schedulers: fix balance-region churn on near-empty clusters (#11137) - #11190
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schedulers: fix balance-region churn on near-empty clusters (#11137)#11190ti-chi-bot wants to merge 8 commits into
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targetStoreScore() only reflected already-pending operators' influence, never the size of the region currently being evaluated for the move. On a cluster where most regions are near-empty (e.g. freshly pre-split, mostly unwritten), the average region size collapses, tolerantResource shrinks with it, and a target store can look artificially light right up until the move lands — letting balance-region pick a target that becomes overloaded the instant the pending region is counted, which triggers a follow-up move to shed it again. Add the candidate region's own approximate size to the target delta (unamplified, since it is not "other pending influence" but the exact size about to be received) so a target's projected post-move score is what actually decides whether it's picked. Recalibrate TestInfluenceAmp's boundary counts: the new term shifted the pre-existing count+size boundary this test pins down by exactly one region-size step. Signed-off-by: bufferflies <1045931706@qq.com>
…t margin getTolerantResource() derived its margin from GetAverageRegionSize(), which averages over every region in the cluster. On a cluster with many freshly-split, unwritten regions, that average collapses toward zero, so the tolerant margin stops damping marginal score differences between otherwise-equivalent stores. Add RegionsInfo.GetNonEmptyAverageRegionSize(), backed by a second pair of incrementally-maintained totals on regionTree (nonEmptyTotalSize / nonEmptyRegionsCnt) alongside the existing totalSize/length(), so it stays O(1). GetAverageRegionSize() itself is untouched; the new method is plumbed through the RegionSetInformer interface and rangeCluster's pass-through wrapper, and getTolerantResource() is switched to use it. Rework TestSingleRegionOnLargeEmptyDiskCanMigrate into TestSingleRegionOnLargeEmptyDiskDoesNotMigrate: with the tolerant margin no longer diluted, a single non-empty region isolated among many empty ones and stores correctly stays put — moving it would not fix a real imbalance and would just relocate the same "which store holds the only real data" state onto a different empty store, inviting the churn reported in tikv#11135. Also drops the region's phantom peer on the target store from the fixture, per review feedback on the prior version of this test. Signed-off-by: bufferflies <1045931706@qq.com>
…ntResource
targetStoreScore added the candidate region's own approximate size on
top of tolerantResource, but tolerantResource (averageRegionSize *
ratio) already represents roughly one region's worth of margin. In any
ordinary cluster where the candidate is close to the average region
size, this summed to about two regions' worth, silently doubling the
score gap required before balance-region would act and rejecting
legitimate moves between equal-sized regions (reported by rleungx:
three 96MiB regions on a source vs an empty target, 192 vs 96 after
the move, no longer scheduled).
Take the larger of the two instead of adding them. This matches the
long-standing but never-implemented intent documented in
shouldBalance()'s own comment ("we use max(regionSize,
averageRegionSize)"): fall back to the candidate's real size only when
it exceeds the average-based margin. Verified against both the
regression case above (now matches pre-PR behavior) and the
originally-motivating churn scenario in
TestSingleRegionOnLargeEmptyDiskDoesNotMigrate (still correctly stays
put). TestInfluenceAmp's boundary counts revert to their original,
pre-PR values, since max() is behaviorally identical to the base
formula whenever the candidate doesn't exceed tolerantResource.
Add TestBalanceRegionOrdinaryMoveNotBlockedByCandidateSize as a
permanent regression test for this, since no existing test previously
covered balancing between several ordinary, similarly-sized regions.
Signed-off-by: bufferflies <1045931706@qq.com>
…k when empty Two review findings from lhy1024: - getTolerantResource()'s size-based branch is shared by any non-count schedule kind, not just balance-region: LeaderKind/WitnessKind with the (legitimate, documented) BySize policy reached it too, so a cluster with one large data region among many empty ones would have its leader-balance tolerance dictated by that one region's size, with no test coverage for that path. Scope GetNonEmptyAverageRegionSize() to RegionKind specifically; LeaderKind/WitnessKind keep using the original GetAverageRegionSize() unconditionally, unaffected by this PR. - GetNonEmptyAverageRegionSize() returned 0 when there were no non-empty regions at all, which silently zeroed out any configured tolerant-size-ratio. scatter-range intentionally schedules over key ranges that can be entirely empty, freshly-split regions (see balance_region.go's explicit empty-region allowance for rangeCluster) and sets its own ratio (2) on top of it. Fall back to the plain all-regions average in that case, matching GetAverageRegionSize()'s value, instead of returning 0. Signed-off-by: bufferflies <1045931706@qq.com>
The regionTree comment named GetAverageRegionSize, but that method was reverted to its original behavior earlier in this PR; the new counters actually back GetNonEmptyAverageRegionSize. Correct the comment. shouldBalance()'s debug log only printed the old, all-regions average while tolerantResource could be computed from either average depending on schedule kind, making the logged number not match what actually drove the decision. Log both averages. Signed-off-by: bufferflies <1045931706@qq.com>
…n tree GetNonEmptyAverageRegionSize() returned a hard 0 when the tree has no regions at all, which would again silently zero out any configured tolerant-size-ratio in that edge case, same class of problem as the "no non-empty regions" fallback added earlier in this PR. GetAverageRegionSize() itself is unaffected (still returns 0 here, as before). Signed-off-by: bufferflies <1045931706@qq.com>
GetNonEmptyAverageRegionSize() fell back to TotalSize()/length() when nonEmptyRegionsCnt was 0 but the tree still had regions. Since "empty" is defined as approximateSize <= EmptyRegionApproximateSize (0 or 1), that fallback could itself compute to 0 - e.g. regions loaded from storage before their first heartbeat can carry approximateSize=0, and even a mix of 0s and 1s can truncate to 0 under integer division. That silently zeroed out any configured tolerant-size-ratio again, the same class of bug this PR already fixed twice. Since nonEmptyRegionsCnt==0 means every region in the tree already has approximateSize <= EmptyRegionApproximateSize, there is nothing meaningful to average in the first place - return EmptyRegionApproximateSize directly, covering the empty-tree, all-size-0, all-size-1, and mixed cases uniformly. Also guard against a nil r.tree (a zero-value RegionsInfo), matching GetAverageRegionSize's existing zero-value behavior. Signed-off-by: bufferflies <1045931706@qq.com>
rleungx found that tolerantResource = GetNonEmptyAverageRegionSize() * tolerantSizeRatio inflates the tolerant margin by up to ~50x on clusters mixing real data with many empty regions, because tolerantSizeRatio's default is still derived from the store's total region count (GetStoreRegionCount(), empty regions included) while the numerator no longer shrinks with them. That mismatch can make shouldBalance() reject moves that plainly should happen. lhy1024 separately found that folding the candidate region's size into only targetDelta (not sourceDelta, which is computed once per source store before a candidate region is even selected) is not symmetric: when the candidate is much larger than tolerantResource, this doesn't guarantee target stays lighter than source after the move (e.g. source=150, target=0, tolerantResource=10, candidate=100 projects to source=50/target=100 post-move, yet was still scheduled). Replace both mechanisms with one: getTolerantResource() reverts to plain GetAverageRegionSize() (no more RegionKind special-casing, so tolerantSizeRatio's count basis matches its numerator again), and a new getRegionScoreDelta() - max(tolerantResource, candidateSize) - is applied identically to both sourceDelta and targetDelta. Making this symmetric requires knowing the candidate before sourceScore is computed, so its computation in balance_region.go's Schedule() moves from once per source store (before a region is selected) to once the candidate region has passed the hot/leader checks. This drops RegionsInfo.GetNonEmptyAverageRegionSize(), its RegionSetInformer/rangeCluster plumbing, and regionTree's nonEmptyTotalSize/nonEmptyRegionsCnt counters, since the non-empty average is no longer used anywhere - the symmetric candidate-size delta now does that job without needing a separate average. shouldBalance()'s debug log also switches to logging the effective getRegionScoreDelta() (not the bare tolerant margin) for RegionKind, since that margin alone no longer matches what actually drove the decision whenever the candidate exceeds it. Add TestBalanceRegionLargeCandidateDoesNotOvershoot (lhy1024's reproduction, now rejected) and TestBalanceRegionRealScheduleDoesNotMoveSoleRegion, which drives the real Schedule() entry point (not the solver methods directly) through the tikv#11135 scenario in both directions, deterministically (the real region is its store's only candidate, so selection can't dodge it). An earlier version of this second test relied on registering the resulting operator's pending OpInfluence and checking that a second Schedule() call produced nothing; rleungx pointed out that AddOperator() never updates the mock cluster, so that check also passes unmodified on pre-PR code - it was only exercising the generic, pre-existing OpController pending-influence suppression, not this fix. The replacement was verified to diverge from pre-PR behavior directly: on the base commit the same setup deterministically produces an operator, since pre-PR scoring never considered candidate size at all. TestInfluenceAmp, TestBalanceRegionOrdinaryMoveNotBlockedByCandidateSize, and TestSingleRegionOnLargeEmptyDiskDoesNotMigrate all continue to pass unmodified against the new formula. Signed-off-by: bufferflies <1045931706@qq.com>
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📝 WalkthroughWalkthroughThe balance-region scheduler now applies candidate-aware score deltas symmetrically and recomputes the source score after candidate selection. New tests cover oversize candidates, ordinary moves, and concentrated data scenarios. ChangesBalance-region score correction
Estimated code review effort: 3 (Moderate) | ~20 minutes Merge Risk: 🔵 Low · up to The scheduler now uses candidate size symmetrically to avoid churn and overshooting moves. Production behavior is covered by targeted tests; the remaining low risk is test-context cleanup being skipped on assertion failures. 🚥 Pre-merge checks | ✅ 4 | ❌ 1❌ Failed checks (1 warning)
✅ Passed checks (4 passed)
Full details: Description checkExplanation The description explains the problem, links issue Full details: Linked Issues checkExplanation The implementation addresses issue
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In `@pkg/schedule/schedulers/balance_region_test.go`:
- Line 228: Defer cancel immediately after prepareSchedulersTest(false) returns
in each test case, ensuring scheduler cleanup also runs when a require assertion
exits early. Keep the existing test setup and assertions unchanged.
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| // directions so the result isn't an artifact of store iteration/sort order. | ||
| func TestBalanceRegionRealScheduleDoesNotMoveSoleRegion(t *testing.T) { | ||
| for _, realOnStore1 := range []bool{true, false} { | ||
| cancel, _, tc, oc := prepareSchedulersTest(false) |
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🩺 Stability & Availability | 🟡 Minor | ⚡ Quick win
Defer cancellation for each test case.
A failed require.NoError or require.Empty call exits the test before line 258. Defer cancel() immediately after prepareSchedulersTest(false) so cleanup runs on failure paths.
Proposed fix
cancel, _, tc, oc := prepareSchedulersTest(false)
+ defer cancel()
re := require.New(t)
@@
- cancel()
}
}As per coding guidelines, "Cancel timers/tickers; close resources with defer and error checks."
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Carefully review the code before committing. Ensure that it accurately replaces the highlighted code, contains no missing lines, and has no issues with indentation. Thoroughly test & benchmark the code to ensure it meets the requirements.
Suggested change
| cancel, _, tc, oc := prepareSchedulersTest(false) | |
| cancel, _, tc, oc := prepareSchedulersTest(false) | |
| defer cancel() |
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In `@pkg/schedule/schedulers/balance_region_test.go` at line 228, Defer cancel
immediately after prepareSchedulersTest(false) returns in each test case,
ensuring scheduler cleanup also runs when a require assertion exits early. Keep
the existing test setup and assertions unchanged.
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This is an automated cherry-pick of #11137
What problem does this PR solve?
Issue Number: Close #11135
balance-regioncould churn/thrash on a cluster with many near-empty regions (e.g. freshly pre-split tables, mostly unwritten) alongside a few regions that actually hold data — observed in production as repeated, low-value peer moves between otherwise-empty stores.Two contributing gaps:
targetStoreScore()'s delta never accounted for the specific region being evaluated for the move, so a target store's score didn't reflect what it was about to receive until after the move landed.getTolerantResource()'s margin is derived fromGetAverageRegionSize(), which averages over every region in the cluster. On a cluster full of empty regions, that average collapses toward zero, so the margin stops damping marginal score differences between otherwise-equivalent stores.What is changed and how does it work?
Check List
Tests
Release note
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