我有一个动作方法,需要根据用户选择的日期完成15~52个长时间运行的SQL查询(所有这些查询都相似,每个查询需要5秒以上才能完成).
在做了大量研究之后,似乎在不阻塞ASP.Net线程的情况下执行此操作的最佳方法是使用async / await任务方法使用SQL查询：
[HttpPost]
public async Task<JsonResult> Action() {
// initialization stuff
// create tasks to run async sql queries
ConcurrentBag<Tuple<DateTime,List<long>>> weeklyObsIdBag =
new ConcurrentBag<Tuple<DateTime,List<long>>>();
Task[] taskList = new Task[reportDates.Count()];
int idx = 0;
foreach (var reportDate in reportDates) { //15 <= reportDates.Count() <= 52
var task = Task.Run(async () => {
using (var sioDbContext = new SioDbContext()) {
var historyEntryQueryable = sioDbContext.HistoryEntries
.AsNoTracking()
.AsQueryable<HistoryEntry>();
var obsIdList = await getObsIdListAsync(
historyEntryQueryable,reportDate
);
weeklyObsIdBag.Add(new Tuple<DateTime,List<long>>(reportDate,obsIdList));
}
});
taskList[idx++] = task;
}
//await for all the tasks to complete
await Task.WhenAll(taskList);
// consume the results from long running sql queries,// which is stored in weeklyObsIdBag
}
private async Task<List<long>> getObsIdListAsync(
IQueryable<HistoryEntry> historyEntryQueryable,DateTime reportDate
) {
//apply reportDate condition to historyEntryQueryable
//run async query
List<long> obsIdList = await historyEntryQueryable.Select(he => he.ObjectId)
.Distinct()
.ToListAsync()
.ConfigureAwait(false);
return obsIdList;
}
进行此更改后,完成此操作所需的时间大大减少,因为现在我可以同时执行多个(15~52)异步SQL查询并等待它们完成而不是按顺序运行它们.但是,用户开始遇到很多超时问题,例如：
(from Elmah error log) "Timeout expired. The timeout period elapsed prior to obtaining a connection from the pool. This may have occurred because all pooled connections were in use and max pool size was reached." "The wait operation timed out"
它是由线程饥饿引起的吗？我感觉我可能在线程池中使用了太多线程来实现我想要的东西,但我认为这应该不是问题,因为我使用async / await来阻止所有线程被阻塞.
如果事情不会以这种方式工作,那么执行多个长时间运行的SQL查询的最佳做法是什么？
解决方法
考虑限制正在执行的并发任务的数量,例如：
int concurrentTasksLimit = 5;
List<Task> taskList = new List<Task>();
foreach (var reportDate in reportDates) { //15 <= reportDates.Count() <= 52
var task = Task.Run(async () => {
using (var sioDbContext = new SioDbContext()) {
var historyEntryQueryable = sioDbContext.HistoryEntries
.AsNoTracking()
.AsQueryable<HistoryEntry>();
var obsIdList = await getObsIdListAsync(
historyEntryQueryable,reportDate
);
weeklyObsIdBag.Add(new Tuple<DateTime,obsIdList));
}
});
taskList.Add(task);
if (concurrentTasksLimit == taskList.Count)
{
await Task.WhenAll(taskList);
// before clearing the list,you should get the results and store in memory (e.g another list) for later usage...
taskList.Clear();
}
}
//await for all the remaining tasks to complete
if (taskList.Any())
await Task.WhenAll(taskList);
请注意,我将taskList更改为实际的List< Task>,它似乎更容易使用它,因为我们需要从列表中添加/删除任务.
此外,您应该在清除taskList之前获取结果,因为您将在以后使用它们.
