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merge: mark file gets as not thread safe (issue5933)...
merge: mark file gets as not thread safe (issue5933) In default installs, this has the effect of disabling the thread-based worker on Windows when manifesting files in the working directory. My measurements have shown that with revlog-based repositories, Mercurial spends a lot of CPU time in revlog code resolving file data. This ends up incurring a lot of context switching across threads and slows down `hg update` operations when going from an empty working directory to the tip of the repo. On mozilla-unified (246,351 files) on an i7-6700K (4+4 CPUs): before: 487s wall after: 360s wall (equivalent to worker.enabled=false) cpus=2: 379s wall Even with only 2 threads, the thread pool is still slower. The introduction of the thread-based worker (02b36e860e0b) states that it resulted in a "~50%" speedup for `hg sparse --enable-profile` and `hg sparse --disable-profile`. This disagrees with my measurement above. I theorize a few reasons for this: 1) Removal of files from the working directory is I/O - not CPU - bound and should benefit from a thread pool (unless I/O is insanely fast and the GIL release is near instantaneous). So tests like `hg sparse --enable-profile` may exercise deletion throughput and aren't good benchmarks for worker tasks that are CPU heavy. 2) The patch was authored by someone at Facebook. The results were likely measured against a repository using remotefilelog. And I believe that revision retrieval during working directory updates with remotefilelog will often use a remote store, thus being I/O and not CPU bound. This probably resulted in an overstated performance gain. Since there appears to be a need to enable the thread-based worker with some stores, I've made the flagging of file gets as thread safe configurable. I've made it experimental because I don't want to formalize a boolean flag for this option and because this attribute is best captured against the store implementation. But we don't have a proper store API for this yet. I'd rather cross this bridge later. It is possible there are revlog-based repositories that do benefit from a thread-based worker. I didn't do very comprehensive testing. If there are, we may want to devise a more proper algorithm for whether to use the thread-based worker, including possibly config options to limit the number of threads to use. But until I see evidence that justifies complexity, simplicity wins. Differential Revision: https://phab.mercurial-scm.org/D3963
Wagner Bruna - - Load All Authors

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bundlespec.txt
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Mercurial supports generating standalone "bundle" files that hold repository
data. These "bundles" are typically saved locally and used later or exchanged
between different repositories, possibly on different machines. Example
commands using bundles are :hg:`bundle` and :hg:`unbundle`.
Generation of bundle files is controlled by a "bundle specification"
("bundlespec") string. This string tells the bundle generation process how
to create the bundle.
A "bundlespec" string is composed of the following elements:
type
A string denoting the bundle format to use.
compression
Denotes the compression engine to use compressing the raw bundle data.
parameters
Arbitrary key-value parameters to further control bundle generation.
A "bundlespec" string has the following formats:
<type>
The literal bundle format string is used.
<compression>-<type>
The compression engine and format are delimited by a hyphen (``-``).
Optional parameters follow the ``<type>``. Parameters are URI escaped
``key=value`` pairs. Each pair is delimited by a semicolon (``;``). The
first parameter begins after a ``;`` immediately following the ``<type>``
value.
Available Types
===============
The following bundle <type> strings are available:
v1
Produces a legacy "changegroup" version 1 bundle.
This format is compatible with nearly all Mercurial clients because it is
the oldest. However, it has some limitations, which is why it is no longer
the default for new repositories.
``v1`` bundles can be used with modern repositories using the "generaldelta"
storage format. However, it may take longer to produce the bundle and the
resulting bundle may be significantly larger than a ``v2`` bundle.
``v1`` bundles can only use the ``gzip``, ``bzip2``, and ``none`` compression
formats.
v2
Produces a version 2 bundle.
Version 2 bundles are an extensible format that can store additional
repository data (such as bookmarks and phases information) and they can
store data more efficiently, resulting in smaller bundles.
Version 2 bundles can also use modern compression engines, such as
``zstd``, making them faster to compress and often smaller.
Available Compression Engines
=============================
The following bundle <compression> engines can be used:
.. bundlecompressionmarker
Examples
========
``v2``
Produce a ``v2`` bundle using default options, including compression.
``none-v1``
Produce a ``v1`` bundle with no compression.
``zstd-v2``
Produce a ``v2`` bundle with zstandard compression using default
settings.
``zstd-v1``
This errors because ``zstd`` is not supported for ``v1`` types.