Add taskList throttling to allow users to limit activities executed per second #432

madhuravi · 2017-11-27T06:11:32Z

Fixes #50

coveralls · 2017-11-27T23:15:42Z

Coverage increased (+0.08%) to 66.321% when pulling 24e626c on tasklist into cadabad on master.

mfateev · 2017-11-27T23:26:43Z

service/matching/taskListManager.go

+func (rl *rateLimiter) shouldUpdate(maxDispatchPerSecond *float64) bool {
+	rl.RLock()
+	defer rl.RUnlock()
+	if maxDispatchPerSecond == nil {


No need to put this check under the lock.

Makes sense

mfateev · 2017-11-27T23:28:05Z

service/matching/taskListManager.go

+		rl.ttl.Reset(rl.ttlRaw)
+		return true
+	default:
+		return *maxDispatchPerSecond < *rl.maxDispatchPerSecond


I think it is the only place that needs the lock.

mfateev · 2017-11-27T23:31:36Z

service/matching/taskListManager.go

+	if ok, _ := c.rateLimiter.TryConsume(1); !ok {
+		// Note: Is a better error counter more appropriate here? We might not want high sevs for this
+		c.metricsClient.IncCounter(scope, metrics.PollErrorsCounter)
+		return nil, createServiceBusyError("TaskList dispatch exceeded limit")


We want to block consumers on long poll and return empty polls to maintain the configured rate. We don't want them receive any busy errors.

Good point. Fixed.

coveralls · 2017-11-28T00:32:13Z

Coverage increased (+0.1%) to 66.351% when pulling e7a1fbd on tasklist into cadabad on master.

venkat1109 · 2017-11-28T00:31:06Z

service/matching/matchingEngine.go

 		e.taskListsLock.RUnlock()
 		return result, nil
 	}
 	e.taskListsLock.RUnlock()
-	mgr := newTaskListManager(e, taskList, e.config)
-	e.taskListsLock.Lock()
+	mgr := newTaskListManager(e, taskList, e.config, maxDispatchPerSecond)


move this after your RUnlock() on line 178

Looks like we had some dup code here in checking the map. Cleaning up.

venkat1109 · 2017-11-28T00:34:48Z

service/matching/taskListManager.go

+	return newTaskListManagerWithRateLimiter(e, taskList, config, rl)
+}
+
+// Only for tests


is this comment accurate ? newTaskListManager uses this helper

This function is for use in tests, will clarify

venkat1109 · 2017-11-28T00:40:58Z

service/matching/taskListManager.go

@@ -323,6 +405,12 @@ func (c *taskListManagerImpl) getTask(ctx context.Context) (*getTaskResult, erro
 		}()
 	}

+	// Wait till long poll expiration for token. If token acquired, proceed.
+	if ok := c.rateLimiter.Consume(1, c.config.LongPollExpirationInterval); !ok {
+		// Note: Is a better error counter more appropriate here? We might not want high sevs for this


yes, emit a metric to indicate the error type i.e. being rate limited

venkat1109 · 2017-11-28T00:45:54Z

service/matching/taskListManager.go

 	String() string
 }

-func newTaskListManager(e *matchingEngineImpl, taskList *taskListID, config *Config) taskListManager {
+type rateLimiter struct {
+	maxDispatchPerSecond *float64


common.TokenBucket doesn't support rates lower than one rps. So, there is no point in taking a float64. We likely want to support rates lower than one rps, so, consider switching to the golang/x/ratelimiter

Great catch. Started out a float assumption but thinking about it a bit more, is there a real use case for wanting it?

venkat1109 · 2017-11-28T00:47:05Z

service/matching/taskListManager.go

+	maxDispatchPerSecond *float64
+	tokenBucket          common.TokenBucket
+	ttl                  *time.Timer
+	sync.RWMutex


by convention, anonymous members / mutex that protects whole struct go at the beginning of the struct

venkat1109 · 2017-11-28T01:01:36Z

service/matching/taskListManager.go

@@ -323,6 +405,12 @@ func (c *taskListManagerImpl) getTask(ctx context.Context) (*getTaskResult, erro
 		}()
 	}

+	// Wait till long poll expiration for token. If token acquired, proceed.
+	if ok := c.rateLimiter.Consume(1, c.config.LongPollExpirationInterval); !ok {


the caller context deadline could technically be less than the LongPollExpirationInterval, this probably needs to be fixed later.

Good catch, fixing.

venkat1109 · 2017-11-28T01:08:22Z

service/matching/taskListManager.go

+}
+
+func (rl *rateLimiter) Consume(count int, timeout time.Duration) bool {
+	rl.RLock()


don't like the fact that we hold the lock while we wait on a caller provided timeout. i.e. if there are 20 go-routines, 19 blocked on Consume() and one blocked on UpdateMaxDispatch(), the latter can starve for a long time (and meanwhile, context deadline could even be exceeded). Here is an alternative - don't hold the lock within the Consume / TryConsumer methods, instead use atomic.Value to hold the token bucket reference. Whenever you change the token bucket, just atomically set the Value to the new token bucket. That way, callers who hold the old token bucket will stick to the old limit. Newer callers will use the new bucket.

Interesting. I would say let's not optimize for that yet and keep it simple, changes are usually rare. Definitely something we can revisit if we hit performance issues.

madhu - this is more than just an optimization. Its a bit of correctness as well, in that, we can block the caller for way more than the deadline on a lock contention. With atomic.Value, you will end up with exactly the same lines of code, so IMO, let's just do it right the first time around and forget about this.

And fyi - its not about the "value changing" - UpdateMaxDispatch is called regardless for every Poll() request

To clarify, it will not affect correctness. Remember that all Poll's will converge on the same throttling value. If one Poll times out, the next one will update it. Also it's not about lines of code, locks are far more common and easier to understand and don't involve type casts. Good read https://texlution.com/post/golang-lock-free-values-with-atomic-value/

Also yes UpdateMaxDispatch is called for every Poll, but write locks are only acquired if the value is changing. Otherwise, only read locks are acquired. Check the shouldUpdate function.

I understand that locks are more more common / standard pattern, but atomic.Values aren't that complicated either. In this case, its clearly the right fit IMO. As for correctness, I understand polls will converge, but you will block the caller for more than the caller provided timeout (deadline). Anyways, will leave it up to you.

Also yes UpdateMaxDispatch is called for every Poll, but write locks are only acquired if the value is changing
yup, you are right.

venkat1109 · 2017-11-28T01:17:03Z

service/matching/taskListManager.go

+type rateLimiter struct {
+	maxDispatchPerSecond *float64
+	tokenBucket          common.TokenBucket
+	ttl                  *time.Timer


consider just using epoch time to check for ttl expiration instead of using a timer i.e. just store expiration as now().nanos+ttl and within your UpdateMaxDispatch just check if time.Now() > expiration.

Timers use the global heap within the go runtime (lock protected afaik) i.e. more expensive than time.Now()

I would keep it simple here, not sure the perf is significant enough to make it more complicated. Also timer uses channels internally, no lock as far as I can see https://golang.org/src/time/sleep.go

madhu - storing the expiry as epoch is a well understood pattern on the server side. And given that this is just couple of lines of code change and it buys us efficiency, why wait until there is a perf issue, lets just do it and be done with it.

Timer is the recommended pattern and simple/easy to understand. What efficiency does epoch buy us? What is the perf advantage here exactly?

recommended ? see this golang/go#15133

venkat1109 · 2017-11-28T01:19:05Z

service/matching/taskListManager.go

@@ -42,20 +42,95 @@ const (
 	done time.Duration = -1
 )

+// NOTE: Is this good enough for stress tests?
+const (
+	_maxDispatchDefault = 100000.0


[ not a blocker ] consider renaming this to _taskListOutboundRPS for clarity

venkat1109 · 2017-11-28T01:20:43Z

service/matching/taskListManager.go

+// NOTE: Is this good enough for stress tests?
+const (
+	_maxDispatchDefault = 100000.0
+	_dispatchLimitTTL   = time.Second


[ not a blocker ] consider renaming this to _taskListRateLimiterTTL

coveralls · 2017-11-28T22:09:44Z

Coverage increased (+0.1%) to 66.363% when pulling 85fac83 on tasklist into cadabad on master.

mfateev · 2017-11-28T22:21:05Z

common/metrics/defs.go

@@ -625,6 +626,7 @@ var MetricDefs = map[ServiceIdx]map[int]metricDefinition{
 		LeaseFailureCounter:           {metricName: "lease.failures"},
 		ConditionFailedErrorCounter:   {metricName: "condition-failed-errors"},
 		RespondQueryTaskFailedCounter: {metricName: "respond-query-failed"},
+		ThrottleErrorCounter:          {metricName: "throttle.errors"},


I think we shouldn't treat these like errors. What about throttle.empty-polls?

Empty polls sounds misleading. It's useful to differentiate between truly empty where no error is returned versus throttling limit is reached. I think it's still an error, but one we might not want in the same monitoring classification. On server side, this should not cause any tickets but users should be able to set alerts on these. There could be potential issues if there are spikes in throttling errors.

mfateev · 2017-11-28T22:28:58Z

service/matching/taskListManager.go

+		c.metricsClient.IncCounter(scope, metrics.ThrottleErrorCounter)
+		msg := fmt.Sprintf("TaskList dispatch exceeded limit: %s", err.Error())
+		// TODO: It's the client that's busy, not the server. Doesn't fit into any other error.
+		return nil, createServiceBusyError(msg)


I don't think we want to return service busy error. Also it is going to be returned even if there is no backlog as we throttle before trying to match. From the poller point of view it is ErrNoTasks.

Same as above, want to differentiate between truly no tasks vs tasks being throttled. Either way I need a new thrift type so user can react to it. ErrNewTasks is not a type, it just has a specific string message.

coveralls · 2017-11-29T01:41:30Z

Coverage increased (+0.2%) to 66.406% when pulling c93e34c on tasklist into cadabad on master.

venkat1109 · 2017-11-29T15:50:30Z

service/matching/matchingEngine.go

 	e.taskListsLock.Lock()
 	if result, ok := e.taskLists[*taskList]; ok {
+		result.UpdateMaxDispatch(maxDispatchPerSecond)


if aren't gonna switch to atomics, consider moving this outside the lock so that, we don't block addition of new task lists, when another tasklist is blocked on UpdateMaxDispatch for multiple seconds if/when they roll out a change in poll limit

mfateev · 2017-11-29T16:06:04Z

service/matching/taskListManager.go

@@ -323,6 +409,16 @@ func (c *taskListManagerImpl) getTask(ctx context.Context) (*getTaskResult, erro
 		}()
 	}

+	// Wait till long poll expiration for token. If token acquired, proceed.
+	stopWatch := c.metricsClient.StartTimer(scope, metrics.PollThrottleLatency)
+	err := c.rateLimiter.Wait(ctx, c.config.LongPollExpirationInterval)


I've just realized that this approach is not going to work for the following scenario:

Rate limit of 10 polls per second

No tasks are scheduled for a minute.

Rate limiter will let 600 pollers to go through.

Tasks are scheduled with 100 per second.

For 6 seconds polls will return 100 tasks per second.

coveralls · 2017-12-21T08:06:24Z

Coverage increased (+0.3%) to 66.871% when pulling 2dff6f6 on tasklist into 26303d5 on master.

coveralls · 2017-12-21T08:16:53Z

Coverage increased (+0.3%) to 66.876% when pulling 2dff6f6 on tasklist into 26303d5 on master.

coveralls · 2017-12-22T22:40:41Z

Coverage increased (+0.4%) to 67.059% when pulling 623475c on tasklist into 26303d5 on master.

coveralls · 2017-12-23T00:48:03Z

Coverage increased (+0.4%) to 67.001% when pulling 744025f on tasklist into 26303d5 on master.

coveralls · 2017-12-28T01:52:25Z

Coverage increased (+0.2%) to 66.999% when pulling 46f4c3e on tasklist into 36c6f54 on master.

samarabbas · 2018-01-02T21:49:33Z

service/matching/matchingEngine.go

 	e.taskListsLock.Lock()
 	if result, ok := e.taskLists[*taskList]; ok {
+		result.UpdateMaxDispatch(maxDispatchPerSecond)


I'm little worried we are switching to a write lock on this hot path. Can we figure out a way which does not require acquiring the write lock on the top level map for TaskListMgr.

There is one way with
RLock
Check map,
if ok,
runlock, update max dispatch
if not ok,
RUnlock
WLock
Again check map
if still not ok, then lock and write

So we will have some duplicate code, we will be checking if it is in the map twice. That way, if it is in map originally, it will onyl write lock. Makes code uglier but avoids write lock in majority of the cases.

We already have that logic in GetTaskList. I'm not sure if this new method is needed. If you move the logic of setting the throttling limit to GetTask than we can simplify this code a lot.

Removed Update method but FYI most of this code is still needed for task list creation.

samarabbas · 2018-01-02T21:54:35Z

service/matching/matchingEngine.go

@@ -341,10 +351,14 @@ pollLoop:
 		}

 		taskList := newTaskListID(domainID, taskListName, persistence.TaskListTypeActivity)
+		var maxDispatch *float64
+		if req.PollRequest.TaskListMetadata != nil {


nit: use "request.TaskListMetadata"

samarabbas · 2018-01-02T21:56:27Z

service/matching/matchingEngine.go

@@ -341,10 +351,14 @@ pollLoop:
 		}

 		taskList := newTaskListID(domainID, taskListName, persistence.TaskListTypeActivity)
+		var maxDispatch *float64
+		if req.PollRequest.TaskListMetadata != nil {
+			maxDispatch = req.PollRequest.TaskListMetadata.MaxTasksPerSecond


nit: use "request.TaskListMetadata"

samarabbas · 2018-01-02T22:23:58Z

service/matching/taskListManager.go

 type taskListManager interface {
 	Start() error
 	Stop()
 	AddTask(execution *s.WorkflowExecution, taskInfo *persistence.TaskInfo) error
 	GetTaskContext(ctx context.Context) (*taskContext, error)
 	SyncMatchQueryTask(ctx context.Context, queryTask *queryTaskInfo) error
 	CancelPoller(pollerID string)
+	UpdateMaxDispatch(maxDispatchPerSecond *float64)


Why do we need a separate API for this? Can you pass in the additional parameter to GetTaskContext?

The task list is constructed before getTaskContext. The max dispatch is a property of the task list, makes more sense to add it as part of the task list fetch.

samarabbas · 2018-01-02T22:42:16Z

service/matching/taskListManager.go

+	if !rsv.OK() {
+		scope := metrics.MatchingTaskListMgrScope
+		c.metricsClient.IncCounter(scope, metrics.AddThrottleCounter)
+		return nil, fmt.Errorf("cannot add to tasklist, limit exceeded")


Can we define a specific error for this? We also need to make sure we don't return back an error to the caller when this happens and instead store it to database.

Are you suggesting returning nil here and not an error at all? Or returning a specific error type that is handled in the handler code. For the second option, we would add a new error type in thrift, and Max was a bit determined that this should not be considered an error which is why I also named the metric a counter instead of an error. Some history from previous thread with Max when I had used a ServiceBusyError classification for this:

mfateev on Nov 28, 2017 Member
I don't think we want to return service busy error. Also it is going to be returned even if there is no backlog as we throttle before trying to match. From the poller point of view it is ErrNoTasks.

@madhuravi

madhuravi on Nov 28, 2017 Member
Same as above, want to differentiate between truly no tasks vs tasks being throttled. Either way I need a new thrift type so user can react to it. ErrNewTasks is not a type, it just has a specific string message.

Pushed a change for this. Let me know what you think.

samarabbas · 2018-01-02T22:51:06Z

service/matching/taskListManager.go

+				c.logger.Warn("Tasklist manager context is cancelled, shutting down")
+				break deliverBufferTasksLoop
+			}
+			c.logger.Warn("Unable to send tasks for poll, limit exceeded")


I'm confused here. I thought Wait will block indefinitely until new tokens are available. What other error will cause Wait to unblock?

It should really only be that error. I'm simply handling this scenario so as to not let it fall through.

samarabbas · 2018-01-02T22:58:43Z

service/matching/taskListManager.go

 func (c *taskListManagerImpl) getTasksPump() {
 	defer close(c.taskBuffer)
 	c.startWG.Wait()

+	go c.deliverBufferTasksForPoll()


Do we need another go routine per tasklist? We already have 2.

How come? There are two goroutines I see, one for the task pump and one to move tasks to poll. So it is 2 with this.
They do need to happen in parallel, so can't bundle them into one goroutine.

Spoke offline.

samarabbas · 2018-01-02T23:21:03Z

service/matching/taskListManager.go

-			return nil, errPumpClosed
+	case result := <-c.tasksForPoll:
+		if result.syncMatch {
+			c.metricsClient.IncCounter(scope, metrics.PollSuccessWithSyncCounter)


Remove any counters which are not used anymore.

What is not used? We still want a metric for successful poll and whether it is due to sync match or not.

samarabbas · 2018-01-02T23:23:41Z

service/matching/taskListManager.go

+			if !ok { // Task list getTasks pump is shutdown
+				break deliverBufferTasksLoop
+			}
+			c.tasksForPoll <- &getTaskResult{task: task}


We should measure the perf impact of this change. Considering we are not serializing all tasks through a non-buffered channel. Can you try to add a bench test to cover this scenario?

Are you talking about preference for buffered tasks over sync tasks? We discussed that and came to the conclusion that even before this, we will still prefer buffer tasks over sync tasks.

I think it is going to prioritize sync match just because sync match uses multiple goroutines to call trySyncMatch while deliverBufferTasksToPoll is called from a single goroutine.

Sync match is not in multiple goroutines. Do you mean there are several parallel calls to AddTask whereas only one goroutine per host adding the buffer tasks? That is true, and that behavior would be true before this code change as well.

samarabbas · 2018-01-02T23:24:32Z

service/matching/taskListManager.go

+				break deliverBufferTasksLoop
+			}
+			c.tasksForPoll <- &getTaskResult{task: task}
+		case <-c.deliverBufferShutdownCh:


Looks like we have 2 separate ways to shutdown this channel.

Spoke offline, this is to satisfy existing tests. There is a comment as well.

coveralls · 2018-01-03T02:01:13Z

Coverage increased (+0.03%) to 66.484% when pulling 006f60c on tasklist into bb03ce5 on master.

coveralls · 2018-01-03T02:02:25Z

Coverage increased (+0.03%) to 66.484% when pulling 006f60c on tasklist into bb03ce5 on master.

coveralls · 2018-01-05T01:27:16Z

Coverage increased (+0.1%) to 66.619% when pulling e4debb3 on tasklist into aa29ebb on master.

samarabbas · 2018-01-05T17:50:58Z

service/matching/taskListManager.go

@@ -50,6 +50,8 @@ const (
 	_defaultTaskDispatchRPSTTL = 60 * time.Second
 )

+var errAddTasklistThrottled = fmt.Errorf("cannot add to tasklist, limit exceeded")


use errors.New

samarabbas

Fix one minor comment and then you can merge it into master.

Addressed/fixed the comments.

coveralls · 2018-01-05T18:18:38Z

Coverage increased (+0.1%) to 66.608% when pulling 310dbdc on tasklist into aa29ebb on master.

madhuravi requested review from mfateev and samarabbas November 27, 2017 06:11

madhuravi force-pushed the tasklist branch from a6e85b5 to 1f90985 Compare November 27, 2017 19:39

mfateev suggested changes Nov 27, 2017

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venkat1109 reviewed Nov 28, 2017

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mfateev reviewed Nov 28, 2017

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madhuravi mentioned this pull request Nov 29, 2017

Add thrift gen files from glide up #436

Merged

venkat1109 reviewed Nov 29, 2017

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mfateev previously requested changes Nov 29, 2017

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madhuravi force-pushed the tasklist branch from c93e34c to 162407f Compare December 20, 2017 22:51

madhuravi force-pushed the tasklist branch from 744025f to 46f4c3e Compare December 28, 2017 01:31

samarabbas suggested changes Jan 2, 2018

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madhuravi force-pushed the tasklist branch from 46f4c3e to a59dc5d Compare January 3, 2018 01:45

madhuravi added 6 commits January 4, 2018 16:39

Add user-defined tasklist throttling

8d1f383

Add tests with 0 throttling limit

4cf7326

Add concurrent publish/consume with varying dispatch throttling

68be53c

Move lock before task list usage

f1e96bf

Tighten tests

812e46b

Remove redundant time.sleep

6874f95

madhuravi added 18 commits January 4, 2018 16:39

Fix race condition in test

993bf87

Add back sleep in sync match tests

2a62e42

Wait for throttling token till long poll expiration

b0f92fc

Switch to use golang rate limiter to accept float

9c84525

Fix lock, take into account context deadline

92a4e91

Add poll throttle latency metric

8650e55

Move throttle to add APIs to limit poll correctly

0fa0e62

Use atomic.Value for rate limiter and fix tests

43c1a44

Fix race condition

b1ad7a8

Fix concurrent activities test

40269a4

Add RLock

7c2cd17

Wrap ratelimiter max dispatch update in lock

ca9b497

Move cancel context to task list to remove extra goroutine

3dba4b0

Undo Makefile changes

b63dbd6

Add separate tests for task list manager

4c13d6a

Fixes from comments

a9c88f8

Avoid write lock in majority of requests

7494dcb

Handle throttle error and not propagate up to AddTask

f69f892

madhuravi force-pushed the tasklist branch from d9c24ea to f69f892 Compare January 5, 2018 00:43

madhuravi added 2 commits January 4, 2018 16:53

Only set dispatch limit on get task

fe0e8cc

Remove ununsed metric

e4debb3

samarabbas reviewed Jan 5, 2018

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samarabbas approved these changes Jan 5, 2018

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Fix error lib

310dbdc

madhuravi merged commit 58a3ad2 into master Jan 5, 2018

madhuravi deleted the tasklist branch January 5, 2018 18:20

madhuravi mentioned this pull request Jan 5, 2018

Set up activity throttle limit as input and propagate to server cadence-workflow/cadence-go-client#331

Merged

Add taskList throttling to allow users to limit activities executed per second #432

Add taskList throttling to allow users to limit activities executed per second #432

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venkat1109 Nov 28, 2017 •

edited

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