Add option for merging cluster updates #3941

rgs1 · 2018-07-24T06:12:27Z

This change introduces a new configuration parameter for clusters,
update_merge_window.

If this is set, health check/metadata/weight changes on endpoints that happen
within a update_merge_window duration will be merged and delivered
at once when the duration ends.

This is useful for big clusters (> 1k endpoints) using the subset LB.

Note, note, note: hosts added/removed won't be merged into a single
update because it's very hard to do this correctly right now.

Partially addresses #3929.

Signed-off-by: Raul Gutierrez Segales rgs@pinterest.com

This change introduces a new configuration parameter for clusters, `time_between_updates`. If this is set, all cluster updates — membership changes, metadata changes on endpoints, or healtcheck state changes — that happen within a `time_between_updates` duration will be merged and delivered at once when the duration ends. This is useful for big clusters (> 1k endpoints) using the subset LB. Partially addresses envoyproxy#3929. Signed-off-by: Raul Gutierrez Segales <rgs@pinterest.com>

htuch · 2018-07-24T16:49:31Z

I think arguably we should just improve the performance of updates rather than requiring end users have to configure workarounds like this. OTOH, this seems expedient, so no objections.

zuercher

Couple questions, but this looks good.

zuercher · 2018-07-24T16:47:04Z

source/common/upstream/cluster_manager_impl.cc

-    postThreadLocalClusterUpdate(cluster, priority, hosts_added, hosts_removed);
+
+    // Should we coalesce updates?
+    if (cluster.info()->lbConfig().has_time_between_updates()) {


Is it worth moving this logic into it's own function to keep this lambda easier to read?

thought about but then decided against since it isn't that big.. I am happy either way, your call.

zuercher · 2018-07-24T17:01:04Z

test/common/upstream/cluster_manager_impl_test.cc

+
+  // Ensure the coalesced updates were applied.
+  timer->callback_();
+  EXPECT_EQ(1, factory_.stats_.counter("cluster_manager.coalesced_updates").value());


Is it worth validating that the hosts were actually removed?

you mean from the intermediate sets? or that added/removed were > 0 (i think they can be 0)?

ah, now i know what you meant

hmm, might be a bit messy

rgs1 · 2018-07-24T17:58:14Z

@zuercher @mattklein123 note that i added the new option, time_between_updates, inside Cluster.CommonLbConfig to avoid additional plumbing... happy to move it up to Cluster if you think that's better.

@htuch I agree, and we are planning on exploring/working-on making the subset LB thread-aware next. But I think this knob is almost a neccessity for very big clusters where you want protection against healthcheck avalanches.

rshriram · 2018-07-24T18:58:05Z

I suggest renaming this to a more intuitive form like cds_update_merge_window or something along those lines?

* move coalesced update logic into its own method * extend the unit test to exercise a follow-up delivery of updates Signed-off-by: Raul Gutierrez Segales <rgs@pinterest.com>

rgs1 · 2018-07-24T19:47:50Z

@rshriram hmm should we just call it update_merge_window (dropping the cds_ prefix, given it's inside the cds.proto and inside Cluster, so there's enough context)?

rgs1 · 2018-07-24T19:51:24Z

source/common/upstream/cluster_manager_impl.h

+    PendingUpdates() {}
+    Event::TimerPtr timer;
+    std::unordered_set<HostSharedPtr> added;
+    std::unordered_set<HostSharedPtr> removed;


hmm, these should probably be ordered so that the merged update sees hosts in the same order it would have seen them if they were delivered immediately?

cc: @zuercher @mattklein123

maybe not that important and not worth the cost...

It's probably easier to debug I think if there is some determinism here. There is no major performance advantage of unordered here most likely.

to keep it ordered we'd have to keep two sets (added/removed) to check uniqueness (e.g.: that a host isn't added/removed twice) and we'd also have to keep two vectors (added/removed) to preserve the order..... or am I missing an ordered set data structure hidding somewhere? :-)

htuch · 2018-07-24T22:28:31Z

update_merge_window +1

htuch

Yeah, overall this is a very economical way of improving efficiency here.

htuch · 2018-07-24T22:29:06Z

api/envoy/api/v2/cds.proto

+    // and delivered in one shot when the duration expires. The start of the duration is when the
+    // first update happens. This is useful for big clusters, with potentially noisy deploys that
+    // might trigger excessive CPU usage due to a constant stream of healthcheck state changes or
+    // membership updates.


... "by default this is not configured and updates apply immediately.".

htuch · 2018-07-24T22:48:34Z

source/common/upstream/cluster_manager_impl.cc

+  updates->added.insert(hosts_added.begin(), hosts_added.end());
+  updates->removed.insert(hosts_removed.begin(), hosts_removed.end());
+
+  // If there's no timer, create one.


Won't this delay the first update after a long period of silence? Should we instead apply immediately in this case? I think we can then just keep track of the last time we applied and only add to a pending queue if < window length.

addressed in the latest commit

* fix documentation * rename config knob to `update_merge_window` * apply updates immediately when outside of window Signed-off-by: Raul Gutierrez Segales <rgs@pinterest.com>

Signed-off-by: Raul Gutierrez Segales <rgs@pinterest.com>

htuch · 2018-07-25T04:23:19Z

source/common/upstream/cluster_manager_impl.cc

+  // Has an update_merge_window gone by since the last update? If so, don't schedule
+  // the update so it can be applied immediately.
+  auto delta = std::chrono::steady_clock::now() - updates->last_updated;
+  uint64_t delta_ms = std::chrono::duration_cast<std::chrono::milliseconds>(delta).count();


Nit: these intermediate vars can be const.

htuch · 2018-07-25T04:24:33Z

source/common/upstream/cluster_manager_impl.cc

+    return false;
+  }
+
+  // Record the updates — preserving order — that should be applied when the timer fires.


FWIW, I was arguing for just a deterministic ordering, e.g. using set vs. unordered_set in my earlier comment, rather than arrival ordering. But, I don't mind if we stick with this.

htuch · 2018-07-25T04:25:22Z

source/common/upstream/cluster_manager_impl.cc

+  }
+
+  // Record the updates — preserving order — that should be applied when the timer fires.
+  for (auto host : hosts_added) {


Nit: const auto& host here and below.

htuch · 2018-07-25T04:26:20Z

source/common/upstream/cluster_manager_impl.cc

+  // Record the updates — preserving order — that should be applied when the timer fires.
+  for (auto host : hosts_added) {
+    if (updates->added_seen.count(host) != 1) {
+      updates->added.push_back(host);


Where does added_seen and removed_seen get inserted into?

oops, good catch

htuch · 2018-07-25T04:27:31Z

source/common/upstream/cluster_manager_impl.cc

+  }
+
+  for (auto host : hosts_removed) {
+    if (updates->removed_seen.count(host) != 1) {


What about sequences of ops in which a host is added, removed and added back again? I'm not sure if we have correctness here.

hmm, becomes trickier with ordering so we should probably avoid that. if we just kept the sets, we can handle cancellations (added/removed). we'd have to decide if want drop cancelled out ops (i say yes).

htuch · 2018-07-25T04:28:34Z

source/common/upstream/cluster_manager_impl.cc

+  cm_stats_.coalesced_updates_.inc();
+
+  // Reset everything.
+  updates->timer = nullptr;


I see, you nuke it here. I'd suggest keeping the timer alive and just doing disable/enable, it probably doesn't matter, but one less wasted allocation/free.

it would be nice to avoid the allocation/free pair, but then we'd have to check if the timer is enabled or not by looking at the last update timestamp... at which point things become a bit more involved. Preferences?

On a related note, the logic to apply an update immediately if the last update happened within a merge window should probably also check if a timer exists (e.g. timer != nullptr). Otherwise, updates could lose their order.

I guess we can add a bool to track the timer's state

htuch · 2018-07-25T04:35:26Z

source/common/upstream/cluster_manager_impl.cc

+                                      PendingUpdatesPtr updates) {
+  // Deliver pending updates.
+  postThreadLocalClusterUpdate(cluster, priority, updates->added, updates->removed);
+  cm_stats_.coalesced_updates_.inc();


Should we also count non-coalesced updates? Or can we just infer this from total?

htuch · 2018-07-25T04:35:46Z

source/common/upstream/cluster_manager_impl.h

@@ -361,6 +362,23 @@ class ClusterManagerImpl : public ClusterManager, Logger::Loggable<Logger::Id::u
  // This map is ordered so that config dumping is consistent.
  typedef std::map<std::string, ClusterDataPtr> ClusterMap;

+  struct PendingUpdates {
+    PendingUpdates() {}


Not needed?

htuch · 2018-07-25T04:36:08Z

source/common/upstream/cluster_manager_impl.h

+    std::unordered_set<HostSharedPtr> removed_seen;
+    MonotonicTime last_updated;
+  };
+  typedef std::shared_ptr<PendingUpdates> PendingUpdatesPtr;


Nit: prefer using here.

htuch · 2018-07-25T04:36:56Z

test/common/upstream/cluster_manager_impl_test.cc

+  // Ensure the coalesced updates were applied again.
+  timer->callback_();
+  EXPECT_EQ(2, factory_.stats_.counter("cluster_manager.coalesced_updates").value());
+}


I think a test that has add, remove, add for the same host would be interesting.

* use const when possible * use using instead of typedef * drop ordering of add/remove insertions * handle cancelled pairs of (add, remove) * remove unneeded struct constructor Will follow-up in another commit tomorrow with: * reuse timer instead of destroying/reallocating * add test for host added/removed/added Signed-off-by: Raul Gutierrez Segales <rgs@pinterest.com>

Signed-off-by: Raul Gutierrez Segales <rgs@pinterest.com>

htuch · 2018-07-25T13:06:37Z

source/common/upstream/cluster_manager_impl.h

@@ -361,6 +362,20 @@ class ClusterManagerImpl : public ClusterManager, Logger::Loggable<Logger::Id::u
  // This map is ordered so that config dumping is consistent.
  typedef std::map<std::string, ClusterDataPtr> ClusterMap;

+  struct PendingUpdates {
+    Event::TimerPtr timer;
+    std::set<HostSharedPtr> added;


This hashes on the pointer value I think. Do we have a guarantee that two identical hosts will have the same pointer value from the caller of scheduleUpdate()?

yeah good point, we should track the host's address instead.

htuch · 2018-07-25T13:07:36Z

source/common/upstream/cluster_manager_impl.cc

+    if (updates->removed.count(host) == 1) {
+      updates->removed.erase(host);
+    } else {
+      updates->added.insert(host);


Do we want to take the latest value for host, since it might have a new health status etc? As this works today, if we do add(unhealthy), add(healthy), only the unhealthy status for the host is set, since insert just returns false the second time.

Again, as with add, remove, add, I think we would benefit from some more tests here.

yes, we want the latest value. and yep, will follow-up with comprehensive testing.

@htuch note that a health change does not get treated as an add/remove:

https://github.com/envoyproxy/envoy/blob/master/source/common/upstream/upstream_impl.cc#L902

That is, if (only) a host's health flipped you'd get called with no new added/removed hosts in which case we'd still do the right thing. Which is, calling postThreadLocalClusterUpdate() with empty vectors for hosts_added and hosts_removed.

Anyway, I'll back all of these claims with tests.

mattklein123 · 2018-07-25T15:02:23Z

Hey just noting that I would like to review thus but I probably won't be able to until Friday. Sorry for the delay.

* track host by address, not by its shared_ptr * ensure we always publish updates with the latest copy of the host Signed-off-by: Raul Gutierrez Segales <rgs@pinterest.com>

Ensure we are calling `postThreadLocalClusterUpdate` at the right time with the right values. I'll follow-up with tests that exercise as many updates combinations as possible: * added/removed/added * unhealthy/healthy/added/removed/added * etc. Just wanted to get the basic helpers in place. Signed-off-by: Raul Gutierrez Segales <rgs@pinterest.com>

rgs1 · 2018-07-25T21:28:31Z

@mattklein123 yeah no worries, things are still moving around and I still need to add significant test coverage for this. Thanks!

Signed-off-by: Raul Gutierrez Segales <rgs@pinterest.com>

* add/remove/add -> add * remove/add/remove -> remove * existing host changes multiple times (e.g.: failed/active/failed -> failed) * metadata/metadata/metadata -> latest metadata * weight/weight/weight -> latest weight Signed-off-by: Raul Gutierrez Segales <rgs@pinterest.com>

rgs1 · 2018-07-25T23:19:17Z

Ok, I am starting to feel a bit more confident about this although it still needs more polish probably. I'll let the paint dry for a bit and then I'll do another pass to add more missing tests.

mattklein123

LGTM, nice work. @htuch can you take a quick pass over the most recent set of changes?

Lets only bump `merged_updates_offwindow` if merging is on. Signed-off-by: Raul Gutierrez Segales <rgs@pinterest.com>

rgs1 · 2018-07-30T21:11:37Z

(sorry, pushed one more 1-line change to avoid a new counter when merging is disabled).

mattklein123 · 2018-07-30T21:23:09Z

@rgs1 for the last commit can you add a stat expectation in one of the tests that would have caught that?

* renamed counters for consistency with existing counters * test for offwindow updates * test that merging related counters don't get updated when merging is disabled Signed-off-by: Raul Gutierrez Segales <rgs@pinterest.com>

rgs1 · 2018-07-30T22:43:40Z

@mattklein123 done — added 2 more tests: one for merging disabled and the other one for offwindow updates that don't trigger a merge. Also, renamed the new counters to make them more consistent with the stats that are already there. LMK what you think.

Signed-off-by: Raul Gutierrez Segales <rgs@pinterest.com>

rgs1 · 2018-07-31T02:11:16Z

So this is how CPU usage looks with & without merging updates:

a) the first 25 minutes are without merging enabled
b) the last 25 minutes are with merging enabled

Throughout the whole test, I am causing ~600 health checks to fail and then to immediately pass again in a loop.

That smoothness is exactly what we were looking for :-) Note that this is with a merge window of 10s.

cc: @htuch @mattklein123

mattklein123

Awesome work, great to see the graph. One last question.

mattklein123 · 2018-07-31T02:47:44Z

source/common/upstream/cluster_manager_impl.cc

+    // a timer is enabled. This race condition is fine, since we'll disable the timer here and
+    // deliver the update immediately.
+
+    // Why wasn't the update scheduled for later delivery?


Sorry what does this comment mean? Is this just tracking the race condition? Is it worth the extra code for this? I think this should be incredibly rare but I agree it can happen.

It's for tracking two things:

a) did we not merge this update because we are out of a merge window? i found this stat useful while testing this out, because it tells you how often merging is not needed
b) did we had merged updates pending that we are cancelling (and delivering immediately)? also useful to know how often a merge window is cancelled before expiring.

I'd like to keep these, I think they are useful. Happy to clarify comments/names. Ideas?

OK makes sense. I would probably go with the stat name @htuch suggested and put the entirety of what you jus wrote in a comment here? I think that would make it clear.

htuch

@rgs1 LGTM with some minor asks. Ready to ship when addressed. Thanks for the graph, nice improvement!

htuch · 2018-07-31T02:46:29Z

api/envoy/api/v2/cds.proto

+    // or metadata updates. By default, this is not configured and updates apply immediately. Also,
+    // the first set of updates to be seen apply immediately as well (e.g.: a new cluster).
+    //
+    // Note: merging does not apply to cluser membership changes (e.g.: adds/removes); this is


Nit: s/cluser/cluster/

htuch · 2018-07-31T02:46:44Z

api/envoy/api/v2/cds.proto

@@ -432,6 +432,17 @@ message Cluster {
      ZoneAwareLbConfig zone_aware_lb_config = 2;
      LocalityWeightedLbConfig locality_weighted_lb_config = 3;
    }
+    // If set, all healthcheck/weight/metadata updates that happen within this duration will be


Nit: s/healthcheck/health check/

htuch · 2018-07-31T02:50:07Z

source/common/upstream/cluster_manager_impl.cc

+  // the update so it can be applied immediately. Ditto if this is not a mergeable update.
+  const auto delta = std::chrono::steady_clock::now() - updates->last_updated_;
+  const uint64_t delta_ms = std::chrono::duration_cast<std::chrono::milliseconds>(delta).count();
+  const bool offwindow = delta_ms > timeout;


Kind of prefer out_of_merge_window for the stats names/vars to be clearer what's happening here. With recent stats optimizations happening, we can use longer names without any memory penalty.

Signed-off-by: Raul Gutierrez Segales <rgs@pinterest.com>

htuch

k-r4d

This clarifies the meaning of the stats we are tracking when we don't schedule an update for merging & later delivery. Signed-off-by: Raul Gutierrez Segales <rgs@pinterest.com>

rgs1 · 2018-07-31T03:45:45Z

@htuch (oops, i stepped on your r+ by pushing updated comments).

mattklein123 · 2018-07-31T15:59:02Z

@rgs1 apologies, one last thing: Can you document the new stats here? https://www.envoyproxy.io/docs/envoy/latest/configuration/cluster_manager/cluster_stats

Signed-off-by: Raul Gutierrez Segales <rgs@pinterest.com>

rgs1 · 2018-07-31T16:29:16Z

@mattklein123 something like d81fb7f?

mattklein123

Awesome, great work! Thank you!

rgs1 · 2018-08-01T03:51:55Z

Tested with prod traffic today.

Deploys without merging updates:

Deploys merging updates:

No more scary spikes :-) I was also thinking that an additional safety step, although this probably gives us almost enough safety for now, would be to add support for increasing the merge window size as a response to CPU going above a threshold (using #3954).

cc: @htuch @mattklein123

htuch · 2018-08-01T11:33:05Z

@rgs1 if there is strong motivation for dynamic window size (not sure if complexity is warranted here), I agree the right way to do it would be to use the overload manager under development, #373. CC @eziskind.

mattklein123 · 2018-08-01T16:12:51Z

@rgs1 awesome on the improvement. I actually wonder if we should eventually default this feature to on at a low threshold, maybe 1s or something? I will open up a follow up issue to track.

rgs1 mentioned this pull request Jul 24, 2018

subset load balancer scalability concerns #3929

Closed

rgs1 force-pushed the coalesce-health-checks branch from 97d3e83 to 139a684 Compare July 24, 2018 06:17

rgs1 force-pushed the coalesce-health-checks branch from 139a684 to 431a634 Compare July 24, 2018 06:19

zuercher reviewed Jul 24, 2018

View reviewed changes

Address review

8e20204

* move coalesced update logic into its own method * extend the unit test to exercise a follow-up delivery of updates Signed-off-by: Raul Gutierrez Segales <rgs@pinterest.com>

rgs1 commented Jul 24, 2018

View reviewed changes

htuch assigned htuch and zuercher Jul 24, 2018

htuch suggested changes Jul 24, 2018

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Raul Gutierrez Segales added 3 commits July 24, 2018 16:59

Address more review comments

c6dbc7d

* fix documentation * rename config knob to `update_merge_window` * apply updates immediately when outside of window Signed-off-by: Raul Gutierrez Segales <rgs@pinterest.com>

Fix docs

9e3288e

Signed-off-by: Raul Gutierrez Segales <rgs@pinterest.com>

Preserve order when merging updates

add79b2

Signed-off-by: Raul Gutierrez Segales <rgs@pinterest.com>

htuch reviewed Jul 25, 2018

View reviewed changes

Raul Gutierrez Segales added 2 commits July 24, 2018 23:13

Skip immediate apply if there's an active timer

0dd31c4

Signed-off-by: Raul Gutierrez Segales <rgs@pinterest.com>

htuch reviewed Jul 25, 2018

View reviewed changes

mattklein123 self-assigned this Jul 25, 2018

Raul Gutierrez Segales added 2 commits July 25, 2018 13:57

Fix a few more issues pointed out by @htuch

0bd58e6

* track host by address, not by its shared_ptr * ensure we always publish updates with the latest copy of the host Signed-off-by: Raul Gutierrez Segales <rgs@pinterest.com>

Raul Gutierrez Segales added 2 commits July 25, 2018 15:21

Move EXPECT_CALL() to the top for clarity

ec643f1

Signed-off-by: Raul Gutierrez Segales <rgs@pinterest.com>

mattklein123 previously approved these changes Jul 30, 2018

View reviewed changes

Increment counter if merging is enabled

31e4fdf

Lets only bump `merged_updates_offwindow` if merging is on. Signed-off-by: Raul Gutierrez Segales <rgs@pinterest.com>

rgs1 dismissed mattklein123’s stale review via 31e4fdf July 30, 2018 21:10

Counter renames + more tests

a5a5391

* renamed counters for consistency with existing counters * test for offwindow updates * test that merging related counters don't get updated when merging is disabled Signed-off-by: Raul Gutierrez Segales <rgs@pinterest.com>

Raul Gutierrez Segales added 2 commits July 30, 2018 15:48

Remove trailing newline

2a264d1

Signed-off-by: Raul Gutierrez Segales <rgs@pinterest.com>

Fix format

25faa2b

Signed-off-by: Raul Gutierrez Segales <rgs@pinterest.com>

mattklein123 reviewed Jul 31, 2018

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htuch suggested changes Jul 31, 2018

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Raul Gutierrez Segales added 2 commits July 30, 2018 20:19

Fix spelling

5affa4c

Signed-off-by: Raul Gutierrez Segales <rgs@pinterest.com>

Rename offwindow → out_of_merge_window

d3464d4

Signed-off-by: Raul Gutierrez Segales <rgs@pinterest.com>

rgs1 changed the title ~~Add option for coalescing cluster updates~~ Add option for merging cluster updates Jul 31, 2018

htuch previously approved these changes Jul 31, 2018

View reviewed changes

Add comment to explain why we keeping some stats

bbbfef8

This clarifies the meaning of the stats we are tracking when we don't schedule an update for merging & later delivery. Signed-off-by: Raul Gutierrez Segales <rgs@pinterest.com>

rgs1 dismissed htuch’s stale review via bbbfef8 July 31, 2018 03:44

htuch previously approved these changes Jul 31, 2018

View reviewed changes

Document new cluster stats

d81fb7f

Signed-off-by: Raul Gutierrez Segales <rgs@pinterest.com>

rgs1 dismissed htuch’s stale review via d81fb7f July 31, 2018 16:28

mattklein123 approved these changes Jul 31, 2018

View reviewed changes

mattklein123 merged commit 62441f9 into envoyproxy:master Jul 31, 2018

mattklein123 mentioned this pull request Aug 1, 2018

Consider turning on cluster update merging by default #4018

Closed

Add option for merging cluster updates #3941

Add option for merging cluster updates #3941

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