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# Copyright (C) 2008 Canonical Ltd
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# This program is free software; you can redistribute it and/or modify
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# it under the terms of the GNU General Public License as published by
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# the Free Software Foundation; either version 2 of the License, or
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# (at your option) any later version.
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# This program is distributed in the hope that it will be useful,
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# but WITHOUT ANY WARRANTY; without even the implied warranty of
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# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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# GNU General Public License for more details.
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# You should have received a copy of the GNU General Public License
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# along with this program; if not, write to the Free Software
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# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
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"""ChunkWriter: write compressed data out with a fixed upper bound."""
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from zlib import Z_FINISH, Z_SYNC_FLUSH
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class ChunkWriter(object):
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"""ChunkWriter allows writing of compressed data with a fixed size.
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If less data is supplied than fills a chunk, the chunk is padded with
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NULL bytes. If more data is supplied, then the writer packs as much
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in as it can, but never splits any item it was given.
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The algorithm for packing is open to improvement! Current it is:
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- write the bytes given
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- if the total seen bytes so far exceeds the chunk size, flush.
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:cvar _max_repack: To fit the maximum number of entries into a node, we
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will sometimes start over and compress the whole list to get tighter
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packing. We get diminishing returns after a while, so this limits the
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number of times we will try.
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The default is to try to avoid recompressing entirely, but setting this
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to something like 20 will give maximum compression.
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:cvar _max_zsync: Another tunable nob. If _max_repack is set to 0, then you
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can limit the number of times we will try to pack more data into a
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node. This allows us to do a single compression pass, rather than
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trying until we overflow, and then recompressing again.
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# In testing, some values for bzr.dev::
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# repack time MB max full
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# zsync time MB repack stop_for_z
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# In testing, some values for mysql-unpacked::
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# repack time MB full stop_for_repack
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# 0 29.5 116.5 0 29782
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# 10 29.4 18.6 195 4526
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# 11 29.2 18.0 421 4143
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# 12 28.0 17.5 702 3738
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# 15 28.9 16.5 1223 2969
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# 20 29.6 15.7 2182 1810
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# 30 31.4 15.4 3891 23
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# Tuple of (num_repack_attempts, num_zsync_attempts)
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# num_zsync_attempts only has meaning if num_repack_attempts is 0.
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_repack_opts_for_speed = (0, 8)
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_repack_opts_for_size = (20, 0)
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def __init__(self, chunk_size, reserved=0, optimize_for_size=False):
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"""Create a ChunkWriter to write chunk_size chunks.
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:param chunk_size: The total byte count to emit at the end of the
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:param reserved: How many bytes to allow for reserved data. reserved
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data space can only be written to via the write(..., reserved=True).
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self.chunk_size = chunk_size
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self.compressor = zlib.compressobj()
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self.bytes_out_len = 0
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# bytes that have been seen, but not included in a flush to out yet
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self.unflushed_in_bytes = 0
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self.unused_bytes = None
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self.reserved_size = reserved
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# Default is to make building fast rather than compact
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self.set_optimize(for_size=optimize_for_size)
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This returns the final compressed chunk, and either None, or the
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bytes that did not fit in the chunk.
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:return: (compressed_bytes, unused_bytes, num_nulls_needed)
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* compressed_bytes: a list of bytes that were output from the
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compressor. If the compressed length was not exactly chunk_size,
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the final string will be a string of all null bytes to pad this
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* unused_bytes: None, or the last bytes that were added, which we
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* num_nulls_needed: How many nulls are padded at the end
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self.bytes_in = None # Free the data cached so far, we don't need it
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out = self.compressor.flush(Z_FINISH)
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self.bytes_list.append(out)
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self.bytes_out_len += len(out)
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if self.bytes_out_len > self.chunk_size:
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raise AssertionError('Somehow we ended up with too much'
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' compressed data, %d > %d'
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% (self.bytes_out_len, self.chunk_size))
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nulls_needed = self.chunk_size - self.bytes_out_len
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self.bytes_list.append("\x00" * nulls_needed)
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return self.bytes_list, self.unused_bytes, nulls_needed
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def set_optimize(self, for_size=True):
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"""Change how we optimize our writes.
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:param for_size: If True, optimize for minimum space usage, otherwise
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optimize for fastest writing speed.
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opts = ChunkWriter._repack_opts_for_size
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opts = ChunkWriter._repack_opts_for_speed
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self._max_repack, self._max_zsync = opts
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def _recompress_all_bytes_in(self, extra_bytes=None):
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"""Recompress the current bytes_in, and optionally more.
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:param extra_bytes: Optional, if supplied we will add it with
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:return: (bytes_out, bytes_out_len, alt_compressed)
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* bytes_out: is the compressed bytes returned from the compressor
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* bytes_out_len: the length of the compressed output
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* compressor: An object with everything packed in so far, and
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compressor = zlib.compressobj()
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append = bytes_out.append
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compress = compressor.compress
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for accepted_bytes in self.bytes_in:
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out = compress(accepted_bytes)
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out = compress(extra_bytes)
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out += compressor.flush(Z_SYNC_FLUSH)
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bytes_out_len = sum(map(len, bytes_out))
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return bytes_out, bytes_out_len, compressor
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def write(self, bytes, reserved=False):
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"""Write some bytes to the chunk.
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If the bytes fit, False is returned. Otherwise True is returned
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and the bytes have not been added to the chunk.
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:param bytes: The bytes to include
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:param reserved: If True, we can use the space reserved in the
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if self.num_repack > self._max_repack and not reserved:
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self.unused_bytes = bytes
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capacity = self.chunk_size
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capacity = self.chunk_size - self.reserved_size
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comp = self.compressor
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# Check to see if the currently unflushed bytes would fit with a bit of
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# room to spare, assuming no compression.
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next_unflushed = self.unflushed_in_bytes + len(bytes)
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remaining_capacity = capacity - self.bytes_out_len - 10
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if (next_unflushed < remaining_capacity):
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# looks like it will fit
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out = comp.compress(bytes)
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self.bytes_list.append(out)
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self.bytes_out_len += len(out)
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self.bytes_in.append(bytes)
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self.unflushed_in_bytes += len(bytes)
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# This may or may not fit, try to add it with Z_SYNC_FLUSH
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# Note: It is tempting to do this as a look-ahead pass, and to
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# 'copy()' the compressor before flushing. However, it seems
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# that Which means that it is the same thing as increasing
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# repack, similar cost, same benefit. And this way we still
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# have the 'repack' knob that can be adjusted, and not depend
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# on a platform-specific 'copy()' function.
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if self._max_repack == 0 and self.num_zsync > self._max_zsync:
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self.unused_bytes = bytes
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out = comp.compress(bytes)
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out += comp.flush(Z_SYNC_FLUSH)
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self.unflushed_in_bytes = 0
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self.bytes_list.append(out)
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self.bytes_out_len += len(out)
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# We are a bit extra conservative, because it seems that you *can*
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# get better compression with Z_SYNC_FLUSH than a full compress. It
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# is probably very rare, but we were able to trigger it.
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if self.num_repack == 0:
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if self.bytes_out_len + safety_margin <= capacity:
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# It fit, so mark it added
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self.bytes_in.append(bytes)
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# We are over budget, try to squeeze this in without any
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(bytes_out, this_len,
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compressor) = self._recompress_all_bytes_in(bytes)
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if self.num_repack >= self._max_repack:
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# When we get *to* _max_repack, bump over so that the
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# earlier > _max_repack will be triggered.
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if this_len + 10 > capacity:
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(bytes_out, this_len,
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compressor) = self._recompress_all_bytes_in()
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self.compressor = compressor
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# Force us to not allow more data
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self.num_repack = self._max_repack + 1
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self.bytes_list = bytes_out
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self.bytes_out_len = this_len
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self.unused_bytes = bytes
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# This fits when we pack it tighter, so use the new packing
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self.compressor = compressor
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self.bytes_in.append(bytes)
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self.bytes_list = bytes_out
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self.bytes_out_len = this_len
added
removed
bzrlib/chunk_writer.py
