revset: improves time complexity of 'roots(xxx)' The canonical way of doing 'roots(X)' is 'X - children(X)'. This is what the implementation used to be. However, computing children is expensive because it is unbounded. Any changesets in the repository may be a children of '0' so you have to look at all changesets in the repository to compute children(0). Moreover the current revsets implementation for children is not lazy, leading to bad performance when fetching the first result. There is a more restricted algorithm to compute roots: roots(X) = [r for r in X if not parents(r) & X] This achieve the same result while only looking for parent/children relation in the X set itself, making the algorithm 'O(len(X))' membership operation. Another advantages is that it turns the check into a simple filter, preserving all laziness property of the underlying revsets. The speed is very significant and some laziness is restored. -) revset without 'roots(...)' to compare to base line 0) before this change 1) after this change revset #0: roots((tip~100::) - (tip~100::tip)) plain min last -) 0.001082 0.000993 0.000790 0) 0.001366 0.001385 0.001339 1) 0.001257 92% 0.001028 74% 0.000821 61% revset #1: roots((0::) - (0::tip)) plain min last -) 0.134551 0.144682 0.068453 0) 0.161822 0.171786 0.157683 1) 0.137583 85% 0.146204 85% 0.070012 44% revset #2: roots(tip~100:) plain min first last -) 0.000219 0.000225 0.000231 0.000229 0) 0.000513 0.000529 0.000507 0.000539 1) 0.000463 90% 0.000269 50% 0.000267 52% 0.000463 85% revset #3: roots(:42) plain min first last -) 0.000119 0.000146 0.000146 0.000146 0) 0.000231 0.000254 0.000253 0.000260 1) 0.000216 93% 0.000186 73% 0.000184 72% 0.000244 93% revset #4: roots(not public()) plain min first -) 0.000478 0.000502 0.000504 0) 0.000611 0.000639 0.000634 1) 0.000604 0.000560 87% 0.000558 revset #5: roots((0:tip)::) plain min max first last -) 0.057795 0.004905 0.058260 0.004908 0.038812 0) 0.132845 0.118931 0.130306 0.114280 0.127742 1) 0.111659 84% 0.005023 4% 0.111658 85% 0.005022 4% 0.092490 72% revset #6: roots(0::tip) plain min max first last -) 0.032971 0.033947 0.033460 0.032350 0.033125 0) 0.083671 0.081953 0.084074 0.080364 0.086069 1) 0.074720 89% 0.035547 43% 0.077025 91% 0.033729 41% 0.083197 revset #7: 42:68 and roots(42:tip) plain min max first last -) 0.006827 0.000251 0.006830 0.000254 0.006771 0) 0.000337 0.000353 0.000366 0.000350 0.000366 1) 0.000318 94% 0.000297 84% 0.000353 0.000293 83% 0.000351 revset #8: roots(0:tip) plain min max first last -) 0.002119 0.000145 0.000147 0.000147 0.000147 0) 0.047441 0.040660 0.045662 0.040284 0.043435 1) 0.038057 80% 0.000187 0% 0.034919 76% 0.000186 0% 0.035097 80% revset #0: roots(:42 + tip~42:) plain min max first last sort -) 0.000321 0.000317 0.000319 0.000308 0.000369 0.000343 0) 0.000772 0.000751 0.000811 0.000750 0.000802 0.000783 1) 0.000632 81% 0.000369 49% 0.000617 76% 0.000358 47% 0.000601 74% 0.000642 81%