android_lookupEventTagNum added. Adds support for creating a new
log tag at runtime, registered to the logd service.
Tested on Hikey, all services stopped, shell only access, CPUs not
locked (there is enough repeatability on this platform).
$ /data/nativetest64/liblog-benchmarks/liblog-benchmarks BM_lookupEventTagNum
iterations ns/op
Precharge: start
Precharge: stop 231
NB: only Tag matching, linear lookup (as reference, before unordered_map)
BM_lookupEventTagNum 1000000 1017
NB: unordered_map with full Tag & Format lookup, but with Tag hashing
BM_lookupEventTagNum 2000000 683
NB: with full Tag & Format hash and lookup for matching
BM_lookupEventTagNum 2000000 814
NB: only Tag matching (Hail Mary path)
BM_lookupEventTagNum 5000000 471
Because the database can now be dynamic, we added reader/writer locks
which adds a 65ns (uncontended) premium on lookups, and switch to
check for an allocation adds 25ns (either open code, or using
string_view, no difference) which means our overall speed takes 90%
as long as the requests did before we switched to unordered_map.
Faster than before where we originally utilized binary lookup on
static content, but not by much. Dynamic updates that are not cached
locally take the following times to acquire long path to logd to
generate.
BM_lookupEventTag 20000000 139
BM_lookupEventTag_NOT 20000000 87
BM_lookupEventFormat 20000000 139
BM_lookupEventTagNum_logd_new 5000 335936
BM_lookupEventTagNum_logd_existing 10000 249226
The long path pickups are mitigated by the built-in caching, and
the public mapping in /dev/event-log-tags.
SideEffects: Event tags and signal handlers do not mix
Test: liblog benchmarks
Bug: 31456426
Change-Id: I69e6489d899cf35cdccffcee0d8d7cad469ada0a
