nix-super/src/libstore/ca-specific-schema.sql

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-- Extension of the sql schema for content-addressed derivations.
-- Won't be loaded unless the experimental feature `ca-derivations`
-- is enabled
create table if not exists Realisations (
id integer primary key autoincrement not null,
drvPath text not null,
outputName text not null, -- symbolic output id, usually "out"
outputPath integer not null,
signatures text, -- space-separated list
foreign key (outputPath) references ValidPaths(id) on delete cascade
);
create index if not exists IndexRealisations on Realisations(drvPath, outputName);
-- We can end-up in a weird edge-case where a path depends on itself because
-- its an output of a CA derivation, that happens to be the same as one of its
-- dependencies.
-- In that case we have a dependency loop (path -> realisation1 -> realisation2
-- -> path) that we need to break by removing the dependencies between the
-- realisations
create trigger if not exists DeleteSelfRefsViaRealisations before delete on ValidPaths
begin
delete from RealisationsRefs where realisationReference in (
select id from Realisations where outputPath = old.id
);
end;
create table if not exists RealisationsRefs (
referrer integer not null,
realisationReference integer,
foreign key (referrer) references Realisations(id) on delete cascade,
foreign key (realisationReference) references Realisations(id) on delete restrict
);
-- used by deletion trigger
create index if not exists IndexRealisationsRefsRealisationReference on RealisationsRefs(realisationReference);
ca-specific-schema.sql: add index on RealisationsRefs(referrer) and (outputPath) For a typical desktop system (~2K packages) we can easily get 100K entries in RealisationsRefs. Without indices query for RealisationsRefs requires linear scan. RealisationsRefs(referrer) -------------------------- Inefficiency is seen as a 100% CPU load of nix-daemon for the following scenario: $ nix edit -f . bash # add unused environment variable, like FOO="1" # populate RealisationsRefs, build fresh system $ nix build -f nixos system --arg config '{ contentAddressedByDefault = true; }' $ nix edit -f . bash # add unused environment variable, like FOO="2" $ time nix build -f nixos system --arg config '{ contentAddressedByDefault = true; }' In this case `bash `will be rebuilt a few times and then rest of CPU time is spent on scanning RealisationsRefs table (about 5 CPU-minutes on my machine). Before the change: $ time nix build -f nixos system ... # step 4 above real 34m3,613s user 0m5,232s sys 0m0,758s Of all this time about 29.5 minutes are taken by nix-daemon's CPU time. After the change: $ time nix build -f nixos system ... # step 4 above real 4m50,061s user 0m5,038s sys 0m0,677s Of all this time about 1 minute is taken by nix-daemon's CPU time. Most of the time is spent polling for non-existent realisations on cache-nixos.org. Realisations(outputPath) ------------------------ After running CA system for two weeks I got ~1M entries in Realisations table. `nix-collect-garbage` became very slow (seemingly 100 path deletions per second). It happens due to a slow cascading delete from Realisations triggered by deletion from ValidPaths. The fix is to add an index on primary key from ValidPaths(id) that triggers cascading deletions. Before the change: $ time nix-collect-garbage -d --max-freed 100G <interrupted before finish, took too long> real 23m32.411s user 17m49.679s sys 4m50.609s Most of time was spent in re-scanning Realisations table on each path deletion. After the change: $ time nix-collect-garbage -d --max-freed 100G real 8m43.226s user 6m16.317s sys 1m40.188s Time is spent scanning sqlite indices and in kernel when unlinking directories.
2021-10-09 20:47:05 +03:00
-- used by QueryRealisationReferences
create index if not exists IndexRealisationsRefs on RealisationsRefs(referrer);
-- used by cascade deletion when ValidPaths is deleted
create index if not exists IndexRealisationsRefsOnOutputPath on Realisations(outputPath);