# HG changeset patch
# User Gregory Szorc <gregory.szorc@gmail.com>
# Date 1415342900 28800
# Node ID 5dcaed20b27ca8e58ddf301b7e9f3711994eba1b
# Parent  bdf7b1ea1dae9daf754e77523c8765f70f6111fc
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 -r bdf7b1ea1dae -r 5dcaed20b27c mercurial/changegroup.py
--- a/mercurial/changegroup.py	Thu Nov 06 22:33:48 2014 -0800
+++ b/mercurial/changegroup.py	Thu Nov 06 22:48:20 2014 -0800
@@ -335,9 +335,6 @@
             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).
@@ -349,7 +346,7 @@
                     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)