schedulers: fix balance-region churn on near-empty clusters - #11137
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Included review availability: Your plan includes up to 4 reviews per rolling hour; 1 remains after this review. 📝 WalkthroughWalkthroughThe balance-region scheduler now tracks non-empty region sizes separately from overall region sizes. It uses this data for tolerant-resource calculations and target scoring. Tests cover ordinary movement and prevent migration of a small data-bearing region between identical large-capacity stores. ChangesRegion balance scoring
Estimated code review effort: 3 (Moderate) | ~20 minutes Merge Risk: 🔵 Low · up to This PR changes region-size averaging to reduce unnecessary balancing churn, but the new exported averaging method may panic if called on a zero-value RegionsInfo. The change is otherwise mergeable with explicit owner awareness or follow-up for that bounded correctness risk. Suggested reviewers: Sequence Diagram(s)sequenceDiagram
participant RegionTree
participant RangeCluster
participant BalanceScheduler
participant BalanceRegionTest
RegionTree->>RangeCluster: provide non-empty average region size
RangeCluster->>BalanceScheduler: expose non-empty average
BalanceScheduler->>BalanceScheduler: calculate tolerance and target score
BalanceRegionTest->>BalanceScheduler: validate migration decisions
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In `@pkg/schedule/schedulers/balance_region_test.go`:
- Around line 121-151: Update the test fixture around AddLeaderRegion so region
1 exists only on store 1 and store 2 does not already host its peer; add one
additional empty region to store 2 to preserve ten regions per store. Keep
solver target selection and the shouldBalance assertion unchanged so the test
validates a schedulable target.
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pkg/schedule/schedulers/balance_region_test.gopkg/schedule/schedulers/utils.go
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>
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Codecov Report❌ Patch coverage is Additional details and impacted files@@ Coverage Diff @@
## master #11137 +/- ##
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+ Coverage 79.43% 79.56% +0.13%
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| // look artificially light just because this move hasn't landed yet. | ||
| // Unlike opInfluence (other, already-pending operators), this is not | ||
| // amplified: it is the literal size the target is about to receive. | ||
| targetDelta := influence*influenceAmp + tolerantResource + p.Region.GetApproximateSize() |
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In the added 10 MiB/6 TiB regression scenario, GetAverageRegionSize() truncates to zero; the populated source's v2 score is slightly above 10 while an empty target's projected score is exactly 10, so this comparison still schedules the peer. After it lands on any empty store, another empty target recreates the same state, allowing the peer to rotate indefinitely and leaving the reported churn unresolved.
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Fixed in 8532b76. getTolerantResource() now uses a new GetNonEmptyAverageRegionSize() (excludes empty regions from the average instead of GetAverageRegionSize()), so it no longer collapses to zero in this scenario. Re-verified the exact case you described: tolerantResource computes to the candidate region's own size instead of truncating to zero, and the target's projected score now exceeds the source's, so shouldBalance() returns false — the region stays put instead of rotating. TestSingleRegionOnLargeEmptyDiskDoesNotMigrate (renamed/inverted from the original test) asserts this directly.
…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>
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| // look artificially light just because this move hasn't landed yet. | ||
| // Unlike opInfluence (other, already-pending operators), this is not | ||
| // amplified: it is the literal size the target is about to receive. | ||
| targetDelta := influence*influenceAmp + tolerantResource + p.Region.GetApproximateSize() |
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tolerantResource already contributes one average region to the target margin. Adding the candidate again rejects legitimate moves when the candidate equals the average: with three 96 MiB regions on the source and an empty target (v1, ratio=1), this head compares 192 vs 192 and schedules nothing, although moving one region leaves 192 vs 96. The base head schedules this move, so this introduces a balance-region regression for ordinary equal-sized regions.
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Confirmed and fixed in dafffd2. You're right that tolerantResource already represents about one region's worth of margin, so adding the candidate's size on top double-counted it. Switched to max(tolerantResource, p.Region.GetApproximateSize()) instead of summing them — this falls back to the candidate's real size only when it exceeds the average-based margin, matching the (previously unimplemented) intent in shouldBalance()'s own comment about max(regionSize, averageRegionSize). Re-ran your exact reproduction (three 96MiB regions on the source, empty target, v1, ratio=1): now scores 192 vs 96 and schedules the move, matching pre-PR/base behavior. Added TestBalanceRegionOrdinaryMoveNotBlockedByCandidateSize as a permanent regression test for this case, since no existing test previously covered balancing between several ordinary, similarly-sized regions.
…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>
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changed the title
| func (r *RegionsInfo) GetNonEmptyAverageRegionSize() int64 { | ||
| r.t.RLock() | ||
| defer r.t.RUnlock() | ||
| if r.tree.nonEmptyRegionsCnt == 0 { |
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Returning 0 when the cluster or selected range contains no non-empty region changes the existing tolerance to zero. This is observable in scatter-range, which intentionally allows 1 MiB empty regions and sets the tolerant ratio to 2: rangeCluster.GetNonEmptyAverageRegionSize() now returns 0, so source gets no tolerance and target only gets the candidate's 1 MiB. The configured ratio is therefore no longer honored and empty-region balancing becomes more aggressive. Please define a fallback, such as retaining GetAverageRegionSize() when the non-empty count is zero, and add an all-empty range regression test.
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Fixed in ec0fec6. GetNonEmptyAverageRegionSize() now falls back to the plain all-regions average (matching GetAverageRegionSize()'s value) when there are no non-empty regions at all, instead of returning 0. Verified: in an all-empty scenario the two methods now return identical values, so scatter-range's configured tolerant-size-ratio is no longer silently zeroed out.
| // Use the non-empty average so a cluster full of freshly-split, | ||
| // unwritten regions doesn't collapse the tolerant margin toward | ||
| // noise levels. | ||
| regionSize := p.GetNonEmptyAverageRegionSize() |
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This branch is shared by all non-count schedule kinds, not just balance-region. In particular, balance-leader with BySize also reaches this call, so a cluster with one large data region and many empty regions will use that large region as the leader tolerance and can suppress legitimate leader transfers. Please either limit this behavior to RegionKind, or document the intended leader behavior and add a regression test.
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Fixed in ec0fec6. Scoped GetNonEmptyAverageRegionSize() to RegionKind specifically in getTolerantResource(); LeaderKind/WitnessKind now unconditionally fall through to the original GetAverageRegionSize() regardless of policy, so leader-schedule-policy=size is unaffected by this PR. Re-ran TestBalanceLeader*/TestShouldBalance to confirm no behavior change on that path.
| totalSize int64 | ||
| // nonEmptyTotalSize and nonEmptyRegionsCnt mirror totalSize/length but | ||
| // exclude empty regions (approximateSize <= EmptyRegionApproximateSize), | ||
| // so GetAverageRegionSize can reflect only regions that actually hold |
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This comment names GetAverageRegionSize, but that method still includes empty regions; these counters are consumed by GetNonEmptyAverageRegionSize. The debug log in pkg/schedule/schedulers/utils.go:174 likewise still reports the old average while tolerantResource uses the new one. Please correct the comment and expose the non-empty average, or rename the log field, so diagnostics match the calculation.
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Fixed in 8203c39. Corrected the comment to name GetNonEmptyAverageRegionSize (the method these counters actually back — GetAverageRegionSize itself was reverted to its original behavior earlier in this PR). Also added a non-empty-average-region-size field to the debug log alongside the existing average-region-size, so both are visible regardless of which one actually fed into tolerantResource for a given schedule kind.
…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>
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Actionable comments posted: 1
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Inline comments:
In `@pkg/core/region.go`:
- Around line 2361-2368: Update GetNonEmptyAverageRegionSize to return 0 when
r.tree is nil before accessing nonEmptyRegionsCnt or calling tree methods, while
preserving the existing behavior for initialized RegionsInfo values.
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pkg/core/region.gopkg/schedule/schedulers/balance_region_test.gopkg/schedule/schedulers/utils.go
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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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The PR description is now materially out of sync with the final diff. It still says this PR adds |
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Updated — title and description now reflect the final implementation (symmetric |
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/ping @rleungx ptal |
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4 participants
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