--consolidator-query-waiter-cap to set the max number of waiter for consolidated query

[jwangace]

Description

This PR proposed to use config ConsolidatorQueryWaiterCap to set the max number of clients Wait() for all consolidated queries, more same queries will fall back to use getConn() where is protected by connection pools.

With configureable ConsolidatorQueryWaiterCap (default to 0 means unlimit, user can adjust this value accordingly), when properly set would help to prevent when the Result of first query been returned, same Result would be sent concurrently to large number of clients, cause memory abruptly went high.

Related Issue(s)

Issue: #17243
Documentation: vitessio/website#1900

Checklist

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Deployment Notes

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[harshit-gangal]

This would also require addition to summary notes for v22 over https://github.com/vitessio/vitess/blob/main/changelog/22.0/22.0.0/summary.md

You can see v21 summary notes to see the template to add new configuration to release notes

[harshit-gangal]

Website docs would also need an update https://vitess.io/docs/22.0/user-guides/configuration-advanced/query-consolidation/

[jwangace]

This would also require addition to summary notes for v22 over https://github.com/vitessio/vitess/blob/main/changelog/22.0/22.0.0/summary.md

You can see v21 summary notes to see the template to add new configuration to release notes

Updated.

[jwangace]

Website docs would also need an update https://vitess.io/docs/22.0/user-guides/configuration-advanced/query-consolidation/

Hi @harshit-gangal This is not in this repo, is it?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.51%. Comparing base (45192d2) to head (e5cdf1a).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17244      +/-   ##
==========================================
- Coverage   67.52%   67.51%   -0.02%     
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  Files        1581     1581              
  Lines      253948   253958      +10     
==========================================
- Hits       171480   171454      -26     
- Misses      82468    82504      +36     

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[jwangace]

This can lead to slowness in query serving path. Either we make it an absolute count than per query count or we add a benchmark test for it.

Pushed new solution to use absolute count.

[harshit-gangal]

nit: this can be ignored to be kept in default config for 0 as default

[jwangace]

I dropped this line and use 0 as default in the flag.

[harshit-gangal]

Adding the waiter in Record is too late. As the wait is already complete in q.Wait().
Post that q.Result() returns the stored result.

c.WaiterCountOfTotal() <= waiterCap - This check will allow concurrent threads to go through without respecting the limit.

[jwangace]

Hi @harshit-gangal Could you please let me know if I understand correctly regarding your concern.

c.WaiterCountOfTotal() <= waiterCap - This check will allow concurrent threads to go through without respecting the limit.

• Same query won't run concurrently, because q is a same *pendingResult object

• Different queries when run concurrently (for those following same queries), this solution uses totalWaiterCount and only record those queries that use Wait() for *pendingResult. For example, if there are 2 different queries, and:
  -- select a from A (1 running, 100 waiting)
  -- select b from B (1 running, 100 waiting)
  Then totalWaiterCount is 100+100=200, I added testCase on this, which is concurrently update totalWaiterCount and verify above theory is correct also verify no data race when on concurrent.

• Uniq queries should only enter if original {...} path, so it does not run c.WaiterCountOfTotal() <= waiterCap check.

c.WaiterCountOfTotal() <= waiterCap - This check will allow concurrent threads to go through without respecting the limit.

WaiterCountOfTotal() is a concurrent safe, so it should be respected the limit.

func (co *consolidator) WaiterCountOfTotal() int64 {
	return atomic.LoadInt64(co.totalWaiterCount)
}

Adding the waiter in Record is too late. As the wait is already complete in q.Wait().
Post that q.Result() returns the stored result.

Record is already used to increase the count of query, for queries passed c.WaiterCountOfTotal() <= waiterCap check, seems a good space to increase totalWaiterCount as well.

[harshit-gangal]

VTTablet: New ResetSequences RPC -> Query Consolidation Waiter Cap

[harshit-gangal]

The waiter count increases on calling the q.Wait() method.
If you run 10s or 100s of request, they can still pass the check c.WaiterCountOfTotal() <= waiterCap before any new waiter not getting consolidated.
A concurrent test will help test if the waiter count exceeds the maximum allowed waiter.

I think the better place to update the wait counter will be in create call

[jwangace]

I will resolve the conflicts once this solution has been approved, to avoid more conflicts.