revlog: use an LRU cache for delta chain bases
Profiling using statprof revealed a hotspot during changegroup
application calculating delta chain bases on generaldelta repos.
Essentially, revlog._addrevision() was performing a lot of redundant
work tracing the delta chain as part of determining when the chain
distance was acceptable. This was most pronounced when adding
revisions to manifests, which can have delta chains thousands of
revisions long.
There was a delta chain base cache on revlogs before, but it only
captured a single revision. This was acceptable before generaldelta,
when _addrevision would build deltas from the previous revision and
thus we'd pretty much guarantee a cache hit when resolving the delta
chain base on a subsequent _addrevision call. However, it isn't
suitable for generaldelta because parent revisions aren't necessarily
the last processed revision.
This patch converts the delta chain base cache to an LRU dict cache.
The cache can hold multiple entries, so generaldelta repos have a
higher chance of getting a cache hit.
The impact of this change when processing changegroup additions is
significant. On a generaldelta conversion of the "mozilla-unified"
repo (which contains heads of the main Firefox repositories in
chronological order - this means there are lots of transitions between
heads in revlog order), this change has the following impact when
performing an `hg unbundle` of an uncompressed bundle of the repo:
before: 5:42 CPU time
after: 4:34 CPU time
Most of this time is saved when applying the changelog and manifest
revlogs:
before: 2:30 CPU time
after: 1:17 CPU time
That nearly a 50% reduction in CPU time applying changesets and
manifests!
Applying a gzipped bundle of the same repo (effectively simulating a
`hg clone` over HTTP) showed a similar speedup:
before: 5:53 CPU time
after: 4:46 CPU time
Wall time improvements were basically the same as CPU time.
I didn't measure explicitly, but it feels like most of the time
is saved when processing manifests. This makes sense, as large
manifests tend to have very long delta chains and thus benefit the
most from this cache.
So, this change effectively makes changegroup application (which is
used by `hg unbundle`, `hg clone`, `hg pull`, `hg unshelve`, and
various other commands) significantly faster when delta chains are
long (which can happen on repos with large numbers of files and thus
large manifests).
In theory, this change can result in more memory utilization. However,
we're caching a dict of ints. At most we have 200 ints + Python object
overhead per revlog. And, the cache is really only populated when
performing read-heavy operations, such as adding changegroups or
scanning an individual revlog. For memory bloat to be an issue, we'd
need to scan/read several revisions from several revlogs all while
having active references to several revlogs. I don't think there are
many operations that do this, so I don't think memory bloat from the
cache will be an issue.
# 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.
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. Here's how it works.
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.)
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.
To work, this extension requires the following of server operators:
* Generating bundle files of repository content (typically periodically,
such as once per day).
* 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 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 clone bundle*. 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.
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.
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".
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
(``experimental.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,
wireproto,
)
testedwith = 'internal'
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.opener.exists('clonebundles.manifest'):
caps.append('clonebundles')
return caps
def extsetup(ui):
extensions.wrapfunction(wireproto, '_capabilities', capabilities)