view mercurial/help/hgignore.txt @ 33171:6d678ab1b10d

revlog: C implementation of delta chain resolution

I've seen revlog._deltachain() appear in a number of performance
profiles. I suspect there are 2 reasons for this:

1. Delta chain resolution performs many index lookups, thus triggering
   population of index tuples. Creating possibly tens of thousands of
   PyObject will have overhead.
2. Delta chain resolution is a tight loop.

By moving delta chain resolution to C, we can defer instantiation
of full index entry tuples and make the loop faster courtesy of
not running in Python.

We can measure the impact to delta chain resolution via
`hg perflogrevision` using the mozilla-central repo with a recent
manifest having delta chain length of 33726:

$ hg perfrevlogrevision -m 364895
! full
! wall 0.367585 comb 0.370000 user 0.340000 sys 0.030000 (best of 27)
! wall 0.357581 comb 0.360000 user 0.350000 sys 0.010000 (best of 28)
! deltachain
! wall 0.010644 comb 0.010000 user 0.010000 sys 0.000000 (best of 270)
! wall 0.000292 comb 0.000000 user 0.000000 sys 0.000000 (best of 8729)

$ hg perfrevlogrevision --cache -m 364895
! deltachain
! wall 0.003904 comb 0.000000 user 0.000000 sys 0.000000 (best of 712)
! wall 0.000284 comb 0.000000 user 0.000000 sys 0.000000 (best of 9926)

The first test measures savings from both not instantiating index
entries and moving to C. The second test (which doesn't clear the
index caches) essentially isolates the benefits of moving from Python
to C. It still shows a 13.7x speedup (versus 36.4x). And there are
multiple milliseconds of savings within the critical path for resolving
revision data. I think that justifies the existence of C code.

A more striking example of the benefits of this change can be
demonstrated by timing `hg debugdeltachain -m` for the mozilla-central
repo:

$ time hg debugdeltachain -m > /dev/null
before:        1057.4s
after:          503.3s
PyPy2.7 5.8.0:  220.0s

It's worth noting that the C code isn't as optimal as it could be.
We're still instantiating a new PyObject for every revision. A future
optimization would be to reuse the PyObject on the cached index tuple.
We could potentially also get wins by using a memory array of raw
integers. There is also room for a delta chain cache on revlog
instances. Of course, the best optimization is to implement revlog
reading outside of Python so Python doesn't need to be concerned
about the relatively expensive index entries and operations on them.

Synopsis
========

The Mercurial system uses a file called ``.hgignore`` in the root
directory of a repository to control its behavior when it searches
for files that it is not currently tracking.

Description
===========

The working directory of a Mercurial repository will often contain
files that should not be tracked by Mercurial. These include backup
files created by editors and build products created by compilers.
These files can be ignored by listing them in a ``.hgignore`` file in
the root of the working directory. The ``.hgignore`` file must be
created manually. It is typically put under version control, so that
the settings will propagate to other repositories with push and pull.

An untracked file is ignored if its path relative to the repository
root directory, or any prefix path of that path, is matched against
any pattern in ``.hgignore``.

For example, say we have an untracked file, ``file.c``, at
``a/b/file.c`` inside our repository. Mercurial will ignore ``file.c``
if any pattern in ``.hgignore`` matches ``a/b/file.c``, ``a/b`` or ``a``.

In addition, a Mercurial configuration file can reference a set of
per-user or global ignore files. See the ``ignore`` configuration
key on the ``[ui]`` section of :hg:`help config` for details of how to
configure these files.

To control Mercurial's handling of files that it manages, many
commands support the ``-I`` and ``-X`` options; see
:hg:`help <command>` and :hg:`help patterns` for details.

Files that are already tracked are not affected by .hgignore, even
if they appear in .hgignore. An untracked file X can be explicitly
added with :hg:`add X`, even if X would be excluded by a pattern
in .hgignore.

Syntax
======

An ignore file is a plain text file consisting of a list of patterns,
with one pattern per line. Empty lines are skipped. The ``#``
character is treated as a comment character, and the ``\`` character
is treated as an escape character.

Mercurial supports several pattern syntaxes. The default syntax used
is Python/Perl-style regular expressions.

To change the syntax used, use a line of the following form::

  syntax: NAME

where ``NAME`` is one of the following:

``regexp``
  Regular expression, Python/Perl syntax.
``glob``
  Shell-style glob.

The chosen syntax stays in effect when parsing all patterns that
follow, until another syntax is selected.

Neither glob nor regexp patterns are rooted. A glob-syntax pattern of
the form ``*.c`` will match a file ending in ``.c`` in any directory,
and a regexp pattern of the form ``\.c$`` will do the same. To root a
regexp pattern, start it with ``^``.

Subdirectories can have their own .hgignore settings by adding
``subinclude:path/to/subdir/.hgignore`` to the root ``.hgignore``. See
:hg:`help patterns` for details on ``subinclude:`` and ``include:``.

.. note::

  Patterns specified in other than ``.hgignore`` are always rooted.
  Please see :hg:`help patterns` for details.

Example
=======

Here is an example ignore file. ::

  # use glob syntax.
  syntax: glob

  *.elc
  *.pyc
  *~

  # switch to regexp syntax.
  syntax: regexp
  ^\.pc/