journal_file_move_to_object() can skip the second
journal_file_move_to() call if the first one already mapped a
sufficiently large area.
Now that mmap_cache_get() returns the size of the mapped area
when asked, ask for the size and only perform the second call if
the required size exceeds the mapped size instead of the object
header size.
This results in a nice performance boost in my testing, even with
a corpus of many small logs burning much CPU time elsewhere:
Before:
# time ./journalctl -b -1 --no-pager > /dev/null
real 0m16.330s
user 0m16.281s
sys 0m0.046s
# time ./journalctl -b -1 --no-pager > /dev/null
real 0m16.409s
user 0m16.358s
sys 0m0.048s
# time ./journalctl -b -1 --no-pager > /dev/null
real 0m16.625s
user 0m16.558s
sys 0m0.061s
After:
# time ./journalctl -b -1 --no-pager > /dev/null
real 0m15.311s
user 0m15.257s
sys 0m0.046s
# time ./journalctl -b -1 --no-pager > /dev/null
real 0m15.201s
user 0m15.135s
sys 0m0.062s
# time ./journalctl -b -1 --no-pager > /dev/null
real 0m15.170s
user 0m15.113s
sys 0m0.053s