Add periodic session refresh for Live Video to Video

[leszko]

To reduce the startup, we can periodically refresh sessions, then it won't happen when the stream comes in. This could save us ~1-2s startup time.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 28 lines in your changes missing coverage. Please review.

Project coverage is 32.13158%. Comparing base (0f9be6a) to head (37485a4).

Files with missing lines	Patch %	Lines
server/ai_session.go	0.00000%	28 Missing ⚠️

Additional details and impacted files

@@                 Coverage Diff                 @@
##              master       #3404         +/-   ##
===================================================
- Coverage   32.15038%   32.13158%   -0.01880%     
===================================================
  Files            147         147                 
  Lines          40830       40857         +27     
===================================================
+ Hits           13127       13128          +1     
- Misses         26928       26954         +26     
  Partials         775         775                 

Files with missing lines	Coverage Δ
server/ai_session.go	2.07715% <0.00000%> (-0.18091%)	⬇️

... and 1 file with indirect coverage changes

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Files with missing lines	Coverage Δ
server/ai_session.go	2.07715% <0.00000%> (-0.18091%)	⬇️

... and 1 file with indirect coverage changes

[j0sh]

Should we try to be a good citizen and tear down the loop when the node shuts down?

[leszko]

Hmm, would be good. But I believe it's not so trivial....heheh. I guess we would need to move this to starter.go, etc. Unless you have some simple idea how to exit this loop when the node shuts down?

[j0sh]

Yeah, or some chan / signal from LivepeerNode ... not a big deal though if it seems more trouble than it is worth.

[ad-astra-video]

Another issue will likely be that ticket params expire in 10 blocks (about 2 minutes). Refreshing the session will add a couple hundred milliseconds on to startup of request.

I don't think would overly work Orchestrators to run this every 110 seconds. Also don't think its unreasonable to increase the ticket params expiration to be 10 minutes. I think 10 blocks was from L1 days with larger variations in gas fees.

go-livepeer/server/ai_session.go

Line 404 in 232df3a

if err := refreshSessionIfNeeded(ctx, sess.BroadcastSession); err != nil {

go-livepeer/server/broadcast.go

Line 1582 in 232df3a

func refreshSessionIfNeeded(ctx context.Context, sess *BroadcastSession) error {

Also think would be nice to save the LatencyScores at the start of the refresh and add back on to the new sessions. The other PR for the new selector used InitialLatencyScore, where does that come from?

[leszko]

Very good points @ad-astra-video !

Another issue will likely be that ticket params expire in 10 blocks (about 2 minutes). Refreshing the session will add a couple hundred milliseconds on to startup of request.
I don't think would overly work Orchestrators to run this every 110 seconds. Also don't think its unreasonable to increase the ticket params expiration to be 10 minutes. I think 10 blocks was from L1 days with larger variations in gas fees.

Hmm, let's maybe do something in between:

• Increase ticket params expire from 10 blocks to 40 blocks Increase ticket params expiration from 10 (~2 min) to 40 (~8 min). #3411
• Change the refresh interval here to 5 min
  Wdyt?

Also think would be nice to save the LatencyScores at the start of the refresh and add back on to the new sessions. The other PR for the new selector used InitialLatencyScore, where does that come from?

IIUC the LatencyScores are not changed, because they are used only in knownSessions and we don't change knownSessions during the refresh. About the initial latency, I think it does not matter which we'll use, if the existing one or the new one, they should be similar.