Start fewer opentracing spans (#8640)

#8567 started a span for every background process. This is good as it means all Synapse code that gets run should be in a span (unless in the sentinel logging context), but it means we generate about 15x the number of spans as we did previously.

This PR attempts to reduce that number by a) not starting one for send commands to Redis, and b) deferring starting background processes until after we're sure they're necessary.

I don't really know how much this will help.

1 files changed, 9 insertions, 3 deletions

diff --git a/synapse/metrics/background_process_metrics.py b/synapse/metrics/background_process_metrics.py
index 08fbf78eee..658f6ecd72 100644
--- a/synapse/metrics/background_process_metrics.py
+++ b/synapse/metrics/background_process_metrics.py
@@ -24,7 +24,7 @@ from prometheus_client.core import REGISTRY, Counter, Gauge
 from twisted.internet import defer
 
 from synapse.logging.context import LoggingContext, PreserveLoggingContext
-from synapse.logging.opentracing import start_active_span
+from synapse.logging.opentracing import noop_context_manager, start_active_span
 
 if TYPE_CHECKING:
     import resource
@@ -167,7 +167,7 @@ class _BackgroundProcess:
         )
 
 
-def run_as_background_process(desc: str, func, *args, **kwargs):
+def run_as_background_process(desc: str, func, *args, bg_start_span=True, **kwargs):
     """Run the given function in its own logcontext, with resource metrics
 
     This should be used to wrap processes which are fired off to run in the
@@ -181,6 +181,9 @@ def run_as_background_process(desc: str, func, *args, **kwargs):
     Args:
         desc: a description for this background process type
         func: a function, which may return a Deferred or a coroutine
+        bg_start_span: Whether to start an opentracing span. Defaults to True.
+            Should only be disabled for processes that will not log to or tag
+            a span.
         args: positional args for func
         kwargs: keyword args for func
 
@@ -199,7 +202,10 @@ def run_as_background_process(desc: str, func, *args, **kwargs):
         with BackgroundProcessLoggingContext(desc) as context:
             context.request = "%s-%i" % (desc, count)
             try:
-                with start_active_span(desc, tags={"request_id": context.request}):
+                ctx = noop_context_manager()
+                if bg_start_span:
+                    ctx = start_active_span(desc, tags={"request_id": context.request})
+                with ctx:
                     result = func(*args, **kwargs)
 
                     if inspect.isawaitable(result):