upstream/mercurial-mirror Files · mercurial/help/internals/requirements.txt

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...

Gregory Szorc - - Load All Authors

File last commit:

r29747:aba2bb2a default


                r30818:4c0a5a25

default

Download file

             requirements.txt
        
                    108 lines
            
             | 3.3 KiB
            
                | text/plain
            
             |
                TextLexer

/ mercurial / help / internals / requirements.txt

History | Source | Raw |Copy content |Copy permalink

Gregory Szorc help: document requirements...	r28523
		Repositories contain a file (``.hg/requires``) containing a list of
		features/capabilities that are required for clients to interface
		with the repository. This file has been present in Mercurial since
		version 0.9.2 (released December 2006).

		One of the first things clients do when opening a repository is read
		``.hg/requires`` and verify that all listed requirements are supported,
		aborting if not. Requirements are therefore a strong mechanism to
		prevent incompatible clients from reading from unknown repository
		formats or even corrupting them by writing to them.

		Extensions may add requirements. When they do this, clients not running
		an extension will be unable to read from repositories.

		The following sections describe the requirements defined by the
		Mercurial core distribution.

		revlogv1
Gregory Szorc help: don't try to render a section on sub-topics...	r29747	========
Gregory Szorc help: document requirements...	r28523
		When present, revlogs are version 1 (RevlogNG). RevlogNG was introduced
		in 2006. The ``revlogv1`` requirement has been enabled by default
		since the ``requires`` file was introduced in Mercurial 0.9.2.

		If this requirement is not present, version 0 revlogs are assumed.

		store
Gregory Szorc help: don't try to render a section on sub-topics...	r29747	=====
Gregory Szorc help: document requirements...	r28523
		The store repository layout should be used.

		This requirement has been enabled by default since the ``requires`` file
		was introduced in Mercurial 0.9.2.

		fncache
Gregory Szorc help: don't try to render a section on sub-topics...	r29747	=======
Gregory Szorc help: document requirements...	r28523
		The fncache repository layout should be used.

		The fncache layout hash encodes filenames with long paths and
		encodes reserved filenames.

		This requirement is enabled by default when the store requirement is
		enabled (which is the default behavior). It was introduced in Mercurial
		1.1 (released December 2008).

		shared
Gregory Szorc help: don't try to render a section on sub-topics...	r29747	======
Gregory Szorc help: document requirements...	r28523
		Denotes that the store for a repository is shared from another location
		(defined by the ``.hg/sharedpath`` file).

		This requirement is set when a repository is created via :hg:`share`.

		The requirement was added in Mercurial 1.3 (released July 2009).

		dotencode
Gregory Szorc help: don't try to render a section on sub-topics...	r29747	=========
Gregory Szorc help: document requirements...	r28523
		The dotencode repository layout should be used.

		The dotencode layout encodes the first period or space in filenames
		to prevent issues on OS X and Windows.

		This requirement is enabled by default when the store requirement
		is enabled (which is the default behavior). It was introduced in
		Mercurial 1.7 (released November 2010).

		parentdelta
Gregory Szorc help: don't try to render a section on sub-topics...	r29747	===========
Gregory Szorc help: document requirements...	r28523
		Denotes a revlog delta encoding format that was experimental and
		replaced by generaldelta. It should not be seen in the wild because
		it was never enabled by default.

		This requirement was added in Mercurial 1.7 and removed in Mercurial
		1.9.

		generaldelta
Gregory Szorc help: don't try to render a section on sub-topics...	r29747	============
Gregory Szorc help: document requirements...	r28523
		Revlogs should be created with the generaldelta flag enabled. The
		generaldelta flag will cause deltas to be encoded against a parent
		revision instead of the previous revision in the revlog.

		Support for this requirement was added in Mercurial 1.9 (released
		July 2011). The requirement was disabled on new repositories by
		default until Mercurial 3.7 (released February 2016).

		manifestv2
Gregory Szorc help: don't try to render a section on sub-topics...	r29747	==========
Gregory Szorc help: document requirements...	r28523
		Denotes that version 2 of manifests are being used.

		Support for this requirement was added in Mercurial 3.4 (released
		May 2015). The requirement is currently experimental and is disabled
		by default.

		treemanifest
Gregory Szorc help: don't try to render a section on sub-topics...	r29747	============
Gregory Szorc help: document requirements...	r28523
		Denotes that tree manifests are being used. Tree manifests are
		one manifest per directory (as opposed to a single flat manifest).

		Support for this requirement was added in Mercurial 3.4 (released
		August 2015). The requirement is currently experimental and is
		disabled by default.

	Site-wide shortcuts
/	Use quick search box
g h	Goto home page
g g	Goto my private gists page
g G	Goto my public gists page
g 0-9	Goto bookmarked items from 0-9
n r	New repository page
n g	New gist page

	Repositories
g s	Goto summary page
g c	Goto changelog page
g f	Goto files page
g F	Goto files page with file search activated
g p	Goto pull requests page
g o	Goto repository settings
g O	Goto repository access permissions settings
t s	Toggle sidebar on some pages