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Fix ExpiringCache.__len__ to be accurate

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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)