fees: enable `CBlockPolicyEstimator` return sub 1 sat/vb fee rate estimates #33199

ismaelsadeeq · 2025-08-15T15:15:26Z

This is a simple PR that updates the block policy estimator’s MIN_BUCKET_FEERATE constant to be identical to the policy DEFAULT_MIN_RELAY_TX_FEE.
It also enables the fee estimator to record transactions with a fee rate up to the DEFAULT_MIN_RELAY_TX_FEE in its stats, which effectively allows the estimator to return sub-1 sat/vB fee rate estimates.

The change is correct because the estimator creates buckets of fee rates from
MIN_BUCKET_FEERATE,
MIN_BUCKET_FEERATE + FEE_SPACING,
MIN_BUCKET_FEERATE + 2 * FEE_SPACING,
… up to MAX_BUCKET_FEERATE.

This means it will record sub-1 sat/vB fee rates in the buckets and may return them as a fee rate estimate when that bucket is the lowest one with sufficient transactions for a given target.

Tying MIN_BUCKET_FEERATE to DEFAULT_MIN_RELAY_TX_FEE also makes it dynamic, so we don’t need to update the fee estimator when the minrelaytxfee default is adjusted in the future.

For the mempool fee rate estimator in #31664, no update is needed since it uses the block assembler and because the node can see sub-1 sat/vB transactions, it can generate block templates with those and therefore return a fee rate estimate accordingly.

While touching this part of the fee estimator code, this PR got rid of the dummy value persisted in the file

DrahtBot · 2025-08-15T15:15:32Z

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For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/33199.

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Type	Reviewers
ACK	polespinasa
Concept ACK	kevkevinpal, musaHaruna

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Conflicts

Reviewers, this pull request conflicts with the following ones:

#34075 (fees: Introduce Mempool Based Fee Estimation to reduce overestimation by ismaelsadeeq)

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polespinasa

concept ACK

This change makes much sense to me, if the min relay fee is lowered the feerate tracking should also take that into account.

test/functional/feature_fee_estimation.py

src/policy/fees/block_policy_estimator.h

kevkevinpal · 2025-08-26T11:59:44Z

Concept ACK 413c81b

It makes sense to update MIN_BUCKET_FEERATE with the recent changes to be sub 1 sat/vbyte

I do think the additional test is a bit redundant but I am okay in adding if others think it makes sense

maflcko · 2025-08-28T10:52:42Z

This picks up #13990? If yes, it may be good to explain the differences.

ismaelsadeeq · 2025-09-01T13:28:01Z

This picks up #13990? If yes, it may be good to explain the differences.

This is different from #13990 in approach:

It omits the consistency check and avoids extending the fee rate buckets when the min bucket fee rate changes.
Instead, it builds on #29702 by simply bumping the fees file version, which ensures that estimates saved in previous files are not read. As such after a restart, the estimator starts afresh.

doc/release-notes-33199.md

test/functional/feature_fee_estimation.py

ismaelsadeeq · 2025-10-29T17:09:42Z

Rebased and fixed nits.

polespinasa

Just some nits and questions :=)

src/policy/fees/block_policy_estimator.h

src/policy/fees/block_policy_estimator.cpp

- This commit deletes a dummy field that is "dummy" and unused.

- This commit also refactors the test the better track confirmed utxos.

ismaelsadeeq · 2025-11-07T14:51:24Z

Just some nits and questions :=)

I force pushed to address the comment, thanks for review @polespinasa

musaHaruna · 2025-12-13T15:08:56Z

Concept ACK (code review up to b767897)

I’m new to fee estimation, but I was able to understand this PR because it is fairly straightforward.

The idea is to make DEFAULT_MIN_RELAY_TX_FEE i.e the minimum feerate a node is willing to relay and accept into its mempool by default equal to MIN_BUCKET_FEERATE, which is used by the fee estimator (CBlockPolicyEstimator).

By default, if a transaction’s feerate is below DEFAULT_MIN_RELAY_TX_FEE, the node:
will not relay it, and will not keep it in its mempool.

Nodes are free to choose a different value via the -minrelaytxfee option, but this value (DEFAULT_MIN_RELAY_TX_FEE) defines the default policy.

MIN_BUCKET_FEERATE lives in the fee estimator and defines the lowest feerate bucket that the estimator tracks. Any transaction with a feerate below this value was previously ignored by the estimator. Importantly, MIN_BUCKET_FEERATE does not reject transactions and does not affect relay or mempool policy it only affects what data the fee estimator records.

Before this PR mempool policy accepts transactions down to DEFAULT_MIN_RELAY_TX_FEE = 100 (i.e. 1 sat/vB). while fee estimator (CBlockPolicyEstimator) Starts tracking fees at a higher minimum:
MIN_BUCKET_FEERATE = 1000 (i.e. 10 sat/vB).

As a result, transactions that were perfectly valid, accepted, and relayed by the node were not recorded by the fee estimator. Therefore the estimator could not learn from low-fee traffic.

By making: MIN_BUCKET_FEERATE == DEFAULT_MIN_RELAY_TX_FEE
the fee estimator now tracks all transactions the node is willing to relay, including sub-1 sat/vB transactions (i.e. transactions with feerates below 1 sat/vB, if the relay policy allows them. Not sure if am correct on that), and
can return lower and more accurate fee estimates.

The change is correct because the estimator creates buckets of fee rates from
MIN_BUCKET_FEERATE,
MIN_BUCKET_FEERATE + FEE_SPACING,
MIN_BUCKET_FEERATE + 2 * FEE_SPACING,
… up to MAX_BUCKET_FEERATE.

Confirmed this behaviour by digging into CBlockPolicyEstimator bucket construction logic.

musaHaruna · 2025-12-13T15:23:40Z

test/functional/feature_fee_estimation.py

    def transact_and_mine(self, numblocks, mining_node):
-        min_fee = Decimal("0.00001")
+        min_fee = MINIMUM_RELAY_FEE_RATE
        # We will now mine numblocks blocks generating on average 100 transactions between each block


nit: feel free to ignore

Suggested change

# We will now mine numblocks blocks generating on average 100 transactions between each block

# We will now mine "numblocks" blocks, generating on average 100 transactions between each block

I think makes it easier to read

polespinasa

code looks good to me, currently trying to test it on mainnet will ack if positive results :)

a small nit for more clarity on the PR

src/policy/fees/block_policy_estimator.cpp

polespinasa

cr and tACK b767897

ismaelsadeeq force-pushed the 08-2025-low-fee-rate-estimate branch from e0da552 to 413c81b Compare August 15, 2025 15:18

This was referenced Aug 16, 2025

fees: rpc: estimatesmartfee now returns a fee rate estimate during low network activity #32395

Closed

Fees: add Fee rate Forecaster Manager #31664

Closed

Fee Estimation via Fee rate Forecasters #30157

Closed

polespinasa reviewed Aug 19, 2025

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kevkevinpal reviewed Aug 26, 2025

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maflcko added the TX fees and policy label Aug 28, 2025

ismaelsadeeq force-pushed the 08-2025-low-fee-rate-estimate branch 2 times, most recently from 30415b6 to 0eb02ea Compare September 1, 2025 13:13

maflcko reviewed Sep 1, 2025

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This was referenced Sep 2, 2025

rpc: require integer verbosity; remove boolean 'verbose' #33214

Open

Fee Estimation: Ignore all transactions that are CPFP'd #30079

Closed

DrahtBot added the Needs rebase label Oct 28, 2025

DrahtBot mentioned this pull request Oct 28, 2025

ci, iwyu: Treat warnings as errors for src/init and src/policy #33725

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ismaelsadeeq force-pushed the 08-2025-low-fee-rate-estimate branch 2 times, most recently from fbf49e0 to 98be7ef Compare October 29, 2025 17:08

DrahtBot removed the Needs rebase label Oct 29, 2025

polespinasa reviewed Oct 29, 2025

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ismaelsadeeq added 5 commits November 7, 2025 15:44

fees: make MIN_BUCKET_FEERATE identical to DEFAULT_MIN_RELAY_TX_FEE

48439a3

fees: bump fees file version

67e9f89

fees: delete unused dummy field

44d25dc

- This commit deletes a dummy field that is "dummy" and unused.

test: ensure fee estimator provide fee rate estimate < 1 s/vb

c56f644

- This commit also refactors the test the better track confirmed utxos.

doc: add release notes

b767897

ismaelsadeeq force-pushed the 08-2025-low-fee-rate-estimate branch from 98be7ef to b767897 Compare November 7, 2025 14:49

musaHaruna reviewed Dec 13, 2025

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fees: Introduce Mempool Based Fee Estimation to reduce overestimation #34075

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polespinasa reviewed Jan 7, 2026

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polespinasa approved these changes Jan 7, 2026

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DrahtBot requested review from kevkevinpal and musaHaruna January 7, 2026 11:57

	# We will now mine numblocks blocks generating on average 100 transactions between each block
	# We will now mine "numblocks" blocks, generating on average 100 transactions between each block

fees: enable CBlockPolicyEstimator return sub 1 sat/vb fee rate estimates #33199

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fees: enable CBlockPolicyEstimator return sub 1 sat/vb fee rate estimates #33199

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fees: enable `CBlockPolicyEstimator` return sub 1 sat/vb fee rate estimates #33199

fees: enable `CBlockPolicyEstimator` return sub 1 sat/vb fee rate estimates #33199

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