Mercurial > hg
view tests/test-empty-group.t @ 40021:c537144fdbef
wireprotov2: support response caching
One of the things I've learned from managing VCS servers over the
years is that they are hard to scale. It is well known that some
companies have very beefy (read: very expensive) servers to power
their VCS needs. It is also known that specialized servers for
various VCS exist in order to facilitate scaling servers. (Mercurial
is in this boat.)
One of the aspects that make a VCS server hard to scale is the
high CPU load incurred by constant client clone/pull operations.
To alleviate the scaling pain associated with data retrieval
operations, I want to integrate caching into the Mercurial wire
protocol server as robustly as possible such that servers can
aggressively cache responses and defer as much server load as
possible.
This commit represents the initial implementation of a general
caching layer in wire protocol version 2.
We define a new interface and behavior for a wire protocol cacher
in repository.py. (This is probably where a reviewer should look
first to understand what is going on.)
The bulk of the added code is in wireprotov2server.py, where we
define how a command can opt in to being cached and integrate
caching into command dispatching.
From a very high-level:
* A command can declare itself as cacheable by providing a callable
that can be used to derive a cache key.
* At dispatch time, if a command is cacheable, we attempt to
construct a cacher and use it for serving the request and/or
caching the request.
* The dispatch layer handles the bulk of the business logic for
caching, making cachers mostly "dumb content stores."
* The mechanism for invalidating cached entries (one of the harder
parts about caching in general) is by varying the cache key when
state changes. As such, cachers don't need to be concerned with
cache invalidation.
Initially, we've hooked up support for caching "manifestdata" and
"filedata" commands. These are the simplest to cache, as they should
be immutable over time. Caching of commands related to changeset
data is a bit harder (because cache validation is impacted by
changes to bookmarks, phases, etc). This will be implemented later.
(Strictly speaking, censoring a file should invalidate caches. I've
added an inline TODO to track this edge case.)
To prove it works, this commit implements a test-only extension
providing in-memory caching backed by an lrucachedict. A new test
showing this extension behaving properly is added. FWIW, the
cacher is ~50 lines of code, demonstrating the relative ease with
which a cache can be added to a server.
While the test cacher is not suitable for production workloads, just
for kicks I performed a clone of just the changeset and manifest data
for the mozilla-unified repository. With a fully warmed cache (of just
the manifest data since changeset data is not cached), server-side
CPU usage dropped from ~73s to ~28s. That's pretty significant and
demonstrates the potential that response caching has on server
scalability!
Differential Revision: https://phab.mercurial-scm.org/D4773
author | Gregory Szorc <gregory.szorc@gmail.com> |
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date | Wed, 26 Sep 2018 17:16:56 -0700 |
parents | eb586ed5d8ce |
children |
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# A B # # 3 4 3 # |\/| |\ # |/\| | \ # 1 2 1 2 # \ / \ / # 0 0 # # if the result of the merge of 1 and 2 # is the same in 3 and 4, no new manifest # will be created and the manifest group # will be empty during the pull # # (plus we test a failure where outgoing # wrongly reported the number of csets) $ hg init a $ cd a $ touch init $ hg ci -A -m 0 adding init $ touch x y $ hg ci -A -m 1 adding x adding y $ hg update 0 0 files updated, 0 files merged, 2 files removed, 0 files unresolved $ touch x y $ hg ci -A -m 2 adding x adding y created new head $ hg merge 1 0 files updated, 0 files merged, 0 files removed, 0 files unresolved (branch merge, don't forget to commit) $ hg ci -A -m m1 $ hg update -C 1 0 files updated, 0 files merged, 0 files removed, 0 files unresolved $ hg merge 2 0 files updated, 0 files merged, 0 files removed, 0 files unresolved (branch merge, don't forget to commit) $ hg ci -A -m m2 created new head $ cd .. $ hg clone -r 3 a b adding changesets adding manifests adding file changes added 4 changesets with 3 changes to 3 files new changesets 5fcb73622933:d15a0c284984 updating to branch default 3 files updated, 0 files merged, 0 files removed, 0 files unresolved $ hg clone -r 4 a c adding changesets adding manifests adding file changes added 4 changesets with 3 changes to 3 files new changesets 5fcb73622933:1ec3c74fc0e0 updating to branch default 3 files updated, 0 files merged, 0 files removed, 0 files unresolved $ hg -R a outgoing b comparing with b searching for changes changeset: 4:1ec3c74fc0e0 tag: tip parent: 1:79f9e10cd04e parent: 2:8e1bb01c1a24 user: test date: Thu Jan 01 00:00:00 1970 +0000 summary: m2 $ hg -R a outgoing c comparing with c searching for changes changeset: 3:d15a0c284984 parent: 2:8e1bb01c1a24 parent: 1:79f9e10cd04e user: test date: Thu Jan 01 00:00:00 1970 +0000 summary: m1 $ hg -R b outgoing c comparing with c searching for changes changeset: 3:d15a0c284984 tag: tip parent: 2:8e1bb01c1a24 parent: 1:79f9e10cd04e user: test date: Thu Jan 01 00:00:00 1970 +0000 summary: m1 $ hg -R c outgoing b comparing with b searching for changes changeset: 3:1ec3c74fc0e0 tag: tip parent: 1:79f9e10cd04e parent: 2:8e1bb01c1a24 user: test date: Thu Jan 01 00:00:00 1970 +0000 summary: m2 $ hg -R b pull a pulling from a searching for changes adding changesets adding manifests adding file changes added 1 changesets with 0 changes to 0 files (+1 heads) new changesets 1ec3c74fc0e0 (run 'hg heads' to see heads, 'hg merge' to merge) $ hg -R c pull a pulling from a searching for changes adding changesets adding manifests adding file changes added 1 changesets with 0 changes to 0 files (+1 heads) new changesets d15a0c284984 (run 'hg heads' to see heads, 'hg merge' to merge)