Askrene: fix constraints #8358
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Askrene: fix constraints #8358
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Lagrang3
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Jul 17, 2025
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Try a new way to refine flows, ie. reduce excess due to MCF accuracy and HTLC max constraints after hop amounts are computed with fees included. Changelog-None. Signed-off-by: Lagrang3 <[email protected]>
Changelog-None Signed-off-by: Lagrang3 <[email protected]>
Disable channels with HTLC min violations so that we don't hit them twice when computing routes. Changelog-None Signed-off-by: Lagrang3 <[email protected]>
that can be useful for us in the mcf.c main loop. Changelog-None Signed-off-by: Lagrang3 <[email protected]>
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by a small amount if the deliver amount is less than the requested amount by X. This step saves runtime by avoiding calling an extra MCF and it helps us solve a small percentage of cases where the only available routes have HTLCmin that is bigger than X. Changelog-None Signed-off-by: Lagrang3 <[email protected]>
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We use a wrapper around the MCF solver that takes care of finding the
best linearization parameters and fixing the flow values to meet the
htlc_min and htlc_max constraints.
We have reworked the current implementation and made it a bit more
similar to renepay's version.
Out of 50000 simulated payment situations distributed accross payment
amounts of 1e2, 1e3, 1e4, 1e5 and 1e6 sats, we find that 133 failed
cases in the master branch turn to success with the current changes,
while only 3 success cases in the master are not solved by the changes.
master
+-------+------+
| S | F |
+---+-------+------+
| S | 46329 | 133 |
changes +---+-------+------+
| F | 3 | 3535 |
+---+-------+------+
Out of the 133 cases that flipped from failure to success the failed
reasons were:
122 -> "Could not find route without excessive cost"
5 -> "We couldn't quite afford it"
5 -> "Amount *msat below minimum"
1 -> tripped an HTLC min check
Changelog-None.
Signed-off-by: Lagrang3 <[email protected]>
and remove unused helper functions. Changelog-None Signed-off-by: Lagrang3 <[email protected]>
that new flows respect the HTLC min/max constraints. Changelog-None Signed-off-by: Lagrang3 <[email protected]>
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rustyrussell
approved these changes
Aug 8, 2025
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Description
In this PR I am refactoring the wrapper of the MCF solver and the
refineprocedure in askreneto fix some issues we have seen when building routes in some common corner cases.
It addresses issues: #8339 and #7960.
Results
We have tested (with this tool https://github.com/Lagrang3/askrene-benchmarks/blob/master/test_askrene_bench.py)
the performance of askrene before and after the changes introduced in this PR.
We have found that in a test set of 50000 payment attempts
(random pairs of nodes and amounts from the set
{1e2, 1e3, 1e4, 1e5, 1e6}) that 133 failed cases using the master branchflip to success in with the changes in this PR, while only 3 cases that were successes in the master become failures
with this PR.
Out of the 133 cases that flipped from failure to success the failed message was attributed to the following:
For completeness we also show the failure cases that remained failure after the changes in this PR:
That means that we are able to fix 5 out of 6 cases of "We couldn't quite afford it".
There is also a significant decrease in runtime for payments around 1000sats (~3x) and significant
less fees for payments around 100sats (~3x):
Tasks
Follow up
In a following PR I will address other issues related to
refineandmcf:This is part of a sequence of refactorings and improvements, last one is #8332.