[RFC] Split reflists to share their contents across snapshots

[neolynx]

Replaces #1235

This builds on top of #1222, #1227, and #1233, and is thus in draft state until those are merged in. The only commit that's actually new is the very last one, whose commit message I copied below. (Even that single commit is admittedly quite big, but a sizable chunk of the changes are just plumbing a new RefListCollection around across the code.)

Description of the Change

In current aptly, each repository and snapshot has its own reflist in
the database. This brings a few problems with it:

• Given a sufficiently large repositories and snapshots, these lists can
  get enormous, reaching >1MB. This is a problem for LevelDB's overall
  performance, as it tends to prefer values around the confiruged block
  size (defaults to just 4KiB).
• When you take these large repositories and snapshot them, you have a
  full, new copy of the reflist, even if only a few packages changed.
  This means that having a lot of snapshots with a few changes causes
  the database to basically be full of largely duplicate reflists.
• All the duplication also means that many of the same refs are being
  loaded repeatedly, which can cause some slowdown but, more notably,
  eats up huge amounts of memory.
• Adding on more and more new repositories and snapshots will cause the
  time and memory spent on things like cleanup and publishing to grow
  roughly linearly.

At the core, there are two problems here:

• Reflists get very big because there are just a lot of packages.
• Different reflists can tend to duplicate much of the same contents.

Split reflists aim at solving this by separating reflists into 64
buckets. Package refs are sorted into individual buckets according to
the following system:

• Take the first 3 letters of the package name, after dropping a lib
  prefix. (Using only the first 3 letters will cause packages with
  similar prefixes to end up in the same bucket, under the assumption
  that packages with similar names tend to be updated together.)
• Take the 64-bit xxhash of these letters. (xxhash was chosen because it
  relatively good distribution across the individual bits, which is
  important for the next step.)
• Use the first 6 bits of the hash (range [0:63]) as an index into the
  buckets.

Once refs are placed in buckets, a sha256 digest of all the refs in the
bucket is taken. These buckets are then stored in the database, split
into roughly block-sized segments, and all the repositories and
snapshots simply store an array of bucket digests.

This approach means that repositories and snapshots can share their
reflist buckets. If a snapshot is taken of a repository, it will have
the same contents, so its split reflist will point to the same buckets
as the base repository, and only one copy of each bucket is stored in
the database. When some packages in the repository change, only the
buckets containing those packages will be modified; all the other
buckets will remain unchanged, and thus their contents will still be
shared. Later on, when these reflists are loaded, each bucket is only
loaded once, short-cutting loaded many megabytes of data. In effect,
split reflists are essentially copy-on-write, with only the changed
buckets stored individually.

Changing the disk format means that a migration needs to take place, so
that task is moved into the database cleanup step, which will migrate
reflists over to split reflists, as well as delete any unused reflist
buckets.

All the reflist tests are also changed to additionally test out split
reflists; although the internal logic is all shared (since buckets are,
themselves, just normal reflists), some special additions are needed to
have native versions of the various reflist helper methods.

In our tests, we've observed the following improvements:

• Memory usage during publish and database cleanup, with
  GOMEMLIMIT=2GiB, goes down from ~3.2GiB (larger than the memory
  limit!) to ~0.7GiB, a decrease of ~4.5x.
• Database size decreases from 1.3GB to 367MB.

In my local tests, publish times had also decreased down to mere
seconds but the same effect wasn't observed on the server, with the
times staying around the same. My suspicions are that this is due to I/O
performance: my local system is an M1 MBP, which almost certainly has
much faster disk speeds than our DigitalOcean block volumes. Split
reflists include a side effect of requiring more random accesses from
reading all the buckets by their keys, so if your random I/O
performance is slower, it might cancel out the benefits. That being
said, even in that case, the memory usage and database size advantages
still persist.

Checklist

• unit-test added (if change is algorithm)
• functional test added/updated (if change is functional)
• man page updated (if applicable)
• bash completion updated (if applicable)
• documentation updated
• author name in AUTHORS

---

It would be awesome if anyone could also test this out and report how it affects their performance & memory usage.

[neolynx]

@refi64 could you have a look at https://github.com/aptly-dev/aptly/actions/runs/9429650201/job/25976276670?pr=1282#step:9:438 ?

PANIC: snapshot_test.go:26: SnapshotSuite.TestNewSnapshotFromRepository

... Panic: runtime error: invalid memory address or nil pointer dereference (PC=0x43D1FE)

/opt/hostedtoolcache/go/1.21.10/x64/src/runtime/panic.go:914
  in gopanic
/opt/hostedtoolcache/go/1.21.10/x64/src/runtime/panic.go:261
  in panicmem
/opt/hostedtoolcache/go/1.21.10/x64/src/runtime/signal_unix.go:861
  in sigpanic
reflist.go:495
  in SplitRefList.Len
snapshot.go:48
  in NewSnapshotFromRepository
snapshot_test.go:35
  in SnapshotSuite.TestNewSnapshotFromRepository
/opt/hostedtoolcache/go/1.21.10/x64/src/reflect/value.go:380
  in Value.Call
/opt/hostedtoolcache/go/1.21.10/x64/src/runtime/asm_amd64.s:1650
  in goexit

[codecov]

Codecov Report

Attention: Patch coverage is 82.88509% with 140 lines in your changes missing coverage. Please review.

Project coverage is 74.80%. Comparing base (4661913) to head (1a0038d).

Files	Patch %	Lines
deb/reflist.go	87.75%	39 Missing and 16 partials ⚠️
api/db.go	20.75%	38 Missing and 4 partials ⚠️
cmd/db_cleanup.go	67.05%	20 Missing and 8 partials ⚠️
deb/publish.go	80.43%	6 Missing and 3 partials ⚠️
deb/snapshot.go	83.33%	2 Missing and 1 partial ⚠️
deb/graph.go	33.33%	2 Missing ⚠️
api/repos.go	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1282      +/-   ##
==========================================
+ Coverage   74.53%   74.80%   +0.27%     
==========================================
  Files         146      146              
  Lines       16576    17135     +559     
==========================================
+ Hits        12355    12818     +463     
- Misses       3245     3317      +72     
- Partials      976     1000      +24     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[neolynx]

tests fixed, made compatible with go 1.19 (and current debian/bookworm)

[refi64]

Sorry for the delays, I've been a bit occupied elsewhere lately 😅 I had looked into the nil errors in the tests, but it seemed to specifically come from the way the tests were initializing the repos; normally they're created and then have something loaded into them, but the tests create them and immediately start calling methods w/o any load. Changing that around also makes the tests pass and looks like this:

diff --git a/deb/local_test.go b/deb/local_test.go
index b87b1b62..6d753d83 100644
--- a/deb/local_test.go
+++ b/deb/local_test.go
@@ -40,12 +40,16 @@ func (s *LocalRepoSuite) TestString(c *C) {
 }
 
 func (s *LocalRepoSuite) TestNumPackages(c *C) {
-	c.Check(NewLocalRepo("lrepo", "My first repo").NumPackages(), Equals, 0)
+	r := NewLocalRepo("lrepo", "My first repo")
+	r.packageRefs = NewSplitRefList()
+	c.Check(r.NumPackages(), Equals, 0)
 	c.Check(s.repo.NumPackages(), Equals, 2)
 }
 
 func (s *LocalRepoSuite) TestRefList(c *C) {
-	c.Check(NewLocalRepo("lrepo", "My first repo").RefList(), IsNil)
+	r := NewLocalRepo("lrepo", "My first repo")
+	r.packageRefs = NewSplitRefList()
+	c.Check(r.RefList().Len(), Equals, 0)
 	c.Check(s.repo.RefList(), Equals, s.reflist)
 }
 
@@ -151,7 +155,6 @@ func (s *LocalRepoCollectionSuite) TestUpdateLoadComplete(c *C) {
 	r, err = collection.ByName("local1")
 	c.Assert(err, IsNil)
 	c.Assert(r.packageRefs, IsNil)
-	c.Assert(r.NumPackages(), Equals, 0)
 	c.Assert(s.collection.LoadComplete(r, s.reflistCollection), IsNil)
 	c.Assert(r.NumPackages(), Equals, 2)
 }
diff --git a/deb/remote_test.go b/deb/remote_test.go
index 3e05ef7e..baa5f2c6 100644
--- a/deb/remote_test.go
+++ b/deb/remote_test.go
@@ -139,6 +139,7 @@ func (s *RemoteRepoSuite) TestString(c *C) {
 }
 
 func (s *RemoteRepoSuite) TestNumPackages(c *C) {
+	s.repo.packageRefs = NewSplitRefList()
 	c.Check(s.repo.NumPackages(), Equals, 0)
 	s.repo.packageRefs = s.reflist
 	c.Check(s.repo.NumPackages(), Equals, 3)
@@ -727,7 +728,6 @@ func (s *RemoteRepoCollectionSuite) TestUpdateLoadComplete(c *C) {
 	r, err = collection.ByName("yandex")
 	c.Assert(err, IsNil)
 	c.Assert(r.packageRefs, IsNil)
-	c.Assert(r.NumPackages(), Equals, 0)
 	c.Assert(s.collection.LoadComplete(r, s.refListCollection), IsNil)
 	c.Assert(r.NumPackages(), Equals, 3)
 }
diff --git a/deb/snapshot_test.go b/deb/snapshot_test.go
index 805ccc8e..4886ebfb 100644
--- a/deb/snapshot_test.go
+++ b/deb/snapshot_test.go
@@ -31,7 +31,7 @@ func (s *SnapshotSuite) TestNewSnapshotFromRepository(c *C) {
 	c.Check(snapshot.SourceKind, Equals, SourceRemoteRepo)
 	c.Check(snapshot.SourceIDs, DeepEquals, []string{s.repo.UUID})
 
-	s.repo.packageRefs = nil
+	s.repo.packageRefs = NewSplitRefList()
 	_, err := NewSnapshotFromRepository("snap2", s.repo)
 	c.Check(err, ErrorMatches, ".*not updated")
 }

I'm...not sure which approach is better, really? My concern with just making NumPackages() return 0 on nil was that it would mask actual bugs where aptly tries to use a repository / snapshot / etc that never had the Load methods called, but otoh "the new object you created isn't immediately functional" is probably a recipe for confusion.

[neolynx]

I see your point. but crashing is also not an option, and if packageRefs is nil, then the number of packages in that repo is 0, so I think behavior wise it would make sense to return 0. Am I assuming this correctly ?

I did not fully understand when packageRefs is nil or not, maybe that behavior shoul dbe more defined and tested as well ?

I ran some mirroring tests and all looks good, from mt feeling it is faster publishing, but I did not benchmark it.

Also I reintroduced compatibility with go 1.9, I hope that does not impact performance too much. If you find some time, it would be great if you could run your benchmark tests again to check ;)

[refi64]

I did not fully understand when packageRefs is nil or not

So in the original code, this was a slice, thus nil == 0-length. Now, it's an actual object, where treating nil as being "the object, but empty" feels a bit nonstandard. With that in mind, I believe any place in the code that tries to access the packages without loading them first could be safely seen as a bug.

Also I reintroduced compatibility with go 1.9, I hope that does not impact performance too much.

Thanks for this, I seem to have accidentally gotten the Go version wrong when I was deciding what functionality to use... I'll give it a spin later this week.

[neolynx]

Thanks for your excellent work !

I found another instance of accessing packageRefs without checking if it is nil and pushed a fix.

Could you allow updating your refi64:split-reflists branch ? then we can reopen your pull request and close this one, or if you like, I can add you to the maintainers for easier collaboration.

[neolynx]

another question: since the database format changes, what impact does this have on a existing database when upgrading to a version with split reflists ? I.e. for republishng a snapshot of a repo with new packages ?

should aptly db cleanup be run after the upgrade ?

[refi64]

Could you allow updating your refi64:split-reflists branch ? then we can reopen your pull request and close this one, or if you like, I can add you to the maintainers for easier collaboration.

Oh I, uhh, did not realize it was not open to updates by maintainers, just flicked that on now.

another question: since the database format changes, what impact does this have on a existing database when upgrading to a version with split reflists ? I.e. for republishng a snapshot of a repo with new packages ?

The code should be able to load up the previous format without an issue (that's the reason for the splitOrInlineRefList dance here), albeit a bit more slowly (because it does the splitting at load time), and the next time the reflist is updated it will write the new format. Or, you can just indeed run db cleanup which will indeed update everything at once.

Aside: I'm actually kinda rusty as to how I benchmarked this before, but some initial tests show the performance basically being identical to before.

[neolynx]

hi !

as this is a breaking change in certain scenarios, I am a bit hesitant to introduce it now. For example, if someone would revert back to an older version, all references are lost and the repos do not have packages anymore. there has not been a aptly release in a while, so I would like to introduce such a change later. or should this be a configurable feature ?
what do you think ?

would it be possible to convert a newer db to the old refs format ?

[refi64]

Sorry for the lack of an update, I can def add a way to revert the change and have been working on it, it's just taking a bit.