As I can see here https://github.com/python/devguide/blob/master/garbage_collector.rst#collecting-the-oldest-generation

> the GC only triggers a full collection of the oldest generation if the ratio long_lived_pending / long_lived_total is above a given value (hardwired to 25%)

But in the python docs here https://docs.python.org/3.10/library/gc.html#gc.set_threshold

> When the number of allocations minus the number of deallocations exceeds threshold0, collection starts. Initially only generation 0 is examined. If generation 0 has been examined more than threshold1 times since generation 1 has been examined, then generation 1 is examined as well. Similarly, threshold2 controls the number of collections of generation 1 before collecting generation 2.

So, which one is correct?

As I can see here https://github.com/python/devguide/blob/master/garbage_collector.rst#collecting-the-oldest-generation

> the GC only triggers a full collection of the oldest generation if the ratio long_lived_pending / long_lived_total is above a given value (hardwired to 25%)

But in the python docs here https://docs.python.org/3.10/library/gc.html#gc.set_threshold

> When the number of allocations minus the number of deallocations exceeds threshold0, collection starts. Initially only generation 0 is examined. If generation 0 has been examined more than threshold1 times since generation 1 has been examined, then generation 1 is examined as well. Similarly, threshold2 controls the number of collections of generation 1 before collecting generation 2.

As I can see here: https://github.com/python/cpython/blob/master/Modules/gcmodule.c#L1456 the first one is correct.

We should probably fix python docs accordingly.

Here's opened PR. https://github.com/python/cpython/pull/21703

New changeset 82ca8fada15b121866530f2cdac1b7055be4a244 by Yaroslav Pankovych in branch 'master':
bpo-41455: Provide a link to how the third generation is collected in the GC docs (GH-21703)
https://github.com/python/cpython/commit/82ca8fada15b121866530f2cdac1b7055be4a244

New changeset 103ce3debf07a48e79319acd0890be0c53e64fff by Miss Islington (bot) in branch '3.9':
bpo-41455: Provide a link to how the third generation is collected in the GC docs (GH-21703)
https://github.com/python/cpython/commit/103ce3debf07a48e79319acd0890be0c53e64fff

New changeset 105cfb5b182da63e8481fcb009e92546d240c6b5 by Miss Islington (bot) in branch '3.8':
bpo-41455: Provide a link to how the third generation is collected in the GC docs (GH-21703)
https://github.com/python/cpython/commit/105cfb5b182da63e8481fcb009e92546d240c6b5

New changeset f3b6f3cd9ac6931ae346cf298fae7b691d5656bb by Miss Islington (bot) in branch '3.7':
bpo-41455: Provide a link to how the third generation is collected in the GC docs (GH-21703) (GH-21788)
https://github.com/python/cpython/commit/f3b6f3cd9ac6931ae346cf298fae7b691d5656bb