unit: reduce heap usage for unit objects

The storage of the unit objects on the heap is currently not very
efficient. For every unit object we allocate a chunk of memory as large
as the biggest unit type, although there are significant differences in
the units' real requirements.
pahole shows the following sizes of structs:
488  Target
496  Snapshot
512  Device
528  Path
560  Timer
576  Automount
1080 Socket
1160 Swap
1168 Service
1280 Mount

Usually there aren't many targets or snapshots in the system, but Device
is one of the most common unit types and for every one we waste
1280 - 512 = 768 bytes.

Fix it by allocating only the right amount for the given unit type.
On my machine (x86_64, with 39 LVM volumes) this decreases systemd's
USS (unique set size) by more than 300 KB.

1 files changed, 4 insertions, 1 deletions

diff --git a/src/unit.h b/src/unit.h
index 19314d6fb..626bdc43a 100644
--- a/src/unit.h
+++ b/src/unit.h
@@ -286,6 +286,9 @@ union Unit {
 struct UnitVTable {
         const char *suffix;
 
+        /* How much memory does an object of this unit type need */
+        size_t object_size;
+
         /* Config file sections this unit type understands, separated
          * by NUL chars */
         const char *sections;
@@ -435,7 +438,7 @@ DEFINE_CAST(SNAPSHOT, Snapshot);
 DEFINE_CAST(SWAP, Swap);
 DEFINE_CAST(PATH, Path);
 
-Unit *unit_new(Manager *m);
+Unit *unit_new(Manager *m, size_t size);
 void unit_free(Unit *u);
 
 int unit_add_name(Unit *u, const char *name);