Reduce cache size by not storing deferreds

Currently the cache descriptors store deferreds rather than raw values,
this is a simple way of triggering only one database hit and sharing the
result if two callers attempt to get the same value.

However, there are a few caches that simply store a mapping from string
to string (or int). These caches can have a large number of entries,
under the assumption that each entry is small. However, the size of a
deferred (specifically the size of ObservableDeferred) is signigicantly
larger than that of the raw value, 2kb vs 32b.

This PR therefore changes the cache descriptors to store the raw values
rather than the deferreds.

As a side effect cached storage function now either return a deferred or
the actual value, as the cached list decriptor already does. This is
fine as we always end up just yield'ing on the returned value
eventually, which handles that case correctly.
This commit is contained in:
Erik Johnston 2017-04-25 10:23:11 +01:00
parent 7af825bae4
commit 119cb9bbcf
2 changed files with 28 additions and 22 deletions

View file

@ -47,10 +47,13 @@ class ReceiptsStore(SQLBaseStore):
# Returns an ObservableDeferred
res = self.get_users_with_read_receipts_in_room.cache.get((room_id,), None)
if res and res.called and user_id in res.result:
# We'd only be adding to the set, so no point invalidating if the
# user is already there
return
if res:
if isinstance(res, defer.Deferred) and res.called:
res = res.result
if user_id in res:
# We'd only be adding to the set, so no point invalidating if the
# user is already there
return
self.get_users_with_read_receipts_in_room.invalidate((room_id,))

View file

@ -19,7 +19,7 @@ from synapse.util import unwrapFirstError, logcontext
from synapse.util.caches.lrucache import LruCache
from synapse.util.caches.treecache import TreeCache, iterate_tree_cache_entry
from . import DEBUG_CACHES, register_cache
from . import register_cache
from twisted.internet import defer
from collections import namedtuple
@ -76,7 +76,7 @@ class Cache(object):
self.cache = LruCache(
max_size=max_entries, keylen=keylen, cache_type=cache_type,
size_callback=(lambda d: len(d.result)) if iterable else None,
size_callback=(lambda d: len(d)) if iterable else None,
)
self.name = name
@ -96,6 +96,17 @@ class Cache(object):
)
def get(self, key, default=_CacheSentinel, callback=None):
"""Looks the key up in the caches.
Args:
key(tuple)
default: What is returned if key is not in the caches. If not
specified then function throws KeyError instead
callback(fn): Gets called when the entry in the cache is invalidated
Returns:
Either a Deferred or the raw result
"""
callbacks = [callback] if callback else []
val = self._pending_deferred_cache.get(key, _CacheSentinel)
if val is not _CacheSentinel:
@ -137,7 +148,7 @@ class Cache(object):
if self.sequence == entry.sequence:
existing_entry = self._pending_deferred_cache.pop(key, None)
if existing_entry is entry:
self.cache.set(key, entry.deferred, entry.callbacks)
self.cache.set(key, result, entry.callbacks)
else:
entry.invalidate()
else:
@ -335,20 +346,10 @@ class CacheDescriptor(_CacheDescriptorBase):
try:
cached_result_d = cache.get(cache_key, callback=invalidate_callback)
observer = cached_result_d.observe()
if DEBUG_CACHES:
@defer.inlineCallbacks
def check_result(cached_result):
actual_result = yield self.function_to_call(obj, *args, **kwargs)
if actual_result != cached_result:
logger.error(
"Stale cache entry %s%r: cached: %r, actual %r",
self.orig.__name__, cache_key,
cached_result, actual_result,
)
raise ValueError("Stale cache entry")
defer.returnValue(cached_result)
observer.addCallback(check_result)
if isinstance(cached_result_d, ObservableDeferred):
observer = cached_result_d.observe()
else:
observer = cached_result_d
except KeyError:
ret = defer.maybeDeferred(
@ -447,7 +448,9 @@ class CacheListDescriptor(_CacheDescriptorBase):
try:
res = cache.get(tuple(key), callback=invalidate_callback)
if not res.has_succeeded():
if not isinstance(res, ObservableDeferred):
results[arg] = res
elif not res.has_succeeded():
res = res.observe()
res.addCallback(lambda r, arg: (arg, r), arg)
cached_defers[arg] = res