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Previouslly, Stmt.connStmt calls DB.connIfFree on each Stmt.css.
Since Stmt.connStmt locks Stmt.mu, a concurrent use of Stmt causes lock
contention on Stmt.mu.
Additionally, DB.connIfFree locks DB.mu which is shared by DB.addDep and
DB.removeDep.
This change removes DB.connIfFree and makes use of a first unused
connection in idle connection pool to reduce lock contention
without making it complicated.
Fixes #9484
On EC2 c3.8xlarge (E5-2680 v2 @ 2.80GHz * 32 vCPU):
benchmark old ns/op new ns/op delta
BenchmarkManyConcurrentQuery-8 40249 34721 -13.73%
BenchmarkManyConcurrentQuery-16 45610 40176 -11.91%
BenchmarkManyConcurrentQuery-32 109831 43179 -60.69%
benchmark old allocs new allocs delta
BenchmarkManyConcurrentQuery-8 25 25 +0.00%
BenchmarkManyConcurrentQuery-16 25 25 +0.00%
BenchmarkManyConcurrentQuery-32 25 25 +0.00%
benchmark old bytes new bytes delta
BenchmarkManyConcurrentQuery-8 3980 3969 -0.28%
BenchmarkManyConcurrentQuery-16 3980 3982 +0.05%
BenchmarkManyConcurrentQuery-32 3993 3990 -0.08%
Change-Id: Ic96296922c465bac38a260018c58324dae1531d9
Reviewed-on: https://go-review.googlesource.com/2207
Reviewed-by: Mikio Hara <mikioh.mikioh@gmail.com>
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