reader: don't decrement total_rdy on message receipt #179

alpaker · 2017-05-10T16:18:11Z

This brings reader behavior into agreement with nsqd behavior (after nsqio/nsq#404) and removes an opportunity for max_in_flight violations (#177).

This block was removed because it's redundant given _redistributed_rdy() and it's easier to maintain invariants without it.

@mreiferson

mreiferson · 2017-05-10T17:40:27Z

Thanks! I'm gonna read through the diff more carefully, but in the meantime tests are failing on Python 3.5 https://travis-ci.org/nsqio/pynsq/jobs/230827968

alpaker · 2017-05-10T18:29:26Z

D'oh! Fixed.

mreiferson · 2017-05-11T17:05:09Z

nsq/reader.py

@@ -358,7 +344,7 @@ def _maybe_update_rdy(self, conn):
 if self.backoff_timer.get_interval() or self.max_in_flight == 0:
 return

- if conn.rdy <= 1 or conn.rdy < int(conn.last_rdy * 0.25):
+ if conn.rdy == 1:


I think I understand this change but want to make sure — this is because we initially set RDY 1 and we want to make sure we adjust to an appropriate per-connection max-in-flight, right?

Perhaps it's worth a comment since it took me a little while to figure this out.

Right. Updating RDY here used to serve two purposes: Periodically reupping RDY when it dipped too low, and going from an initial throttled RDY 1 on startup to full-throttle connection-max-in-flight. Now only the second case is relevant.

mreiferson · 2017-05-11T17:09:02Z

nsq/reader.py

@@ -665,10 +646,21 @@ def _redistribute_rdy_state(self):
 logger.info('[%s:%s] idle connection, giving up RDY count', conn.id, self.name)
 self._send_rdy(conn, 0)

+ conns = self.conns.values()
+
+ in_flight = [c for c in conns if c.in_flight]


I still don't quite understand why we need to use in_flight when total_rdy should represent the same thing? The accounting for both total_rdy and in_flight used to happen in 2 subsequent lines in the same block.

This might be further complicated because the max_in_flight variable below is terribly named. Perhaps a better name would be available_rdy?

mreiferson · 2017-05-11T17:11:19Z

nsq/reader.py

@@ -677,7 +669,7 @@ def _redistribute_rdy_state(self):
 # We also don't attempt to avoid the connections who previously might have had RDY 1
 # because it would be overly complicated and not actually worth it (ie. given enough
 # redistribution rounds it doesn't matter).
- possible_conns = list(self.conns.values())
+ possible_conns = [c for c in conns if not (c.in_flight or c.rdy)]


Same comment here re: use of in_flight

Consider a case where max_in_flight is less than the connection count and a task takes longer than low_rdy_idle_timeout. Then we'll set RDY 0 on a connection even while it's still processing a message (https:/nsqio/pynsq/blob/master/nsq/reader.py#L664). So we can end up in a situation where total_rdy is strictly less than the total count of in-flight messages. If we go from from RDY 0 to RDY 1 on a connection that has messages available for delivery, we guarantee a max-in-flight violation.

The above last sentence should be:

If we go from from RDY 0 to RDY 1 on a connection (other than the one just set to RDY 0) that has messages available for delivery, we guarantee a max-in-flight violation.

Note that this is not difficult to trigger. All you need is:

max_in_flight less than the number of connections

typical task duration longer than low_rdy_idle_timeout

msgs available on each connection

OK, I think that makes sense, but what about the section above? The sum of in_flight can also be less than total_rdy, in which case we'd enter this loop below thinking we have more than we actually do to give out?

Sorry, I think I misunderstood our earlier discussion to have meant that that was the kind of case you weren't worried about. The possibility you mention would be taken care of with:

in_flight_or_rdy = sum(max(c.in_flight, c.rdy) for c in conns) if backoff_interval: max_in_flight = max(0, 1 - in_flight_or_rdy) else: max_in_flight = self.max_in_flight - in_flight_or_rdy

That's essentially the logic I proposed in #177 (comment).

With the proposed changes, the full diff would look like https:/nsqio/pynsq/compare/master...alpaker:no-decr-total-rdy-2?diff=unified&expand=1&name=no-decr-total-rdy-2#diff-2a8bc85bf9c95f482da1eb0490a60251R654

Cool, let's update this PR with that change.

As per above, I also think we should prefix the max_in_flight variable in this method with available_.

Thanks!

alpaker · 2017-05-12T20:30:35Z

@mreiferson Updated as discussed. (I went with your earlier suggestion of available_rdy for the variable name in _redistribute_rdy().)

mreiferson · 2017-05-13T13:49:24Z

nsq/reader.py

+ c = random.choice([c for c in conns if c.in_flight])
+ logger.info('[%s:%s] too many msgs in flight, giving up RDY count', c.id, self.name)
+ self._send_rdy(c, 0)
+ except IndexError:


Prefer not using exceptions for flow control here.

mreiferson · 2017-05-13T13:49:50Z

nsq/reader.py

 else:
- max_in_flight = self.max_in_flight - self.total_rdy
+ available_rdy = self.max_in_flight - in_flight_or_rdy


Now that we're using in_flight and rdy, I think this needs a max(0, ...) too

Both issues fixed.

redistribution logic accordingly. This brings reader behavior into agreement with nsqd behavior (compare nsqio/nsq#404) and removes an opportunity for max_in_flight violations (nsqio#177).

mreiferson

LGTM

alpaker force-pushed the no-decr-total-rdy branch from 9d2f54c to fa348cf Compare May 10, 2017 17:55

mreiferson added the bug label May 10, 2017

mreiferson reviewed May 11, 2017

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alpaker force-pushed the no-decr-total-rdy branch 3 times, most recently from d7867f4 to ba6c2b4 Compare May 12, 2017 19:28

mreiferson requested changes May 13, 2017

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reader: Don't decrement total_rdy on message receipt. Adjust RDY

47d1693

redistribution logic accordingly. This brings reader behavior into agreement with nsqd behavior (compare nsqio/nsq#404) and removes an opportunity for max_in_flight violations (nsqio#177).

alpaker force-pushed the no-decr-total-rdy branch from ba6c2b4 to 47d1693 Compare May 13, 2017 16:25

mreiferson approved these changes May 13, 2017

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mreiferson merged commit d24ee1c into nsqio:master May 13, 2017

alpaker deleted the no-decr-total-rdy branch May 13, 2017 16:43

mreiferson mentioned this pull request May 13, 2017

reader: max_in_flight check in _send_rdy() doesn't take in-flight msgs into account #177

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mreiferson mentioned this pull request Jun 3, 2017

consumer: redistribute RDY when connections are active nsqio/go-nsq#208

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mreiferson mentioned this pull request Mar 8, 2019

consumer: cleanup RDY handling; fix pausing nsqio/go-nsq#249

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alpaker mentioned this pull request Sep 14, 2020

reader: don't send RDY 1 on a new connection if that would violate max-in-flight #254

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reader: don't decrement total_rdy on message receipt #179

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