changegroup: increase write buffer size to 128k By default, Python defers to the operating system for choosing the default buffer size on opened files. On my Linux machine, the default is 4k, which is really small for 2016. This patch bumps the write buffer size when writing changegroups/bundles to 128k. This matches the 128k read buffer we already use on revlogs. It's worth noting that this only impacts when writing to an explicit file (such as during `hg bundle`). Buffers when writing to bundle files via the repo vfs or to a temporary file are not impacted. When producing a none-v2 bundle file of the mozilla-unified repository, this change caused the number of write() system calls to drop from 952,449 to 29,788. After this change, the most frequent system calls are fstat(), read(), lseek(), and open(). There were 2,523,672 system calls after this patch (so a net decrease of ~950k is statistically significant). This change shows no performance change on my system. But I have a high-end system with a fast SSD. It is quite possible this change will have a significant impact on network file systems, where extra network round trips due to excessive I/O system calls could introduce significant latency.

--- a/mercurial/changegroup.py	Fri Oct 14 01:31:11 2016 +0200
+++ b/mercurial/changegroup.py	Sun Oct 16 13:35:23 2016 -0700
@@ -93,7 +93,9 @@
             if vfs:
                 fh = vfs.open(filename, "wb")
             else:
-                fh = open(filename, "wb")
+                # Increase default buffer size because default is usually
+                # small (4k is common on Linux).
+                fh = open(filename, "wb", 131072)
         else:
             fd, filename = tempfile.mkstemp(prefix="hg-bundle-", suffix=".hg")
             fh = os.fdopen(fd, "wb")