Summary:
This is for https://github.com/facebook/rocksdb/issues/9737
I have wasted more than a few hours running db_bench benchmarks where --seed was not set
and getting better than expected results because cache hit rates are great because
multiple invocations of db_bench used the same value for --seed or did not set it,
and then all used 0. The result is that all see the same sequence of keys.
Others have done the same. The problem is worse in that it is easy to miss and the result is a benchmark with results that are misleading.
A good way to avoid this is to set it to the equivalent of gettimeofday() when either
--seed is not set or it is set to 0 (the default).
With this change the actual seed is printed when it was 0 at process start:
Set seed to 1647992570365606 because --seed was 0
Pull Request resolved: https://github.com/facebook/rocksdb/pull/9740
Test Plan:
Perf results:
./db_bench --benchmarks=fillseq,readrandom --num=1000000 --reads=4000000
readrandom : 6.469 micros/op 154583 ops/sec; 17.1 MB/s (4000000 of 4000000 found)
./db_bench --benchmarks=fillseq,readrandom --num=1000000 --reads=4000000 --seed=0
readrandom : 6.565 micros/op 152321 ops/sec; 16.9 MB/s (4000000 of 4000000 found)
./db_bench --benchmarks=fillseq,readrandom --num=1000000 --reads=4000000 --seed=1
readrandom : 6.461 micros/op 154777 ops/sec; 17.1 MB/s (4000000 of 4000000 found)
./db_bench --benchmarks=fillseq,readrandom --num=1000000 --reads=4000000 --seed=2
readrandom : 6.525 micros/op 153244 ops/sec; 17.0 MB/s (4000000 of 4000000 found)
Reviewed By: jay-zhuang
Differential Revision: D35145361
Pulled By: mdcallag
fbshipit-source-id: 2b35b153ccec46b27d7c9405997523555fc51267
1a130fa3c1