Mercurial > hg
view hgext/clonebundles.py @ 39506:b66ea3fc3a86
sparse-revlog: set max delta chain length to on thousand
The new snapshot system used in the sparse-revlog case gave us some small size
benefit so far. However its most important property is to gracefully handle
harder limit on delta chainlength.
Long delta chain has a very detrimental impact on read (and write) performance
in revlog. Being able to shorter them provide a great boost. However, shorting
delta used to result significantly lower compression ratio. The intermediate
snapshots effectively suppress most of this effect (even all in some case).
# Effect on the test repository
The repository we use for test is not "realistic" but can still show this in
action using an unreasonably low chain limit. Limiting the chain length show a
sizeable increase but stay under control: +6% for limit=15; +15% for limit=10.
Without the snapshot system the increase is significantly bigger: +45% for
limit=15; +80% for limit=10. Even slightly larger than without delta chain
limit, the resulting size is still smaller than before we started doing
snapshots.
Here is a table for comparison. *Since the repository is not branchy, the
initial sparse-revlog version does not bring much benefit compare to the
non-sparse one):
chain length limit | none | limit=15 | limit=10 |
without sparse-revlog | 62 818 987 | 112 664 615 | 131 222 574 |
without snapshot | 74 365 490 | 108 211 410 | 133 857 764 |
with snapshot | 59 230 936 | 63 002 924 | 68 415 329 |
# Effect On Real Life Repositories
The series provides significant benefits on all kind of repositories.
Using `hg debugupgraderepo -o redeltaparent --run`, we recomputed delta chain
for various repositories with different settings:
- delta chain length: unlimited or 1000 limit
- sparse-revlog: enabled or disabled
- this series: applied or not applied
We can observe multiple types of effect:
- On very branchy repositories:
* The delta chain limit as low impact on the repo size.
* Intermediate snapshot greatly reduces manifest size:
- pypy: -80%
- netbeans: -95%
* The delta chain limit is effective, without a size impact:
- netbeans average: 613 -> 282
- private #1 average: 1 068 -> 307
- On more linear repository:
* Intermediate snapshot limit the impact of delta chain limit:
- mozilla:
without the series: +360%
with the series: +25%
* The delta chain limit provides large improvement:
- mozilla's average chain length:
unlimited: 15 338
limited: 469
* Despite the chain length limit, the manifest size is reduced:
- mercurial: -25%
- mozilla: -30%
It is clear that the use of chains of intermediate snapshots provide large
benefits both in storage size and delta chains quality. We should now switch our
effort toward making sure the write performance are acceptable. Then,
`sparse-revlog` will be a suitable format for all new repository.
# Raw Statistic
* no-sparse: general delta repository not using sparse-revlog
* no-snapshot: sparse-revlog repository not using this series
* snapshot: sparse-revlog repository using this series
mercurial
Manifest Size:
limit | none | 1000
------------|-------------|------------
no-sparse | 8 021 373 | 8 199 366
no-snapshot | 8 103 561 | 8 259 719
snapshot | 6 137 116 | 6 126 433
Manifest Chain length data
limit || none || 1000 ||
value || average | max || average | max ||
------------||---------|---------||---------|---------||
no-sparse || 307 | 1456 || 279 | 1000 ||
no-snapshot || 312 | 1456 || 283 | 1000 ||
snapshot || 248 | 1208 || 241 | 1000 ||
Full Store Size
limit | none | 1000
------------|-------------|------------
no-sparse | 51 013 198 | 51 201 574
no-snapshot | 50 930 795 | 51 141 006
snapshot | 48 072 037 | 48 093 572
pypy
Manifest Size:
limit | none | 1000
------------|-------------|------------
no-sparse | 193 987 784 | 193 987 784
no-snapshot | 163 171 745 | 163 312 229
snapshot | 34 605 900 | 34 600 750
Manifest Chain length data
limit || none || 1000 ||
value || average | max || average | max ||
------------||---------|---------||---------|---------||
no-sparse || 101 | 692 || 101 | 692 ||
no-snapshot || 151 | 1307 || 148 | 1000 ||
snapshot || 128 | 1309 || 125 | 1000 ||
Full Store Size
limit | none | 1000
------------|-------------|------------
no-sparse | 495 931 473 | 495 931 473
no-snapshot | 465 441 017 | 465 581 501
snapshot | 355 467 301 | 355 472 451
Mozilla
Manifest Size:
limit | none | 1000
------------|----------------|---------------
no-sparse | 416 757 148 | 1 869 009 668
no-snapshot | 401 592 370 | 1 843 493 795
snapshot | 224 359 521 | 284 615 500
Manifest Chain length data
limit || none || 1000 ||
value || average | max || average | max ||
------------||---------|---------||---------|---------||
no-sparse || 15 333 | 58 980 || 468 | 1 000 ||
no-snapshot || 15 336 | 58 980 || 469 | 1 000 ||
snapshot || 15 338 | 58 983 || 469 | 1 000 ||
Full Store Size
limit | none | 1000
------------|----------------|---------------
no-sparse | 2 712 477 887 | 4 164 995 451
no-snapshot | 2 698 887 835 | 4 141 054 304
snapshot | 2 518 130 385 | 2 578 587 596
Netbeans
Manifest Size:
limit | none | 1000
------------|----------------|---------------
no-sparse | 4 766 794 101 | 4 870 642 687
no-snapshot | 4 334 806 082 | 4 428 681 309
snapshot | 232 659 666 | 240 330 665
Manifest Chain length data
limit || none || 1000 ||
value || average | max || average | max ||
------------||---------|---------||---------|---------||
no-sparse || 597 | 6802 || 254 | 1 000 ||
no-snapshot || 648 | 6 802 || 305 | 1 000 ||
snapshot || 613 | 6 804 || 282 | 1 000 ||
Full Store Size
limit | none | 1000
------------|----------------|---------------
no-sparse | 5 807 347 998 | 5 911 196 584
no-snapshot | 5 375 398 602 | 5 469 273 829
snapshot | 1 282 519 928 | 1 290 190 927
Private repo #1
Manifest Size:
limit | none | 1000
------------|-----------------|---------------
no-sparse | 41 389 010 840 | 41 398 162 091
no-snapshot | 9 737 319 435 | 10 223 773 150
snapshot | 744 215 807 | 747 961 822
Manifest Chain length data
limit || none || 1000 ||
value || average | max || average | max ||
------------||---------|---------||---------|---------||
no-sparse || 245 | 8 885 || 81 | 1 000 ||
no-snapshot || 1 225 | 8 885 || 336 | 1 000 ||
snapshot || 1 068 | 7 909 || 307 | 1 000 ||
Full Store Size
limit | none | 1000
------------|----------------|---------------
no-sparse | 49 646 065 126 | 49 655 216 377
no-snapshot | 17 924 862 856 | 18 411 316 571
snapshot | 9 009 024 710 | 9 012 770 725
Private repo #2
We currently have less data available for that repository.
* Before is a sparse-revlog repository without this series
* After is a sparse-revlog repository with this series + 1000 chain limit
Manifest Size:
Before: 1 531 485 040 bytes
After: 1 091 422 451 bytes
Manifest Chain:
Before: 2 218 avg; 6 575 Max
After: 442 avg; 1 000 Max
Full Store Size
Before: 15 203 955 615
after: 8 207 180 693
author | Boris Feld <boris.feld@octobus.net> |
---|---|
date | Fri, 07 Sep 2018 11:18:45 -0400 |
parents | b4d85bc122bd |
children | 2372284d9457 |
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# This software may be used and distributed according to the terms of the # GNU General Public License version 2 or any later version. """advertise pre-generated bundles to seed clones "clonebundles" is a server-side extension used to advertise the existence of pre-generated, externally hosted bundle files to clients that are cloning so that cloning can be faster, more reliable, and require less resources on the server. "pullbundles" is a related feature for sending pre-generated bundle files to clients as part of pull operations. Cloning can be a CPU and I/O intensive operation on servers. Traditionally, the server, in response to a client's request to clone, dynamically generates a bundle containing the entire repository content and sends it to the client. There is no caching on the server and the server will have to redundantly generate the same outgoing bundle in response to each clone request. For servers with large repositories or with high clone volume, the load from clones can make scaling the server challenging and costly. This extension provides server operators the ability to offload potentially expensive clone load to an external service. Pre-generated bundles also allow using more CPU intensive compression, reducing the effective bandwidth requirements. Here's how clone bundles work: 1. A server operator establishes a mechanism for making bundle files available on a hosting service where Mercurial clients can fetch them. 2. A manifest file listing available bundle URLs and some optional metadata is added to the Mercurial repository on the server. 3. A client initiates a clone against a clone bundles aware server. 4. The client sees the server is advertising clone bundles and fetches the manifest listing available bundles. 5. The client filters and sorts the available bundles based on what it supports and prefers. 6. The client downloads and applies an available bundle from the server-specified URL. 7. The client reconnects to the original server and performs the equivalent of :hg:`pull` to retrieve all repository data not in the bundle. (The repository could have been updated between when the bundle was created and when the client started the clone.) This may use "pullbundles". Instead of the server generating full repository bundles for every clone request, it generates full bundles once and they are subsequently reused to bootstrap new clones. The server may still transfer data at clone time. However, this is only data that has been added/changed since the bundle was created. For large, established repositories, this can reduce server load for clones to less than 1% of original. Here's how pullbundles work: 1. A manifest file listing available bundles and describing the revisions is added to the Mercurial repository on the server. 2. A new-enough client informs the server that it supports partial pulls and initiates a pull. 3. If the server has pull bundles enabled and sees the client advertising partial pulls, it checks for a matching pull bundle in the manifest. A bundle matches if the format is supported by the client, the client has the required revisions already and needs something from the bundle. 4. If there is at least one matching bundle, the server sends it to the client. 5. The client applies the bundle and notices that the server reply was incomplete. It initiates another pull. To work, this extension requires the following of server operators: * Generating bundle files of repository content (typically periodically, such as once per day). * Clone bundles: A file server that clients have network access to and that Python knows how to talk to through its normal URL handling facility (typically an HTTP/HTTPS server). * A process for keeping the bundles manifest in sync with available bundle files. Strictly speaking, using a static file hosting server isn't required: a server operator could use a dynamic service for retrieving bundle data. However, static file hosting services are simple and scalable and should be sufficient for most needs. Bundle files can be generated with the :hg:`bundle` command. Typically :hg:`bundle --all` is used to produce a bundle of the entire repository. :hg:`debugcreatestreamclonebundle` can be used to produce a special *streaming clonebundle*. These are bundle files that are extremely efficient to produce and consume (read: fast). However, they are larger than traditional bundle formats and require that clients support the exact set of repository data store formats in use by the repository that created them. Typically, a newer server can serve data that is compatible with older clients. However, *streaming clone bundles* don't have this guarantee. **Server operators need to be aware that newer versions of Mercurial may produce streaming clone bundles incompatible with older Mercurial versions.** A server operator is responsible for creating a ``.hg/clonebundles.manifest`` file containing the list of available bundle files suitable for seeding clones. If this file does not exist, the repository will not advertise the existence of clone bundles when clients connect. For pull bundles, ``.hg/pullbundles.manifest`` is used. The manifest file contains a newline (\\n) delimited list of entries. Each line in this file defines an available bundle. Lines have the format: <URL> [<key>=<value>[ <key>=<value>]] That is, a URL followed by an optional, space-delimited list of key=value pairs describing additional properties of this bundle. Both keys and values are URI encoded. For pull bundles, the URL is a path under the ``.hg`` directory of the repository. Keys in UPPERCASE are reserved for use by Mercurial and are defined below. All non-uppercase keys can be used by site installations. An example use for custom properties is to use the *datacenter* attribute to define which data center a file is hosted in. Clients could then prefer a server in the data center closest to them. The following reserved keys are currently defined: BUNDLESPEC A "bundle specification" string that describes the type of the bundle. These are string values that are accepted by the "--type" argument of :hg:`bundle`. The values are parsed in strict mode, which means they must be of the "<compression>-<type>" form. See mercurial.exchange.parsebundlespec() for more details. :hg:`debugbundle --spec` can be used to print the bundle specification string for a bundle file. The output of this command can be used verbatim for the value of ``BUNDLESPEC`` (it is already escaped). Clients will automatically filter out specifications that are unknown or unsupported so they won't attempt to download something that likely won't apply. The actual value doesn't impact client behavior beyond filtering: clients will still sniff the bundle type from the header of downloaded files. **Use of this key is highly recommended**, as it allows clients to easily skip unsupported bundles. If this key is not defined, an old client may attempt to apply a bundle that it is incapable of reading. REQUIRESNI Whether Server Name Indication (SNI) is required to connect to the URL. SNI allows servers to use multiple certificates on the same IP. It is somewhat common in CDNs and other hosting providers. Older Python versions do not support SNI. Defining this attribute enables clients with older Python versions to filter this entry without experiencing an opaque SSL failure at connection time. If this is defined, it is important to advertise a non-SNI fallback URL or clients running old Python releases may not be able to clone with the clonebundles facility. Value should be "true". heads Used for pull bundles. This contains the ``;`` separated changeset hashes of the heads of the bundle content. bases Used for pull bundles. This contains the ``;`` separated changeset hashes of the roots of the bundle content. This can be skipped if the bundle was created without ``--base``. Manifests can contain multiple entries. Assuming metadata is defined, clients will filter entries from the manifest that they don't support. The remaining entries are optionally sorted by client preferences (``ui.clonebundleprefers`` config option). The client then attempts to fetch the bundle at the first URL in the remaining list. **Errors when downloading a bundle will fail the entire clone operation: clients do not automatically fall back to a traditional clone.** The reason for this is that if a server is using clone bundles, it is probably doing so because the feature is necessary to help it scale. In other words, there is an assumption that clone load will be offloaded to another service and that the Mercurial server isn't responsible for serving this clone load. If that other service experiences issues and clients start mass falling back to the original Mercurial server, the added clone load could overwhelm the server due to unexpected load and effectively take it offline. Not having clients automatically fall back to cloning from the original server mitigates this scenario. Because there is no automatic Mercurial server fallback on failure of the bundle hosting service, it is important for server operators to view the bundle hosting service as an extension of the Mercurial server in terms of availability and service level agreements: if the bundle hosting service goes down, so does the ability for clients to clone. Note: clients will see a message informing them how to bypass the clone bundles facility when a failure occurs. So server operators should prepare for some people to follow these instructions when a failure occurs, thus driving more load to the original Mercurial server when the bundle hosting service fails. """ from __future__ import absolute_import from mercurial import ( extensions, wireprotov1server, ) testedwith = 'ships-with-hg-core' def capabilities(orig, repo, proto): caps = orig(repo, proto) # Only advertise if a manifest exists. This does add some I/O to requests. # But this should be cheaper than a wasted network round trip due to # missing file. if repo.vfs.exists('clonebundles.manifest'): caps.append('clonebundles') return caps def extsetup(ui): extensions.wrapfunction(wireprotov1server, '_capabilities', capabilities)