changegroup: sparsely populate fnodes Previously, fnodes had a key and empty dict value for every element in changedfiles. This is somewhat wasteful. Empty dicts in CPython consume a lot more memory than you would expect - 280 bytes. On mozilla-central, which has ~190,000 files/fnodes keys, the previous loop populating fnodes allocated 91,924 KB of memory, most of that for the empty dicts. With this patch in place, our peak RSS during mozilla-central clone drops: before: 364,356 KB after: 326,008 KB delta: -38,348 KB When combined with the previous patch, total peak RSS decrease is now 190,116 KB.

changegroup: don't store unused value on fnodes (issue4443) The contents of fnodes are only accessed once per key. It is wasteful to cache the value since nobody will use it. Before this patch, the caching of unused data in fnodes was effectively causing a memory leak during the file streaming part of bundle creation. On mozilla-central (which has ~190,000 entries in fnodes), this patch has a significant impact on RSS at the end of generate(): before: 516,124 KB after: 364,356 KB delta: -151,768 KB The origin of this code can be traced back to 627cd7842e5d and has been with us since the 2.7 release.

changegroup: don't define lookupmf() until it is needed lookupmf() is currently defined earlier than when it is needed. Future patches further refactoring this code will be easier to read when lookupmf() is in its new home.