Yield during large v2 state res. (#7735)

State res v2 across large data sets can be very CPU intensive, and if
all the relevant events are in the cache the algorithm will run from
start to finish within a single reactor tick. This can result in
blocking the reactor tick for several seconds, which can have major
repercussions on other requests.

To fix this we simply add the occaisonal `sleep(0)` during iterations to
yield execution until the next reactor tick. The aim is to only do this
for large data sets so that we don't impact otherwise quick resolutions.=

1 files changed, 1 insertions, 0 deletions

diff --git a/changelog.d/7735.bugfix b/changelog.d/7735.bugfix
new file mode 100644
index 0000000000..86959a5ca4
--- /dev/null
+++ b/changelog.d/7735.bugfix
@@ -0,0 +1 @@
+Fix large state resolutions from stalling Synapse for seconds at a time.