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localrepo: experimental support for non-zlib revlog compression...
localrepo: experimental support for non-zlib revlog compression The final part of integrating the compression manager APIs into revlog storage is the plumbing for repositories to advertise they are using non-zlib storage and for revlogs to instantiate a non-zlib compression engine. The main intent of the compression manager work was to zstd all of the things. Adding zstd to revlogs has proved to be more involved than other places because revlogs are... special. Very small inputs and the use of delta chains (which are themselves a form of compression) are a completely different use case from streaming compression, which bundles and the wire protocol employ. I've conducted numerous experiments with zstd in revlogs and have yet to formalize compression settings and a storage architecture that I'm confident I won't regret later. In other words, I'm not yet ready to commit to a new mechanism for using zstd - or any other compression format - in revlogs. That being said, having some support for zstd (and other compression formats) in revlogs in core is beneficial. It can allow others to conduct experiments. This patch introduces *highly experimental* support for non-zlib compression formats in revlogs. Introduced is a config option to control which compression engine to use. Also introduced is a namespace of "exp-compression-*" requirements to denote support for non-zlib compression in revlogs. I've prefixed the namespace with "exp-" (short for "experimental") because I'm not confident of the requirements "schema" and in no way want to give the illusion of supporting these requirements in the future. I fully intend to drop support for these requirements once we figure out what we're doing with zstd in revlogs. A good portion of the patch is teaching the requirements system about registered compression engines and passing the requested compression engine as an opener option so revlogs can instantiate the proper compression engine for new operations. That's a verbose way of saying "we can now use zstd in revlogs!" On an `hg pull` conversion of the mozilla-unified repo with no extra redelta settings (like aggressivemergedeltas), we can see the impact of zstd vs zlib in revlogs: $ hg perfrevlogchunks -c ! chunk ! wall 2.032052 comb 2.040000 user 1.990000 sys 0.050000 (best of 5) ! wall 1.866360 comb 1.860000 user 1.820000 sys 0.040000 (best of 6) ! chunk batch ! wall 1.877261 comb 1.870000 user 1.860000 sys 0.010000 (best of 6) ! wall 1.705410 comb 1.710000 user 1.690000 sys 0.020000 (best of 6) $ hg perfrevlogchunks -m ! chunk ! wall 2.721427 comb 2.720000 user 2.640000 sys 0.080000 (best of 4) ! wall 2.035076 comb 2.030000 user 1.950000 sys 0.080000 (best of 5) ! chunk batch ! wall 2.614561 comb 2.620000 user 2.580000 sys 0.040000 (best of 4) ! wall 1.910252 comb 1.910000 user 1.880000 sys 0.030000 (best of 6) $ hg perfrevlog -c -d 1 ! wall 4.812885 comb 4.820000 user 4.800000 sys 0.020000 (best of 3) ! wall 4.699621 comb 4.710000 user 4.700000 sys 0.010000 (best of 3) $ hg perfrevlog -m -d 1000 ! wall 34.252800 comb 34.250000 user 33.730000 sys 0.520000 (best of 3) ! wall 24.094999 comb 24.090000 user 23.320000 sys 0.770000 (best of 3) Only modest wins for the changelog. But manifest reading is significantly faster. What's going on? One reason might be data volume. zstd decompresses faster. So given more bytes, it will put more distance between it and zlib. Another reason is size. In the current design, zstd revlogs are *larger*: debugcreatestreamclonebundle (size in bytes) zlib: 1,638,852,492 zstd: 1,680,601,332 I haven't investigated this fully, but I reckon a significant cause of larger revlogs is that the zstd frame/header has more bytes than zlib's. For very small inputs or data that doesn't compress well, we'll tend to store more uncompressed chunks than with zlib (because the compressed size isn't smaller than original). This will make revlog reading faster because it is doing less decompression. Moving on to bundle performance: $ hg bundle -a -t none-v2 (total CPU time) zlib: 102.79s zstd: 97.75s So, marginal CPU decrease for reading all chunks in all revlogs (this is somewhat disappointing). $ hg bundle -a -t <engine>-v2 (total CPU time) zlib: 191.59s zstd: 115.36s This last test effectively measures the difference between zlib->zlib and zstd->zstd for revlogs to bundle. This is a rough approximation of what a server does during `hg clone`. There are some promising results for zstd. But not enough for me to feel comfortable advertising it to users. We'll get there...

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phases.txt
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What are phases?
================
Phases are a system for tracking which changesets have been or should
be shared. This helps prevent common mistakes when modifying history
(for instance, with the mq or rebase extensions).
Each changeset in a repository is in one of the following phases:
- public : changeset is visible on a public server
- draft : changeset is not yet published
- secret : changeset should not be pushed, pulled, or cloned
These phases are ordered (public < draft < secret) and no changeset
can be in a lower phase than its ancestors. For instance, if a
changeset is public, all its ancestors are also public. Lastly,
changeset phases should only be changed towards the public phase.
How are phases managed?
=======================
For the most part, phases should work transparently. By default, a
changeset is created in the draft phase and is moved into the public
phase when it is pushed to another repository.
Once changesets become public, extensions like mq and rebase will
refuse to operate on them to prevent creating duplicate changesets.
Phases can also be manually manipulated with the :hg:`phase` command
if needed. See :hg:`help -v phase` for examples.
To make yours commits secret by default, put this in your
configuration file::
[phases]
new-commit = secret
Phases and servers
==================
Normally, all servers are ``publishing`` by default. This means::
- all draft changesets that are pulled or cloned appear in phase
public on the client
- all draft changesets that are pushed appear as public on both
client and server
- secret changesets are neither pushed, pulled, or cloned
.. note::
Pulling a draft changeset from a publishing server does not mark it
as public on the server side due to the read-only nature of pull.
Sometimes it may be desirable to push and pull changesets in the draft
phase to share unfinished work. This can be done by setting a
repository to disable publishing in its configuration file::
[phases]
publish = False
See :hg:`help config` for more information on configuration files.
.. note::
Servers running older versions of Mercurial are treated as
publishing.
.. note::
Changesets in secret phase are not exchanged with the server. This
applies to their content: file names, file contents, and changeset
metadata. For technical reasons, the identifier (e.g. d825e4025e39)
of the secret changeset may be communicated to the server.
Examples
========
- list changesets in draft or secret phase::
hg log -r "not public()"
- change all secret changesets to draft::
hg phase --draft "secret()"
- forcibly move the current changeset and descendants from public to draft::
hg phase --force --draft .
- show a list of changeset revision and phase::
hg log --template "{rev} {phase}\n"
- resynchronize draft changesets relative to a remote repository::
hg phase -fd "outgoing(URL)"
See :hg:`help phase` for more information on manually manipulating phases.