diff mercurial/help/config.txt @ 27895:2d6a89e79b48

scmutil: support background file closing

Closing files that have been appended to is relatively slow on
Windows/NTFS. This makes several Mercurial operations slower on
Windows.

The workaround to this issue is conceptually simple: use multiple
threads for I/O. Unfortunately, Python doesn't scale well to multiple
threads because of the GIL. And, refactoring our code to use threads
everywhere would be a huge undertaking. So, we decide to tackle this
problem by starting small: establishing a thread pool for closing
files.

This patch establishes a mechanism for closing file handles on separate
threads. The coordinator object is basically a queue of file handles to
operate on and a thread pool consuming from the queue.

When files are opened through the VFS layer, the caller can specify
that delay closing is allowed.

A proxy class for file handles has been added. We must use a proxy
because it isn't possible to modify __class__ on built-in types. This
adds some overhead. But as future patches will show, this overhead
is cancelled out by the benefit of closing file handles on background
threads.

--- a/mercurial/help/config.txt	Tue Jan 12 23:56:48 2016 +0900
+++ b/mercurial/help/config.txt	Thu Jan 14 13:34:59 2016 -0800
@@ -2037,3 +2037,27 @@
     Number of CPUs to use for parallel operations. A zero or
     negative value is treated as ``use the default``.
     (default: 4 or the number of CPUs on the system, whichever is larger)
+
+``backgroundclose``
+    Whether to enable closing file handles on background threads during certain
+    operations. Some platforms aren't very efficient at closing file
+    handles that have been written or appened to. By performing file closing
+    on background threads, file write rate can increase substantially.
+    (default: true on Windows, false elsewhere)
+
+``backgroundcloseminfilecount``
+    Minimum number of files required to trigger background file closing.
+    Operations not writing this many files won't start background close
+    threads.
+    (default: 2048)
+
+``backgroundclosemaxqueue``
+    The maximum number of opened file handles waiting to be closed in the
+    background. This option only has an effect if ``backgroundclose`` is
+    enabled.
+    (default: 384)
+
+``backgroundclosethreadcount``
+    Number of threads to process background file closes. Only relevant if
+    ``backgroundclose`` is enabled.
+    (default: 4)