mirror of
https://github.com/borgbackup/borg.git
synced 2024-12-27 02:08:54 +00:00
103 lines
2.8 KiB
Python
103 lines
2.8 KiB
Python
"""
|
|
Compute hashtable sizes with nices properties
|
|
- prime sizes (for small to medium sizes)
|
|
- 2 prime-factor sizes (for big sizes)
|
|
- fast growth for small sizes
|
|
- slow growth for big sizes
|
|
|
|
Note:
|
|
this is just a tool for developers.
|
|
within borgbackup, it is just used to generate hash_sizes definition for _hashindex.c.
|
|
"""
|
|
|
|
from collections import namedtuple
|
|
|
|
K, M, G = 2**10, 2**20, 2**30
|
|
|
|
# hash table size (in number of buckets)
|
|
start, end_p1, end_p2 = 1 * K, 127 * M, 2 * G - 10 * M # stay well below 2^31 - 1
|
|
|
|
Policy = namedtuple("Policy", "upto grow")
|
|
|
|
policies = [
|
|
# which growth factor to use when growing a hashtable of size < upto
|
|
# grow fast (*2.0) at the start so we do not have to resize too often (expensive).
|
|
# grow slow (*1.1) for huge hash tables (do not jump too much in memory usage)
|
|
Policy(256*K, 2.0),
|
|
Policy(2*M, 1.7),
|
|
Policy(16*M, 1.4),
|
|
Policy(128*M, 1.2),
|
|
Policy(2*G-1, 1.1),
|
|
]
|
|
|
|
|
|
# slightly modified version of:
|
|
# http://www.macdevcenter.com/pub/a/python/excerpt/pythonckbk_chap1/index1.html?page=2
|
|
def eratosthenes():
|
|
"""Yields the sequence of prime numbers via the Sieve of Eratosthenes."""
|
|
D = {} # map each composite integer to its first-found prime factor
|
|
q = 2 # q gets 2, 3, 4, 5, ... ad infinitum
|
|
while True:
|
|
p = D.pop(q, None)
|
|
if p is None:
|
|
# q not a key in D, so q is prime, therefore, yield it
|
|
yield q
|
|
# mark q squared as not-prime (with q as first-found prime factor)
|
|
D[q * q] = q
|
|
else:
|
|
# let x <- smallest (N*p)+q which wasn't yet known to be composite
|
|
# we just learned x is composite, with p first-found prime factor,
|
|
# since p is the first-found prime factor of q -- find and mark it
|
|
x = p + q
|
|
while x in D:
|
|
x += p
|
|
D[x] = p
|
|
q += 1
|
|
|
|
|
|
def two_prime_factors(pfix=65537):
|
|
"""Yields numbers with 2 prime factors pfix and p."""
|
|
for p in eratosthenes():
|
|
yield pfix * p
|
|
|
|
|
|
def get_grow_factor(size):
|
|
for p in policies:
|
|
if size < p.upto:
|
|
return p.grow
|
|
|
|
|
|
def find_bigger_prime(gen, i):
|
|
while True:
|
|
p = next(gen)
|
|
if p >= i:
|
|
return p
|
|
|
|
|
|
def main():
|
|
sizes = []
|
|
i = start
|
|
|
|
gen = eratosthenes()
|
|
while i < end_p1:
|
|
grow_factor = get_grow_factor(i)
|
|
p = find_bigger_prime(gen, i)
|
|
sizes.append(p)
|
|
i = int(i * grow_factor)
|
|
|
|
gen = two_prime_factors() # for lower ram consumption
|
|
while i < end_p2:
|
|
grow_factor = get_grow_factor(i)
|
|
p = find_bigger_prime(gen, i)
|
|
sizes.append(p)
|
|
i = int(i * grow_factor)
|
|
|
|
print("""\
|
|
static int hash_sizes[] = {
|
|
%s
|
|
};
|
|
""" % ', '.join(str(size) for size in sizes))
|
|
|
|
|
|
if __name__ == '__main__':
|
|
main()
|