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Fix ExpiringCache.__len__ to be accurate 

It used to try and produce an estimate, which was sometimes negative.
This caused metrics to be sad, so lets always just calculate it from
scratch.

(This appears to have been a longstanding bug, but one which has been made more
of a problem by #3932 and #3933).

(This was originally done by Erik as part of #3933. I'm cherry-picking it
because really it's a fix in its own right)
This commit is contained in:
Erik Johnston 2018-09-21 16:25:42 +01:00 committed by Richard van der Hoff
parent f65163627f
commit 3baf6e1667
1 changed files with 7 additions and 10 deletions
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17
synapse/util/caches/expiringcache.py
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@ -16,6 +16,8 @@
import logging import logging
from collections import OrderedDict from collections import OrderedDict
from six import itervalues
from synapse.metrics.background_process_metrics import run_as_background_process from synapse.metrics.background_process_metrics import run_as_background_process
from synapse.util.caches import register_cache from synapse.util.caches import register_cache
@ -54,8 +56,6 @@ class ExpiringCache(object):
self.iterable = iterable self.iterable = iterable
self._size_estimate = 0
self.metrics = register_cache("expiring", cache_name, self) self.metrics = register_cache("expiring", cache_name, self)
if not self._expiry_ms: if not self._expiry_ms:
@ -74,16 +74,11 @@ class ExpiringCache(object):
now = self._clock.time_msec() now = self._clock.time_msec()
self._cache[key] = _CacheEntry(now, value) self._cache[key] = _CacheEntry(now, value)
if self.iterable:
self._size_estimate += len(value)
# Evict if there are now too many items # Evict if there are now too many items
while self._max_len and len(self) > self._max_len: while self._max_len and len(self) > self._max_len:
_key, value = self._cache.popitem(last=False) _key, value = self._cache.popitem(last=False)
if self.iterable: if self.iterable:
removed_len = len(value.value) self.metrics.inc_evictions(len(value.value))
self.metrics.inc_evictions(removed_len)
self._size_estimate -= removed_len
else: else:
self.metrics.inc_evictions() self.metrics.inc_evictions()
@ -134,7 +129,9 @@ class ExpiringCache(object):
for k in keys_to_delete: for k in keys_to_delete:
value = self._cache.pop(k) value = self._cache.pop(k)
if self.iterable: if self.iterable:
self._size_estimate -= len(value.value) self.metrics.inc_evictions(len(value.value))
else:
self.metrics.inc_evictions()
logger.debug( logger.debug(
"[%s] _prune_cache before: %d, after len: %d", "[%s] _prune_cache before: %d, after len: %d",
@ -143,7 +140,7 @@ class ExpiringCache(object):
def __len__(self): def __len__(self):
if self.iterable: if self.iterable:
return self._size_estimate return sum(len(entry.value) for entry in itervalues(self._cache))
else: else:
return len(self._cache) return len(self._cache)