[PATCH 3 of 3 stable] changegroup: sparsely populate fnodes

[Gregory Szorc]

# HG changeset patch
# User Gregory Szorc <gregory.szorc at gmail.com>
# Date 1415342900 28800
#      Thu Nov 06 22:48:20 2014 -0800
# Node ID 1c646d31cdd16a0f7dddaf869bc0ea59d7c0c25c
# Parent  87945ee4a50d8e639a737d72f8e5c1e8a45441da
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.

diff --git a/mercurial/changegroup.py b/mercurial/changegroup.py
--- a/mercurial/changegroup.py
+++ b/mercurial/changegroup.py
@@ -343,10 +343,8 @@ class cg1packer(object):
                                 reorder=reorder):
             yield chunk
         progress(msgbundling, None)
 
-        for f in changedfiles:
-            fnodes[f] = {}
 
         # Callback for the manifest, used to collect linkrevs for filelog
         # revisions.
         # Returns the linkrev node (collected in lookupcl).
@@ -357,9 +355,9 @@ class cg1packer(object):
                 for f, n in mdata.iteritems():
                     if f in changedfiles:
                         # record the first changeset introducing this filelog
                         # version
-                        fnodes[f].setdefault(n, clnode)
+                        fnodes.setdefault(f, {}).setdefault(n, clnode)
             return clnode
 
         mfnodes = self.prune(mf, mfs, commonrevs, source)
         for chunk in self.group(mfnodes, mf, lookupmf, units=_('manifests'),