[go-nuts] how to reduce the connection pool size strictly? (database.sql.DB)

DB.SetMaxOpenConns could reduce the connection pool size, but the
unnecssary connections won't close until DB's free connection size >= DB's
idle connections size.
That is to say, when the pool is busy (lots of requests), the unnecessary
connections won't be closed.

I found the key point code in src/database/sql/sql.go

func (db *DB) putConnDBLocked(dc *driverConn, err error) bool {
if c := len(db.connRequests); c > 0 {
req := db.connRequests[0]
// This copy is O(n) but in practice faster than a linked list.
// TODO: consider compacting it down less often and
// moving the base instead?
copy(db.connRequests, db.connRequests[1:])
db.connRequests = db.connRequests[:c-1]
if err == nil {
dc.inUse = true
}
req <- connRequest{
conn: dc,
err: err,
}
return true
} else if err == nil && !db.closed && db.maxIdleConnsLocked() >
len(db.freeConn) {
db.freeConn = append(db.freeConn, dc)
return true
}
return false
}


And what I want:
func (db *DB) putConnDBLocked(dc *driverConn, err error) bool {
if db.numOpen > db.maxOpen {
return false
}
if c := len(db.connRequests); c > 0 {
req := db.connRequests[0]
........

I want to know that whether I should modify that code to achive my goal.
Is that the right way?

--
You received this message because you are subscribed to the Google Groups
"golang-nuts" group.
To unsubscribe from this group and stop receiving emails from it, send an
email to golang-nuts...@googlegroups.com <javascript:>.
For more options, visit https://groups.google.com/d/optout.

On Friday, December 26, 2014 4:35:01 AM UTC+2, Lynx Lo wrote:

I wrote a sample:

Your suggested change would cause requests (in db.connRequests) to be
satisfied late (or not at all). So, not a good change.

DB.SetMaxOpenConns will also reduce maxIdleConns if it is greater than
the new maxOpenConns. This will cause all idle conns over the limit to be
closed immediately.

Can you provide more detail on what problem you are trying to solve?

-Tad
--

I wrote a sample:

package main

import (
"database/sql"
"fmt"
_ "github.com/go-sql-driver/mysql"
"log"
)

func wait(tip string) {
var s string
fmt.Printf("%s >", tip)
fmt.Scanln(&s)
}

func main() {
db, err := sql.Open("mysql", "user:passwd@/mydb")
if err != nil {
log.Fatal(err)
}
defer db.Close()

db.SetMaxIdleConns(4)
db.SetMaxOpenConns(4)
tx1, _ := db.Begin()
tx2, _ := db.Begin()
tx3, _ := db.Begin()
tx4, _ := db.Begin()
wait("tx 1,2,3,4 ok") // numOpen: 4

db.SetMaxOpenConns(2)
wait("maxOpenConns set to 2") // numOpen: 4

// closed 2 of conns, they should be closed
tx1.Rollback()
tx2.Rollback()
wait("tx closed 1,2") // numOpen: 4, expect 2

// simulate high load, there are many pending requests
tx5, _ := db.Begin()
tx6, _ := db.Begin()
wait("tx opened 5,6") // numOpen: 4, expect 2

tx3.Rollback()
tx4.Rollback()
wait("tx closed 3,4") // numOpen: 4, expect 2

tx5.Rollback()
tx6.Rollback()
}

The unnecssary connections won't be closed until they are idle.
If the pool is being busy for a long time, they will keep opening.

在 2014年12月26日星期五UTC+8上午3时00分40秒，Tad Glines写道：
You received this message because you are subscribed to the Google Groups
"golang-nuts" group.
To unsubscribe from this group and stop receiving emails from it, send an
email to golang-nuts...@googlegroups.com <javascript:>.
For more options, visit https://groups.google.com/d/optout.