles abstraction, and at this point there is no user visible
+facilities.
+
+Documentation
+=============
+
+See DESIGN in the groupcompress osurc.e
+"""
+def test_suite():
+ # Thunk across to load_tests for niceness with older bzr versions
+ from bzrlib.tests import TestLoader
+ loader = TestLoader()
+ return loader.loadTestsFromModuleNames(['bzrlib.plugins.groupcompress'])
+
+
+def load_tests(standard_tests, module, loader):
+ standard_tests.addTests(loader.loadTestsFromModuleNames(
+ ['bzrlib.plugins.groupcompress.tests']))
+ return standard_tests
=== added file 'errors.py'
--- a/errors.py 1970-01-01 00:00:00 +0000
+++ b/errors.py 2008-07-05 18:15:40 +0000
@@ -0,0 +1,20 @@
+# groupcompress, a bzr plugin providing experimental compression logic
+# Copyright (C) 2008 Canonical Limited.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License version 2 as published
+# by the Free Software Foundation.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
+#
+
+"""Error objects for compression functions."""
+
+from bzrlib.errors import BzrError
=== added file 'groupcompress.py'
--- a/groupcompress.py 1970-01-01 00:00:00 +0000
+++ b/groupcompress.py 2008-07-05 18:15:40 +0000
@@ -0,0 +1,110 @@
+# groupcompress, a bzr plugin providing new compression logic.
+# Copyright (C) 2008 Canonical Limited.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License version 2 as published
+# by the Free Software Foundation.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
+#
+
+"""Core compression logic for compressing streams of related files."""
+
+
+from bzrlib import pack
+from bzrlib.knit import _DirectPackAccess
+from bzrlib.plugins.index2.repofmt import InMemoryBTree
+from bzrlib.versionedfile import VersionedFiles
+
+
+def make_pack_factory(graph, delta, keylength):
+ """Create a factory for creating a pack based groupcompress.
+
+ This is only functional enough to run interface tests, it doesn't try to
+ provide a full pack environment.
+
+ :param graph: Store a graph.
+ :param delta: Delta compress contents.
+ :param keylength: How long should keys be.
+ """
+ def factory(transport):
+ parents = graph or delta
+ ref_length = 0
+ if graph:
+ ref_length += 1
+ if delta:
+ ref_length += 1
+ max_delta_chain = 200
+ else:
+ max_delta_chain = 0
+ graph_index = InMemoryBTree(reference_lists=ref_length,
+ key_elements=keylength)
+ stream = transport.open_write_stream('newpack')
+ writer = pack.ContainerWriter(stream.write)
+ writer.begin()
+ index = _GCGraphIndex(graph_index, lambda:True, parents=parents,
+ deltas=delta, add_callback=graph_index.add_nodes)
+ access = _DirectPackAccess({})
+ access.set_writer(writer, graph_index, (transport, 'newpack'))
+ result = GroupCompressVersionedFiles(index, access,
+ max_delta_chain=max_delta_chain)
+ result.stream = stream
+ result.writer = writer
+ return result
+ return factory
+
+
+def cleanup_pack_group(versioned_files):
+ versioned_files.stream.close()
+ versioned_files.writer.end()
+
+
+class GroupCompressVersionedFiles(VersionedFiles):
+ """A group-compress based VersionedFiles implementation."""
+
+ def __init__(self, index, access, max_delta_chain=-1):
+ """Create a GroupCompressVersionedFiles object.
+
+ :param index: The index object storing access and graph data.
+ :param access: The access object storing raw data.
+ """
+
+
+class _GCGraphIndex(object):
+ """Mapper from GroupCompressVersionedFiles needs into GraphIndex storage."""
+
+ def __init__(self, graph_index, is_locked, deltas=False, parents=True,
+ add_callback=None):
+ """Construct a _GCGraphIndex on a graph_index.
+
+ :param graph_index: An implementation of bzrlib.index.GraphIndex.
+ :param is_locked: A callback to check whether the object should answer
+ queries.
+ :param deltas: Allow delta-compressed records.
+ :param parents: If True, record knits parents, if not do not record
+ parents.
+ :param add_callback: If not None, allow additions to the index and call
+ this callback with a list of added GraphIndex nodes:
+ [(node, value, node_refs), ...]
+ :param is_locked: A callback, returns True if the index is locked and
+ thus usable.
+ """
+ self._add_callback = add_callback
+ self._graph_index = graph_index
+ self._deltas = deltas
+ self._parents = parents
+ if deltas and not parents:
+ # XXX: TODO: Delta tree and parent graph should be conceptually
+ # separate.
+ raise KnitCorrupt(self, "Cannot do delta compression without "
+ "parent tracking.")
+ self.has_graph = parents
+ self._is_locked = is_locked
+
=== added file 'setup.py'
--- a/setup.py 1970-01-01 00:00:00 +0000
+++ b/setup.py 2008-07-05 18:15:40 +0000
@@ -0,0 +1,21 @@
+#!/usr/bin/env python
+from distutils.core import setup
+
+bzr_plugin_name = 'groupcompress'
+
+bzr_plugin_version = (1, 6, 0, 'dev', 0)
+
+
+if __name__ == '__main__':
+ setup(name="bzr groupcompress",
+ version="1.6.0dev0",
+ description="bzr group compression.",
+ author="Robert Collins",
+ author_email="bazaar at lists.canonical.com",
+ license = "GNU GPL v2",
+ url="https://launchpad.net/bzr-groupcompress",
+ packages=['bzrlib.plugins.groupcompress',
+ 'bzrlib.plugins.groupcompress.tests',
+ ],
+ package_dir={'bzrlib.plugins.groupcompress': '.'},
+ }
=== added directory 'tests'
=== added file 'tests/__init__.py'
--- a/tests/__init__.py 1970-01-01 00:00:00 +0000
+++ b/tests/__init__.py 2008-07-05 18:15:40 +0000
@@ -0,0 +1,30 @@
+# groupcompress, a bzr plugin providing new compression logic.
+# Copyright (C) 2008 Canonical Limited.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License version 2 as published
+# by the Free Software Foundation.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
+#
+
+
+"""Tests for the groupcompress bzr plugin."""
+
+
+def load_tests(standard_tests, module, loader):
+ test_modules = [
+ 'errors',
+ 'groupcompress',
+ ]
+ standard_tests.addTests(loader.loadTestsFromModuleNames(
+ ['bzrlib.plugins.groupcompress.tests.test_' + name for
+ name in test_modules]))
+ return standard_tests
=== added file 'tests/test_errors.py'
--- a/tests/test_errors.py 1970-01-01 00:00:00 +0000
+++ b/tests/test_errors.py 2008-07-05 18:15:40 +0000
@@ -0,0 +1,21 @@
+# groupcompress, a bzr plugin providing new compression logic.
+# Copyright (C) 2008 Canonical Limited.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License version 2 as published
+# by the Free Software Foundation.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
+#
+
+"""Tests for error formatting."""
+
+from bzrlib.plugins.groupcompress import errors
+from bzrlib.tests import TestCaseWithTransport
=== added file 'tests/test_groupcompress.py'
--- a/tests/test_groupcompress.py 1970-01-01 00:00:00 +0000
+++ b/tests/test_groupcompress.py 2008-07-05 18:15:40 +0000
@@ -0,0 +1,49 @@
+# groupcompress, a bzr plugin providing new compression logic.
+# Copyright (C) 2008 Canonical Limited.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License version 2 as published
+# by the Free Software Foundation.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
+#
+
+"""Tests for group compression."""
+
+import zlib
+
+from bzrlib import tests
+from bzrlib.plugins.groupcompress import errors, groupcompress
+from bzrlib.tests import (
+ TestCaseWithTransport,
+ TestScenarioApplier,
+ adapt_tests,
+ )
+from bzrlib.transport import get_transport
+
+
+def load_tests(standard_tests, module, loader):
+ from bzrlib.tests.test_versionedfile import TestVersionedFiles
+ vf_interface_tests = loader.loadTestsFromTestCase(TestVersionedFiles)
+ cleanup_pack_group = groupcompress.cleanup_pack_group
+ make_pack_factory = groupcompress.make_pack_factory
+ group_scenario = ('groupcompress-nograph', {
+ 'cleanup':cleanup_pack_group,
+ 'factory':make_pack_factory(False, False, 1),
+ 'graph': False,
+ 'key_length':1,
+ }
+ )
+ applier = TestScenarioApplier()
+ applier.scenarios = [group_scenario]
+ adapt_tests(vf_interface_tests, applier, standard_tests)
+ return standard_tests
+
+
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