# HG changeset patch
# User Gregory Szorc <gregory.sz...@gmail.com>
# Date 1476650123 25200
#      Sun Oct 16 13:35:23 2016 -0700
# Node ID 4582f12754622ae049afefee05176ef107d99a7e
# Parent  9e2f957b05ac5c76595280a6084ba01d7b369a05
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.

diff --git a/mercurial/changegroup.py b/mercurial/changegroup.py
--- a/mercurial/changegroup.py
+++ b/mercurial/changegroup.py
@@ -88,17 +88,19 @@ def writechunks(ui, chunks, filename, vf
     fh = None
     cleanup = None
         if filename:
             if vfs:
                 fh = vfs.open(filename, "wb")
-                fh = open(filename, "wb")
+                # Increase default buffer size because default is usually
+                # small (4k is common on Linux).
+                fh = open(filename, "wb", 131072)
             fd, filename = tempfile.mkstemp(prefix="hg-bundle-", suffix=".hg")
             fh = os.fdopen(fd, "wb")
         cleanup = filename
         for c in chunks:
         cleanup = None
         return filename
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