Boris Feld <boris.feld@octobus.net> [Fri, 07 Sep 2018 11:18:45 -0400] rev 39505
snapshot: also consider the snapshot chain of one unrelated revision
To maximize the chance of good delta chain reuse, we inject an unrelated delta
chain into our search. To do so, we search for the highest revision unrelated
to the parents of the current revision and use its snapshot chain too.
Adding this extra snapshot into the mix can have a performance impact. We'll
deal with performance impact in a later series.
Boris Feld <boris.feld@octobus.net> [Fri, 07 Sep 2018 11:17:37 -0400] rev 39504
snapshot: extract parent chain computation
The final step of this series is to include chain related to "prev" in the
search. Before adding that code we do some simple code movement to clarify the
next diff.
Boris Feld <boris.feld@octobus.net> [Fri, 07 Sep 2018 11:17:36 -0400] rev 39503
snapshot: refine candidate snapshot base upward
Once we found a suitable snapshot base it is useful to check if it has a
"children" snapshot that would provide a better diff. This is useful when base
not directly related to stored revision are picked. In those case, we "jumped"
to this new chain at an arbitrary point, checking if a higher point is more
appropriate will help to provide better results and increase snapshot reuse.
Boris Feld <boris.feld@octobus.net> [Fri, 07 Sep 2018 11:17:35 -0400] rev 39502
snapshot: try to refine new snapshot base down the chain
There are cases where doing a diff against a snapshot's parent will be shorter
than against the snapshot itself. Reusing snapshot not directly related to the
revision we are trying to store increase this odd.
So once we found a possible candidate, we check the snapshots lower in the
chain.
This will involve extra processing, but this extra processing will only happen
when we are doing building a snapshot, a rare situation.
Boris Feld <boris.feld@octobus.net> [Fri, 07 Sep 2018 11:17:34 -0400] rev 39501
snapshot: make sure we'll never refine delta base from a reused source
The point of reusing delta from the source is to avoid doing computation when
applying a bundle. Refining such delta would go against that spirit.
We do not have refining logic in place yet. This code needed to be moved out
of the way before we could start adding such logic.
Boris Feld <boris.feld@octobus.net> [Fri, 07 Sep 2018 11:17:34 -0400] rev 39500
snapshot: turn _refinedgroups into a coroutine
We are now almost ready to start adding refining logic.
Boris Feld <boris.feld@octobus.net> [Fri, 07 Sep 2018 11:17:33 -0400] rev 39499
snapshot: also use None as a stop value for `_refinegroup`
This is yet another small step toward turning `_refinegroups` into a co-routine.
Boris Feld <boris.feld@octobus.net> [Fri, 07 Sep 2018 11:17:33 -0400] rev 39498
snapshot: add refining logic at the findeltainfo level
Once we found a delta, we want to have the candidates logic challenge it,
searching for a better candidate.
The logic at the lower level is still missing. We'll introduce it later.
Adding small changes in individual commits make it simpler to explain the code
change.
This is another small step toward turning `_refinegroups` into a co-routine.
Boris Feld <boris.feld@octobus.net> [Fri, 07 Sep 2018 11:17:32 -0400] rev 39497
snapshot: use None as a stop value when looking for a good delta
Having clear stop value should help keep clear logic around the co-routine.
The alternative of using a StopIteration exception give a messier result.
This is one small step toward turning `_refinegroups` into a co-routine.
Boris Feld <boris.feld@octobus.net> [Fri, 07 Sep 2018 11:17:32 -0400] rev 39496
snapshot: introduce an intermediate `_refinedgroups` generator
This method will be used to improve the search for a good snapshot base. To
keep things simpler, we introduce the necessary function before doing any
delta base logic change. The next handful of commits will focus on refactoring
the code to let that new logic land as clearly as possible.
# General Idea
Right now, the search for a good delta base stop whenever we found a good one.
However, when using sparse-revlog, we should probably try a bit harder.
We do significant effort to increase delta re-use by jumping on "unrelated"
delta chains that provide better results. Moving to another chain for a better
result is good, but we have no guarantee we jump at a reasonable point in that
new chain. When we consider over the chains related to the parents, we start
from the higher-level snapshots. This is a way to consider the snapshot closer
to the current revision that has the best chance to produce a small delta. We
do benefit from this walk order when jumping to a better "unrelated" stack.
To counter-balance this, we'll introduce a way to "refine" the result. After a
good delta have been found, we'll keep searching for a better delta, using the
current best one as a starting point.
# Target Setup
The `finddeltainfo` method is responsible for the general search for a good
delta. It requests candidates base from `_candidategroups` and decides which
one are usable.
The `_candidategroups` generator act as a top-level filter, it does not care
about how we pick candidates, it just does basic filtering, excluding
revisions that have been tested already or that are an obvious misfit.
The `_rawgroups` generator is the one with the actual ancestors walking logic,
It does not care about what would do a good delta and what was already tested,
it just issues the initial candidates.
We introduce a new `_refinedgroup` function to bridge the gap between
`_candidategroups` and `_rawgroups`. It delegates the initial iteration logic
and then performing relevant refining of the valid base once found. (This
logic is yet to be added to function)
All these logics are fairly independent and easier to understand when standing
alone, not mixed with each other. It also makes it easy to test and try
different approaches for one of those four layers without affecting the other
ones.
# Technical details
To communicate `finddeltainfo` choice of "current best delta base" to the
`_refinegroup` logic, we plan to use python co-routine feature. The
`_candidategroups` and `_refinegroup` generators will become co-routine. This
will allow `_refinegroup` to detect when a good delta have been found and
triggers various refining steps.
For now, `_candidategroups` will just pass the value down the stack.
After poking at various option, the co-routine appears the best to keep each
layers focus on its duty, without the need to spread implementation details
across layers.
Boris Feld <boris.feld@octobus.net> [Fri, 07 Sep 2018 11:17:31 -0400] rev 39495
snapshot: consider unrelated snapshots at a similar level first
This new step is inserted before considering using a level-N snapshot as a
base for a level-N+1 snapshot. We first check if existing level-N+1 snapshots
using the same base would be a suitable base for a level-N+2 snapshot.
This increases snapshot reuse and limits the risk of snapshot explosion in
very branchy repositories.
Using a "deeper" snapshot as the base also results in a smaller snapshot since
it builds a level-N+2 intermediate snapshot instead of an N+1 one.
This logic is similar for the one we added in a previous commit. In that
previous commit is only applied to level-0 "siblings".
We can see this effect in the test repository. Snapshots moved from lower
levels to higher levels.
Boris Feld <boris.feld@octobus.net> [Fri, 07 Sep 2018 11:17:30 -0400] rev 39494
snapshot: consider all snapshots in the parents' chains
There are no reasons to only consider full snapshot as a possible base for an
intermediate snapshot. Now that the basic principles have been set, we can
start adding more levels of snapshots.
We now consider all snapshots in the parent's chains (full or intermediate).
This creates a chain of intermediate snapshots, each smaller than the previous
one.
# Effect On The Test Repository
In the test repository, we can see a decrease in the revlog size and slightly
shorter delta chain.
However, that approach creates snapshots more frequently, increasing the risk
of ending into problematic cases in very branchy repositories (not triggered
by the test repository). The next changesets will remove that risk by adding
logic that increases deltas reuse.
Boris Feld <boris.feld@octobus.net> [Fri, 07 Sep 2018 11:17:30 -0400] rev 39493
snapshot: search for unrelated but reusable full-snapshot
# New Strategy Step: Reusing Snapshot Outside Of Parents' Chain.
If no suitable bases were found in the parent's chains, see if we could reuse
a full snapshot not directly related to the current revision. Such search can
be expensive, so we only search for snapshots appended to the revlog *after*
the bases used by the parents of the current revision (the one we just
tested). We assume the parent's bases were created because the previous
snapshots were unsuitable, so there are low odds they would be useful now.
This search gives a chance to reuse a delta chain unrelated to the current
revision. Without this re-use, topological branches would keep reopening new
full chains. Creating more and more snapshots as the repository grow.
In repositories with many topological branches, the lack of delta reuse can
create too many snapshots reducing overall compression to nothing. This
results in a very large repository and other usability issues.
For now, we still focus on creating level-1 snapshots. However, this principle
will play a large part in how we avoid snapshot explosion once we have more
snapshot levels.
# Effects On The Test Repository
In the test repository we created, we can see the beneficial effect of such
reuse. We need very few level-0 snapshots and the overall revlog size has
decreased.
The `hg debugrevlog` call, show a "lvl-2" snapshot. It comes from the existing
delta logic using the `prev` revision (revlog's tip) as the base. In this
specific case, it turns out the tip was a level-1 snapshot. This is a
coincidence that can be ignored.
Finding and testing against all these unrelated snapshots can have a
performance impact at write time. We currently focus on building good deltas
chain we build. Performance concern will be dealt with later in another
series.
Boris Feld <boris.feld@octobus.net> [Fri, 07 Sep 2018 11:17:29 -0400] rev 39492
snapshot: try intermediate snapshot against parents' base
# Regarding The Series Started By This Changeset
This is the first changesets of a group adjusting delta chain strategy to
build a useful chain of intermediate snapshots. The series will introduce a
full strategy to produce chains of multiple snapshots on top of which a
"usual" delta chain will be built.
That strategy will have multiple steps to maximize snapshot reuse, avoiding
pathological cases and improving overall compression in very branchy
repositories. An important property of sparse-revlog using such snapshot-chain
is that they can use very short delta chain without problematic impact on the
resulting compression. Shorter delta chains are important to achieve good
performance.
To make each step clear, we'll introduce them one by one.
See the end of this series for full details.
# Regarding This Changeset
Before this change, if we cannot store the current revision as a delta against
a "simple" candidate (p1, p2, prev), we created a new level-0 snapshot (also
called full snapshot).
As the first step, we introduce a simple strategy: try an intermediate level-1
snapshot against the chain base of the "current revision" parents.
The "current revision" is the one we are currently trying to store in the
revlog, triggering this search for a good delta base.
The first item in the chain is always a level-0 snapshot.
# Effect On The Test Repository
We can already see the effect on the test-repository. Most of the snapshots
have shifted from level 0 to level 1. The overall size has slightly decreased.
(However, keep in mind that this repository only emulates real data)
# Regarding Statistic
The current series focuses on improving the chain built. Improving the
performance of this logic will be done as a second step. Sparse-revlog is
still experimental and disabled by default.
We'll provide more statistic about resulting size and delta chain at the end
of this series.
Boris Feld <boris.feld@octobus.net> [Mon, 10 Sep 2018 09:08:24 -0700] rev 39491
sparse-revlog: add a test checking revlog deltas for a churning file
The test repository contains 5000 revisions and is therefore slow to build:
five minutes with CHG, over fifteen minutes without. It is too slow to build
during the test. Bundling all content produce a sizeable result, 20BM, too
large to be committed. Instead, we commit a script to build the expected
bundle and the test checks if the bundle is available. Any run of the script
will produce the same repository content, using resulting in the same hashes.
Using smaller repositories was tried, however, it misses most of the cases we
are planning to improve. Having them in a 5000 repository is already nice, we
usually see these case in repositories in the order of magnitude of one
million revisions.
This test will be very useful to check various changes strategy for building
delta to store in a sparse-revlog.
In this series we will focus our attention on the following metrics:
The ones that will impact the final storage performance (size, space):
* size of the revlog data file (".hg/store/data/*.d")
* chain length info
The ones that describe the deltas patterns:
* number of snapshot revision (and their level)
* size taken by snapshot revision (and their level)