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discovery: slowly increase sampling size...
discovery: slowly increase sampling size Some pathological discovery runs can requires many roundtrip. When this happens things can get very slow. To make the algorithm more resilience again such pathological case. We slowly increase the sample size with each roundtrip (+5%). This will have a negligible impact on "normal" discovery with few roundtrips, but a large positive impact of case with many roundtrips. Asking more question per roundtrip helps to reduce the undecided set faster. Instead of reducing the undecided set a linear speed (in the worst case), we reduce it as a guaranteed (small) exponential rate. The data below show this slow ramp up in sample size: round trip | 1 | 5 | 10 | 20 | 50 | 100 | 130 | sample size | 200 | 254 | 321 | 517 | 2 199 | 25 123 | 108 549 | covered nodes | 200 | 1 357 | 2 821 | 7 031 | 42 658 | 524 530 | 2 276 755 | To be a bit more concrete, lets take a very pathological case as an example. We are doing discovery from a copy of Mozilla-try to a more recent version of mozilla-unified. Mozilla-unified heads are unknown to the mozilla-try repo and there are over 1 million "missing" changesets. (the discovery is "local" to avoid network interference) Without this change, the discovery: - last 1858 seconds (31 minutes), - does 1700 round trip, - asking about 340 000 nodes. With this change, the discovery: - last 218 seconds (3 minutes, 38 seconds a -88% improvement), - does 94 round trip (-94%), - asking about 344 211 nodes (+1%). Of course, this is an extreme case (and 3 minutes is still slow). However this give a good example of how this sample size increase act as a safety net catching any bad situations. We could image a steeper increase than 5%. For example 10% would give the following number: round trip | 1 | 5 | 10 | 20 | 50 | 75 | 100 | sample size | 200 | 321 | 514 | 1 326 | 23 060 | 249 812 | 2 706 594 | covered nodes | 200 | 1 541 | 3 690 | 12 671 | 251 871 | 2 746 254 | 29 770 966 | In parallel, it is useful to understand these pathological cases and improve them. However the current change provides a general purpose safety net to smooth the impact of pathological cases. To avoid issue with older http server, the increase in sample size only occurs if the protocol has not limit on command argument size.

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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.