mirror of
https://github.com/borgbackup/borg.git
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103 lines
2.8 KiB
Python
103 lines
2.8 KiB
Python
"""
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Compute hashtable sizes with nices properties
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- prime sizes (for small to medium sizes)
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- 2 prime-factor sizes (for big sizes)
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- fast growth for small sizes
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- slow growth for big sizes
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Note:
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this is just a tool for developers.
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within borgbackup, it is just used to generate hash_sizes definition for _hashindex.c.
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"""
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from collections import namedtuple
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K, M, G = 2**10, 2**20, 2**30
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# hash table size (in number of buckets)
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start, end_p1, end_p2 = 1 * K, 127 * M, 2 * G - 10 * M # stay well below 2^31 - 1
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Policy = namedtuple("Policy", "upto grow")
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policies = [
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# which growth factor to use when growing a hashtable of size < upto
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# grow fast (*2.0) at the start so we do not have to resize too often (expensive).
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# grow slow (*1.1) for huge hash tables (do not jump too much in memory usage)
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Policy(256*K, 2.0),
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Policy(2*M, 1.7),
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Policy(16*M, 1.4),
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Policy(128*M, 1.2),
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Policy(2*G-1, 1.1),
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]
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# slightly modified version of:
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# http://www.macdevcenter.com/pub/a/python/excerpt/pythonckbk_chap1/index1.html?page=2
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def eratosthenes():
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"""Yields the sequence of prime numbers via the Sieve of Eratosthenes."""
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D = {} # map each composite integer to its first-found prime factor
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q = 2 # q gets 2, 3, 4, 5, ... ad infinitum
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while True:
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p = D.pop(q, None)
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if p is None:
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# q not a key in D, so q is prime, therefore, yield it
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yield q
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# mark q squared as not-prime (with q as first-found prime factor)
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D[q * q] = q
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else:
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# let x <- smallest (N*p)+q which wasn't yet known to be composite
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# we just learned x is composite, with p first-found prime factor,
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# since p is the first-found prime factor of q -- find and mark it
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x = p + q
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while x in D:
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x += p
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D[x] = p
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q += 1
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def two_prime_factors(pfix=65537):
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"""Yields numbers with 2 prime factors pfix and p."""
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for p in eratosthenes():
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yield pfix * p
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def get_grow_factor(size):
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for p in policies:
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if size < p.upto:
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return p.grow
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def find_bigger_prime(gen, i):
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while True:
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p = next(gen)
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if p >= i:
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return p
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def main():
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sizes = []
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i = start
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gen = eratosthenes()
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while i < end_p1:
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grow_factor = get_grow_factor(i)
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p = find_bigger_prime(gen, i)
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sizes.append(p)
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i = int(i * grow_factor)
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gen = two_prime_factors() # for lower ram consumption
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while i < end_p2:
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grow_factor = get_grow_factor(i)
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p = find_bigger_prime(gen, i)
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sizes.append(p)
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i = int(i * grow_factor)
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print("""\
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static int hash_sizes[] = {
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%s
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};
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""" % ', '.join(str(size) for size in sizes))
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if __name__ == '__main__':
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main()
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