Synapse takes forever to send large responses - dead time gap after `encode_json_response`

[MadLittleMods]

Synapse takes forever to send large responses. It takes us longer to send just the response than it does for us to process and encode_json_response in some cases.

Examples

98s to process the request and encode_json_response but the request isn't finished sending until 484s (8 minutes) which is 6.5 minutes of dead-time. The response size is 36MB

Jaeger trace: 4238bdbadd9f3077.json

59s to process and finishes after 199s. The response size is 36MB

Jaeger trace: 2149cc5e59306446.json

I've come across this before and it's not a new thing. For example in #13620 (original issue), I described as the "mystery gap at the end after we encode the JSON response (encode_json_response)" but never encountered it being this egregious.

It can also happen for small requests. 2s to process and finishes after 5s. The response size is 120KB

Investigation

@kegsay pointed out _write_bytes_to_request which runs after encode_json_response and has comments like "Write until there's backpressure telling us to stop." that definitely hint at some areas of interest.

synapse/synapse/http/server.py

Lines 869 to 873 in 03937a1

	with start_active_span("encode_json_response"):
	span = active_span()
	json_str = await defer_to_thread(request.reactor, encode, span)
	_write_bytes_to_request(request, json_str)

The JSON serialization is done in a background thread because it can block the reactor for many seconds. This part seems normal and fast (no problem).

But we also use _ByteProducer to send the bytes down to the client. Using a producer ensures we can send down all of the bytes to the client without hitting a 60s timeout (see context in comments below)

synapse/synapse/http/server.py

Lines 883 to 889 in d40bc27

	# The problem with dumping all of the response into the `Request` object at
	# once (via `Request.write`) is that doing so starts the timeout for the
	# next request to be received: so if it takes longer than 60s to stream back
	# the response to the client, the client never gets it.
	#
	# The correct solution is to use a Producer; then the timeout is only
	# started once all of the content is sent over the TCP connection.

This logic was added in:

• Iteratively encode JSON responses matrix-org/synapse#8013
• Switch the JSON byte producer from a pull to a push producer matrix-org/synapse#8116

Some extra time is expected as we're working with the reactor instead of blocking it but it seems like something isn't tuned optimally (chunk size, starting/stopping too much, etc)

[reivilibre]

It'd probably be good to check if the worker was overloaded at the time the trace was taken (i.e. it's just CPU contention), or if this is something different.

[MadLittleMods]

I've updated the description with the Jaeger traces if you're curious about the timings. Seems like the CPU was busy but not totally overloaded.

https://grafana.matrix.org/d/000000012/synapse?orgId=1&var-datasource=default&var-bucket_size=$__auto_interval_bucket_size&var-instance=matrix.org&var-job=synapse_sliding_sync&var-index=All&from=1726513200000&to=1726516800000

Hour where the requests happened:

The whole day yesterday:

[kegsay]

Additional data point:

for an initial /sync in SSS.

[ara4n]

(crosslinking to #13356)