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# Copyright (C) 2008 Canonical Ltd
#
# This program is free software; you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation; either version 2 of the License, or
# (at your option) any later version.
#
# 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 Street, Fifth Floor, Boston, MA 02110-1301 USA
#
"""ChunkWriter: write compressed data out with a fixed upper bound."""
from __future__ import absolute_import
import zlib
from zlib import Z_FINISH, Z_SYNC_FLUSH
class ChunkWriter(object):
"""ChunkWriter allows writing of compressed data with a fixed size.
If less data is supplied than fills a chunk, the chunk is padded with
NULL bytes. If more data is supplied, then the writer packs as much
in as it can, but never splits any item it was given.
The algorithm for packing is open to improvement! Current it is:
- write the bytes given
- if the total seen bytes so far exceeds the chunk size, flush.
:cvar _max_repack: To fit the maximum number of entries into a node, we
will sometimes start over and compress the whole list to get tighter
packing. We get diminishing returns after a while, so this limits the
number of times we will try.
The default is to try to avoid recompressing entirely, but setting this
to something like 20 will give maximum compression.
:cvar _max_zsync: Another tunable nob. If _max_repack is set to 0, then you
can limit the number of times we will try to pack more data into a
node. This allows us to do a single compression pass, rather than
trying until we overflow, and then recompressing again.
"""
# In testing, some values for bzr.dev::
# repack time MB max full
# 1 7.5 4.6 1140 0
# 2 8.4 4.2 1036 1
# 3 9.8 4.1 1012 278
# 4 10.8 4.1 728 945
# 20 11.1 4.1 0 1012
# repack = 0
# zsync time MB repack stop_for_z
# 0 5.0 24.7 0 6270
# 1 4.3 13.2 0 3342
# 2 4.9 9.6 0 2414
# 5 4.8 6.2 0 1549
# 6 4.8 5.8 1 1435
# 7 4.8 5.5 19 1337
# 8 4.4 5.3 81 1220
# 10 5.3 5.0 260 967
# 11 5.3 4.9 366 839
# 12 5.1 4.8 454 731
# 15 5.8 4.7 704 450
# 20 5.8 4.6 1133 7
# In testing, some values for mysql-unpacked::
# next_bytes estim
# repack time MB full stop_for_repack
# 1 15.4 0 3913
# 2 35.4 13.7 0 346
# 20 46.7 13.4 3380 0
# repack=0
# zsync stop_for_z
# 0 29.5 116.5 0 29782
# 1 27.8 60.2 0 15356
# 2 27.8 42.4 0 10822
# 5 26.8 25.5 0 6491
# 6 27.3 23.2 13 5896
# 7 27.5 21.6 29 5451
# 8 27.1 20.3 52 5108
# 10 29.4 18.6 195 4526
# 11 29.2 18.0 421 4143
# 12 28.0 17.5 702 3738
# 15 28.9 16.5 1223 2969
# 20 29.6 15.7 2182 1810
# 30 31.4 15.4 3891 23
# Tuple of (num_repack_attempts, num_zsync_attempts)
# num_zsync_attempts only has meaning if num_repack_attempts is 0.
_repack_opts_for_speed = (0, 8)
_repack_opts_for_size = (20, 0)
def __init__(self, chunk_size, reserved=0, optimize_for_size=False):
"""Create a ChunkWriter to write chunk_size chunks.
:param chunk_size: The total byte count to emit at the end of the
chunk.
:param reserved: How many bytes to allow for reserved data. reserved
data space can only be written to via the write(..., reserved=True).
"""
self.chunk_size = chunk_size
self.compressor = zlib.compressobj()
self.bytes_in = []
self.bytes_list = []
self.bytes_out_len = 0
# bytes that have been seen, but not included in a flush to out yet
self.unflushed_in_bytes = 0
self.num_repack = 0
self.num_zsync = 0
self.unused_bytes = None
self.reserved_size = reserved
# Default is to make building fast rather than compact
self.set_optimize(for_size=optimize_for_size)
def finish(self):
"""Finish the chunk.
This returns the final compressed chunk, and either None, or the
bytes that did not fit in the chunk.
:return: (compressed_bytes, unused_bytes, num_nulls_needed)
* compressed_bytes: a list of bytes that were output from the
compressor. If the compressed length was not exactly chunk_size,
the final string will be a string of all null bytes to pad this
to chunk_size
* unused_bytes: None, or the last bytes that were added, which we
could not fit.
* num_nulls_needed: How many nulls are padded at the end
"""
self.bytes_in = None # Free the data cached so far, we don't need it
out = self.compressor.flush(Z_FINISH)
self.bytes_list.append(out)
self.bytes_out_len += len(out)
if self.bytes_out_len > self.chunk_size:
raise AssertionError('Somehow we ended up with too much'
' compressed data, %d > %d'
% (self.bytes_out_len, self.chunk_size))
nulls_needed = self.chunk_size - self.bytes_out_len
if nulls_needed:
self.bytes_list.append("\x00" * nulls_needed)
return self.bytes_list, self.unused_bytes, nulls_needed
def set_optimize(self, for_size=True):
"""Change how we optimize our writes.
:param for_size: If True, optimize for minimum space usage, otherwise
optimize for fastest writing speed.
:return: None
"""
if for_size:
opts = ChunkWriter._repack_opts_for_size
else:
opts = ChunkWriter._repack_opts_for_speed
self._max_repack, self._max_zsync = opts
def _recompress_all_bytes_in(self, extra_bytes=None):
"""Recompress the current bytes_in, and optionally more.
:param extra_bytes: Optional, if supplied we will add it with
Z_SYNC_FLUSH
:return: (bytes_out, bytes_out_len, alt_compressed)
* bytes_out: is the compressed bytes returned from the compressor
* bytes_out_len: the length of the compressed output
* compressor: An object with everything packed in so far, and
Z_SYNC_FLUSH called.
"""
compressor = zlib.compressobj()
bytes_out = []
append = bytes_out.append
compress = compressor.compress
for accepted_bytes in self.bytes_in:
out = compress(accepted_bytes)
if out:
append(out)
if extra_bytes:
out = compress(extra_bytes)
out += compressor.flush(Z_SYNC_FLUSH)
append(out)
bytes_out_len = sum(map(len, bytes_out))
return bytes_out, bytes_out_len, compressor
def write(self, bytes, reserved=False):
"""Write some bytes to the chunk.
If the bytes fit, False is returned. Otherwise True is returned
and the bytes have not been added to the chunk.
:param bytes: The bytes to include
:param reserved: If True, we can use the space reserved in the
constructor.
"""
if self.num_repack > self._max_repack and not reserved:
self.unused_bytes = bytes
return True
if reserved:
capacity = self.chunk_size
else:
capacity = self.chunk_size - self.reserved_size
comp = self.compressor
# Check to see if the currently unflushed bytes would fit with a bit of
# room to spare, assuming no compression.
next_unflushed = self.unflushed_in_bytes + len(bytes)
remaining_capacity = capacity - self.bytes_out_len - 10
if (next_unflushed < remaining_capacity):
# looks like it will fit
out = comp.compress(bytes)
if out:
self.bytes_list.append(out)
self.bytes_out_len += len(out)
self.bytes_in.append(bytes)
self.unflushed_in_bytes += len(bytes)
else:
# This may or may not fit, try to add it with Z_SYNC_FLUSH
# Note: It is tempting to do this as a look-ahead pass, and to
# 'copy()' the compressor before flushing. However, it seems
# that Which means that it is the same thing as increasing
# repack, similar cost, same benefit. And this way we still
# have the 'repack' knob that can be adjusted, and not depend
# on a platform-specific 'copy()' function.
self.num_zsync += 1
if self._max_repack == 0 and self.num_zsync > self._max_zsync:
self.num_repack += 1
self.unused_bytes = bytes
return True
out = comp.compress(bytes)
out += comp.flush(Z_SYNC_FLUSH)
self.unflushed_in_bytes = 0
if out:
self.bytes_list.append(out)
self.bytes_out_len += len(out)
# We are a bit extra conservative, because it seems that you *can*
# get better compression with Z_SYNC_FLUSH than a full compress. It
# is probably very rare, but we were able to trigger it.
if self.num_repack == 0:
safety_margin = 100
else:
safety_margin = 10
if self.bytes_out_len + safety_margin <= capacity:
# It fit, so mark it added
self.bytes_in.append(bytes)
else:
# We are over budget, try to squeeze this in without any
# Z_SYNC_FLUSH calls
self.num_repack += 1
(bytes_out, this_len,
compressor) = self._recompress_all_bytes_in(bytes)
if self.num_repack >= self._max_repack:
# When we get *to* _max_repack, bump over so that the
# earlier > _max_repack will be triggered.
self.num_repack += 1
if this_len + 10 > capacity:
(bytes_out, this_len,
compressor) = self._recompress_all_bytes_in()
self.compressor = compressor
# Force us to not allow more data
self.num_repack = self._max_repack + 1
self.bytes_list = bytes_out
self.bytes_out_len = this_len
self.unused_bytes = bytes
return True
else:
# This fits when we pack it tighter, so use the new packing
self.compressor = compressor
self.bytes_in.append(bytes)
self.bytes_list = bytes_out
self.bytes_out_len = this_len
return False
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