1 """Discussion of bloom constants for bup:
3 There are four basic things to consider when building a bloom filter:
4 The size, in bits, of the filter
5 The capacity, in entries, of the filter
6 The probability of a false positive that is tolerable
7 The number of bits readily available to use for addressing filter bits
9 There is one major tunable that is not directly related to the above:
10 k: the number of bits set in the filter per entry
12 Here's a wall of numbers showing the relationship between k; the ratio between
13 the filter size in bits and the entries in the filter; and pfalse_positive:
15 mn|k=3 |k=4 |k=5 |k=6 |k=7 |k=8 |k=9 |k=10 |k=11
16 8|3.05794|2.39687|2.16792|2.15771|2.29297|2.54917|2.92244|3.41909|4.05091
17 9|2.27780|1.65770|1.40703|1.32721|1.34892|1.44631|1.61138|1.84491|2.15259
18 10|1.74106|1.18133|0.94309|0.84362|0.81937|0.84555|0.91270|1.01859|1.16495
19 11|1.36005|0.86373|0.65018|0.55222|0.51259|0.50864|0.53098|0.57616|0.64387
20 12|1.08231|0.64568|0.45945|0.37108|0.32939|0.31424|0.31695|0.33387|0.36380
21 13|0.87517|0.49210|0.33183|0.25527|0.21689|0.19897|0.19384|0.19804|0.21013
22 14|0.71759|0.38147|0.24433|0.17934|0.14601|0.12887|0.12127|0.12012|0.12399
23 15|0.59562|0.30019|0.18303|0.12840|0.10028|0.08523|0.07749|0.07440|0.07468
24 16|0.49977|0.23941|0.13925|0.09351|0.07015|0.05745|0.05049|0.04700|0.04587
25 17|0.42340|0.19323|0.10742|0.06916|0.04990|0.03941|0.03350|0.03024|0.02870
26 18|0.36181|0.15765|0.08392|0.05188|0.03604|0.02748|0.02260|0.01980|0.01827
27 19|0.31160|0.12989|0.06632|0.03942|0.02640|0.01945|0.01549|0.01317|0.01182
28 20|0.27026|0.10797|0.05296|0.03031|0.01959|0.01396|0.01077|0.00889|0.00777
29 21|0.23591|0.09048|0.04269|0.02356|0.01471|0.01014|0.00759|0.00609|0.00518
30 22|0.20714|0.07639|0.03473|0.01850|0.01117|0.00746|0.00542|0.00423|0.00350
31 23|0.18287|0.06493|0.02847|0.01466|0.00856|0.00555|0.00392|0.00297|0.00240
32 24|0.16224|0.05554|0.02352|0.01171|0.00663|0.00417|0.00286|0.00211|0.00166
33 25|0.14459|0.04779|0.01957|0.00944|0.00518|0.00316|0.00211|0.00152|0.00116
34 26|0.12942|0.04135|0.01639|0.00766|0.00408|0.00242|0.00157|0.00110|0.00082
35 27|0.11629|0.03595|0.01381|0.00626|0.00324|0.00187|0.00118|0.00081|0.00059
36 28|0.10489|0.03141|0.01170|0.00515|0.00259|0.00146|0.00090|0.00060|0.00043
37 29|0.09492|0.02756|0.00996|0.00426|0.00209|0.00114|0.00069|0.00045|0.00031
38 30|0.08618|0.02428|0.00853|0.00355|0.00169|0.00090|0.00053|0.00034|0.00023
39 31|0.07848|0.02147|0.00733|0.00297|0.00138|0.00072|0.00041|0.00025|0.00017
40 32|0.07167|0.01906|0.00633|0.00250|0.00113|0.00057|0.00032|0.00019|0.00013
42 Here's a table showing available repository size for a given pfalse_positive
43 and three values of k (assuming we only use the 160 bit SHA1 for addressing the
44 filter and 8192bytes per object):
46 pfalse|obj k=4 |cap k=4 |obj k=5 |cap k=5 |obj k=6 |cap k=6
47 2.500%|139333497228|1038.11 TiB|558711157|4262.63 GiB|13815755|105.41 GiB
48 1.000%|104489450934| 778.50 TiB|436090254|3327.10 GiB|11077519| 84.51 GiB
49 0.125%| 57254889824| 426.58 TiB|261732190|1996.86 GiB| 7063017| 55.89 GiB
51 This eliminates pretty neatly any k>6 as long as we use the raw SHA for
54 filter size scales linearly with repository size for a given k and pfalse.
56 Here's a table of filter sizes for a 1 TiB repository:
58 pfalse| k=3 | k=4 | k=5 | k=6
59 2.500%| 138.78 MiB | 126.26 MiB | 123.00 MiB | 123.37 MiB
60 1.000%| 197.83 MiB | 168.36 MiB | 157.58 MiB | 153.87 MiB
61 0.125%| 421.14 MiB | 307.26 MiB | 262.56 MiB | 241.32 MiB
64 * We want the bloom filter to fit in memory; if it doesn't, the k pagefaults
65 per lookup will be worse than the two required for midx.
66 * We want the pfalse_positive to be low enough that the cost of sometimes
67 faulting on the midx doesn't overcome the benefit of the bloom filter.
68 * We have readily available 160 bits for addressing the filter.
69 * We want to be able to have a single bloom address entire repositories of
72 Based on these parameters, a combination of k=4 and k=5 provides the behavior
73 that bup needs. As such, I've implemented bloom addressing, adding and
74 checking functions in C for these two values. Because k=5 requires less space
75 and gives better overall pfalse_positive performance, it is preferred if a
76 table with k=5 can represent the repository.
78 None of this tells us what max_pfalse_positive to choose.
80 Brandon Low <lostlogic@lostlogicx.com> 2011-02-04
83 from __future__ import absolute_import
84 import os, math, struct
86 from bup import _helpers
87 from bup.compat import pending_raise
88 from bup.helpers import (debug1, debug2, log, mmap_read, mmap_readwrite,
89 mmap_readwrite_private, unlink)
93 MAX_BITS_EACH = 32 # Kinda arbitrary, but 4 bytes per entry is pretty big
94 MAX_BLOOM_BITS = {4: 37, 5: 29} # 160/k-log2(8)
95 MAX_PFALSE_POSITIVE = 1. # Totally arbitrary, needs benchmarking
100 bloom_contains = _helpers.bloom_contains
101 bloom_add = _helpers.bloom_add
103 # FIXME: check bloom create() and ShaBloom handling/ownership of "f".
104 # The ownership semantics should be clarified since the caller needs
105 # to know who is responsible for closing it.
108 """Wrapper which contains data from multiple index files. """
109 def __init__(self, filename, f=None, readwrite=False, expected=-1):
112 self.readwrite = readwrite
115 assert(filename.endswith(b'.bloom'))
118 self.file = f = f or open(filename, 'r+b')
121 # Decide if we want to mmap() the pages as writable ('immediate'
122 # write) or else map them privately for later writing back to
123 # the file ('delayed' write). A bloom table's write access
124 # pattern is such that we dirty almost all the pages after adding
125 # very few entries. But the table is so big that dirtying
126 # *all* the pages often exceeds Linux's default
127 # /proc/sys/vm/dirty_ratio or /proc/sys/vm/dirty_background_ratio,
128 # thus causing it to start flushing the table before we're
129 # finished... even though there's more than enough space to
130 # store the bloom table in RAM.
132 # To work around that behaviour, if we calculate that we'll
133 # probably end up touching the whole table anyway (at least
134 # one bit flipped per memory page), let's use a "private" mmap,
135 # which defeats Linux's ability to flush it to disk. Then we'll
136 # flush it as one big lump during close().
137 pages = os.fstat(f.fileno()).st_size // 4096 * 5 # assume k=5
138 self.delaywrite = expected > pages
139 debug1('bloom: delaywrite=%r\n' % self.delaywrite)
141 self.map = mmap_readwrite_private(self.file, close=False)
143 self.map = mmap_readwrite(self.file, close=False)
145 self.file = f or open(filename, 'rb')
146 self.map = mmap_read(self.file)
149 log('Warning: invalid BLOM header (%r) in %r\n' % (got, filename))
152 ver = struct.unpack('!I', self.map[4:8])[0]
153 if ver < BLOOM_VERSION:
154 log('Warning: ignoring old-style (v%d) bloom %r\n'
158 if ver > BLOOM_VERSION:
159 log('Warning: ignoring too-new (v%d) bloom %r\n'
164 self.bits, self.k, self.entries = struct.unpack('!HHI', self.map[8:16])
165 idxnamestr = self.map[16 + 2**self.bits:]
167 self.idxnames = idxnamestr.split(b'\0')
171 def _init_failed(self):
173 self.bits = self.entries = 0
174 self.map, tmp_map = None, self.map
175 self.file, tmp_file = None, self.file
179 finally: # This won't handle pending exceptions correctly in py2
184 return self.map and self.bits
189 if self.map and self.readwrite:
190 debug2("bloom: closing with %d entries\n" % self.entries)
191 self.map[12:16] = struct.pack('!I', self.entries)
194 self.file.write(self.map)
197 self.file.seek(16 + 2**self.bits)
199 self.file.write(b'\0'.join(self.idxnames))
200 finally: # This won't handle pending exceptions correctly in py2
209 def __exit__(self, type, value, traceback):
210 with pending_raise(value, rethrow=False):
213 def pfalse_positive(self, additional=0):
214 n = self.entries + additional
217 return 100*(1-math.exp(-k*float(n)/m))**k
220 """Add the hashes in ids (packed binary 20-bytes) to the filter."""
222 raise Exception("Cannot add to closed bloom")
223 self.entries += bloom_add(self.map, ids, self.bits, self.k)
225 def add_idx(self, ix):
226 """Add the object to the filter."""
227 self.add(ix.shatable)
228 self.idxnames.append(os.path.basename(ix.name))
230 def exists(self, sha):
231 """Return nonempty if the object probably exists in the bloom filter.
233 If this function returns false, the object definitely does not exist.
234 If it returns true, there is a small probability that it exists
235 anyway, so you'll have to check it some other way.
237 global _total_searches, _total_steps
241 found, steps = bloom_contains(self.map, sha, self.bits, self.k)
242 _total_steps += steps
246 return int(self.entries)
249 def create(name, expected, delaywrite=None, f=None, k=None):
250 """Create and return a bloom filter for `expected` entries."""
251 bits = int(math.floor(math.log(expected * MAX_BITS_EACH // 8, 2)))
252 k = k or ((bits <= MAX_BLOOM_BITS[5]) and 5 or 4)
253 if bits > MAX_BLOOM_BITS[k]:
254 log('bloom: warning, max bits exceeded, non-optimal\n')
255 bits = MAX_BLOOM_BITS[k]
256 debug1('bloom: using 2^%d bytes and %d hash functions\n' % (bits, k))
257 f = f or open(name, 'w+b')
259 f.write(struct.pack('!IHHI', BLOOM_VERSION, bits, k, 0))
260 assert(f.tell() == 16)
261 # NOTE: On some systems this will not extend+zerofill, but it does on
262 # darwin, linux, bsd and solaris.
263 f.truncate(16+2**bits)
265 if delaywrite != None and not delaywrite:
266 # tell it to expect very few objects, forcing a direct mmap
268 return ShaBloom(name, f=f, readwrite=True, expected=expected)
271 def clear_bloom(dir):
272 unlink(os.path.join(dir, b'bup.bloom'))