import json
import mmap
import os
import struct
_INITIAL_MMAP_SIZE = 1 << 20
_pack_integer_func = struct.Struct(b'i').pack
_pack_double_func = struct.Struct(b'd').pack
_unpack_integer = struct.Struct(b'i').unpack_from
_unpack_double = struct.Struct(b'd').unpack_from
# struct.pack_into has atomicity issues because it will temporarily write 0 into
# the mmap, resulting in false reads to 0 when experiencing a lot of writes.
# Using direct assignment solves this issue.
def _pack_double(data, pos, value):
data[pos:pos + 8] = _pack_double_func(value)
def _pack_integer(data, pos, value):
data[pos:pos + 4] = _pack_integer_func(value)
def _read_all_values(data, used=0):
"""Yield (key, value, pos). No locking is performed."""
if used <= 0:
# If not valid `used` value is passed in, read it from the file.
used = _unpack_integer(data, 0)[0]
pos = 8
while pos < used:
encoded_len = _unpack_integer(data, pos)[0]
# check we are not reading beyond bounds
if encoded_len + pos > used:
raise RuntimeError('Read beyond file size detected, file is corrupted.')
pos += 4
encoded_key = data[pos : pos + encoded_len]
padded_len = encoded_len + (8 - (encoded_len + 4) % 8)
pos += padded_len
value = _unpack_double(data, pos)[0]
yield encoded_key.decode('utf-8'), value, pos
pos += 8
class MmapedDict(object):
"""A dict of doubles, backed by an mmapped file.
The file starts with a 4 byte int, indicating how much of it is used.
Then 4 bytes of padding.
There's then a number of entries, consisting of a 4 byte int which is the
size of the next field, a utf-8 encoded string key, padding to a 8 byte
alignment, and then a 8 byte float which is the value.
Not thread safe.
"""
def __init__(self, filename, read_mode=False):
self._f = open(filename, 'rb' if read_mode else 'a+b')
self._fname = filename
capacity = os.fstat(self._f.fileno()).st_size
if capacity == 0:
self._f.truncate(_INITIAL_MMAP_SIZE)
capacity = _INITIAL_MMAP_SIZE
self._capacity = capacity
self._m = mmap.mmap(self._f.fileno(), self._capacity,
access=mmap.ACCESS_READ if read_mode else mmap.ACCESS_WRITE)
self._positions = {}
self._used = _unpack_integer(self._m, 0)[0]
if self._used == 0:
self._used = 8
_pack_integer(self._m, 0, self._used)
else:
if not read_mode:
for key, _, pos in self._read_all_values():
self._positions[key] = pos
@staticmethod
def read_all_values_from_file(filename):
with open(filename, 'rb') as infp:
# Read the first block of data, including the first 4 bytes which tell us
# how much of the file (which is preallocated to _INITIAL_MMAP_SIZE bytes) is occupied.
data = infp.read(mmap.PAGESIZE)
used = _unpack_integer(data, 0)[0]
if used > len(data): # Then read in the rest, if needed.
data += infp.read(used - len(data))
return _read_all_values(data, used)
def _init_value(self, key):
"""Initialize a value. Lock must be held by caller."""
encoded = key.encode('utf-8')
# Pad to be 8-byte aligned.
padded = encoded + (b' ' * (8 - (len(encoded) + 4) % 8))
value = struct.pack('i{0}sd'.format(len(padded)).encode(), len(encoded), padded, 0.0)
while self._used + len(value) > self._capacity:
self._capacity *= 2
self._f.truncate(self._capacity)
self._m = mmap.mmap(self._f.fileno(), self._capacity)
self._m[self._used:self._used + len(value)] = value
# Update how much space we've used.
self._used += len(value)
_pack_integer(self._m, 0, self._used)
self._positions[key] = self._used - 8
def _read_all_values(self):
"""Yield (key, value, pos). No locking is performed."""
return _read_all_values(data=self._m, used=self._used)
def read_all_values(self):
"""Yield (key, value). No locking is performed."""
for k, v, _ in self._read_all_values():
yield k, v
def read_value(self, key):
if key not in self._positions:
self._init_value(key)
pos = self._positions[key]
# We assume that reading from an 8 byte aligned value is atomic
return _unpack_double(self._m, pos)[0]
def write_value(self, key, value):
if key not in self._positions:
self._init_value(key)
pos = self._positions[key]
# We assume that writing to an 8 byte aligned value is atomic
_pack_double(self._m, pos, value)
def close(self):
if self._f:
self._m.close()
self._m = None
self._f.close()
self._f = None
def mmap_key(metric_name, name, labelnames, labelvalues):
"""Format a key for use in the mmap file."""
# ensure labels are in consistent order for identity
labels = dict(zip(labelnames, labelvalues))
return json.dumps([metric_name, name, labels], sort_keys=True)
