"的foreach"循环：使用R中的所有核心（特别是如果我们在foreach循环内发送sql查询）

问题描述

我打算使用foreach来使用我CPU中的所有内核。赶上是我需要在循环内发送一个SQL查询。该脚本正常工作正常的'for'循环，但是当我将其更改为foreach时出现以下错误。
错误是：

$ p $ select：中断的系统调用
select：中断的系统调用
select：Interrupted system call
{：task 1 failed - expired MySQLConnection


我使用的代码是：

$ $ $ $ $ b $ library $（$）$ b $ library（foreach）
library（doMC）
library（RMySQL）
library（multicore）
registerDoMC（cores = 6）
m con list< -dbListTables（con ）
foreach（i = 1：（length（list））％dopar％{
query t }

尽管'foreach'在我的系统中可以正常工作，但是只有在sql查询的时候才会出错。是否有一种方法可以在'foreach'循环中发送sql查询？

将数据库查询移到循环之外，并锁定访问权限，以便不执行并行数据库查询。我认为这也会加快速度，因为你不会有并行磁盘访问，同时仍然可以进行并行处理。

含义（伪代码）
db =连接数据库
threadlock = lock（）;

parfor {
threadlock.lock
result = db query（把这里的所有数据都拉出来，因为你在加载的时候不能保持数据库的锁定而不能处理）
thread.unlock
处理结果数据（现在只是数据，而不是sql对象）。
}

I intend to use "foreach" to uitlize all the cores in my CPU. The catch is i need to send a sql query inside the loop. The script is working fine with normal 'for' loop, but it is giving following error when i change it to 'foreach'.The error is :

select: Interrupted system call
select: Interrupted system call
select: Interrupted system call
Error in { : task 1 failed - "expired MySQLConnection"

The code i used is :

library(foreach)
library(doMC)
library(RMySQL)
library(multicore)
registerDoMC(cores=6)
m <- dbDriver("MySQL", max.con = 100)
con <- dbConnect(m, user="*****", password = "******", host ="**.**.***",dbname="dbname")
list<-dbListTables(con)
foreach(i = 1:(length(list))%dopar%{
  query<-paste("SELECT * FROM ",list[i]," WHERE `CLOSE` BETWEEN 1 AND 100",sep="")
  t<-dbGetQuery(con,query)
}

Though 'foreach' is working fine in my system for all other purposes, it is giving error only in case of sql queries. Is there a way to send sql queries inside 'foreach' loop?

解决方案

My suggestion is this:Move the database queries outside the loop, and lock access so you dont do parallel database queries. I think that will speed things up too, as you won't have parallel disk access, while still being able to do parallel processing.

Meaning (pseudo code)db = connect to databasethreadlock = lock();

parfor { threadlock.lock result = db query (pull all data here, as you cant process while you load without keeping the database locked) thread.unlock process resulting data (which is now just data, and not a sql object).}

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