systemd / systemd / 30181146251

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github

Committed by yuwata

Commit Message

shared/dropin: don't re-derive drop-in name candidates per lookup dir

unit_file_find_dirs() is called once for every (unit name or alias,
lookup directory, drop-in suffix) combination while enumerating units
at boot, to check whether that unit has a ".d", ".wants", ".requires"
or ".upholds" drop-in directory in that particular lookup path. On a
typical system with ~270 loaded units and ~12 directories in the unit
search path, this adds up to tens of thousands of calls.

For every one of those calls, the function used to independently
re-derive the full chain of candidate unit names to check for that one
directory: the name itself, its template if it is a template instance,
and its "-" prefix chain (e.g. for "foo-bar-waldo.service" also
"foo-bar-.service" and "foo-.service"), recursively expanding further
where applicable. That derivation only depends on the unit name itself
and does not involve the lookup directory at all, so it produces the
exact same list of candidate names regardless of which of the 12
lookup directories is currently being checked. Despite this, it was
being fully recomputed for every single directory, doing several small
allocations and unit-name parsing calls (unit_name_template(),
unit_name_to_prefix(), unit_name_build_from_type(), ...) each time.

Split the name-derivation logic out into its own function,
unit_file_expand_dropin_names(), and compute it once per unit
name/alias, then reuse the resulting candidate list across all lookup
directories instead of re-deriving it for each of them. The order in
which candidate directories end up being added is unchanged, so this
is not expected to alter drop-in resolution behaviour: I confirmed this
by comparing the sorted unit load state, fragment path and drop-in path
output of "systemd --test --system" before and after this change on the
same unit tree, which is byte-for-byte identical.

I measured the effect by instrumenting manager_enumerate() with
CLOCK_MONOTONIC timestamps and r... (continued)

Coverage Stats

27 of 30 new or added lines in 1 file covered. (90.0%)

2927 existing lines in 51 files now uncovered.

348282 of 474820 relevant lines covered (73.35%)

1314830.72 hits per line