Apache benchmark: always a few "super late" requests, why ?

Perhaps no matter if i change number of workers, or worker connections,
there's always some super late connections on my "ab" tests..

Am very new to benchmark, but the way am doing now is:

ab -n 8000 -c 1000 http://address_to_a_plain_text_file

ab -n 8000 -c 1000
http://address_to_a_rails_address_that_queries_the_database

Almost all tests i do ends up with very good results up to 90% of the
requests ( both, with plain text and with rails script ), but then
sometimes in this last 10% there's very slow requests ( 10x more than the
fastest )..

For instance : ab -n 8000 -c 1000 http://address_to_a_plain_text_file

Percentage of the requests served within a certain time (ms)
50% 142
66% 152
75% 156
80% 158
90% 169
95% 181
98% 4180
99% 5236
100% 5485 (longest request)

The ones that goes to the database sometimes get very slow compared to
plain text files, so am guessing i should enlarge somehow the pipe betweens
rails and the db.

For instance : ab -n 8000 -c 1000 http://address_to_rails_touching_db_file

Percentage of the requests served within a certain time (ms)
50% 10
66% 10
75% 10
80% 11
90% 12
95% 13
98% 16
99% 25
100% 13237 (longest request)


Even my plain text files are suffering, so i would guess that is a problem
in my worker_process / worker_connections and keepalive_timeout ?


And for the file that touchs the database i would guess i should be
tweaking some database configuration between my rails and mongodb...


Any advices?
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