リストスレッドの安全性
-
14-11-2019 - |
質問
以下のコードを使っています
var processed = new List<Guid>();
Parallel.ForEach(items, item =>
{
processed.Add(SomeProcessingFunc(item));
});
.
上記のコードスレッドは安全ですか?リストが破損しているかを処理させるチャンスはありますか?または追加する前にロックを使用する必要がありますか?
var processed = new List<Guid>();
Parallel.ForEach(items, item =>
{
lock(items.SyncRoot)
processed.Add(SomeProcessingFunc(item));
});
.
ありがとう。
解決
No! It is not safe at all, because processed.Add
is not. You can do following:
items.AsParallel().Select(item => SomeProcessingFunc(item)).ToList();
Keep in mind that Parallel.ForEach
was created mostly for imperative operations for each element of sequence. What you do is map: project each value of sequence. That is what Select
was created for. AsParallel
scales it across threads in most efficient manner.
This code works correctly:
var processed = new List<Guid>();
Parallel.ForEach(items, item =>
{
lock(items.SyncRoot)
processed.Add(SomeProcessingFunc(item));
});
but makes no sense in terms of multithreading. lock
ing at each iteration forces totally sequential execution, bunch of threads will be waiting for single thread.
他のヒント
To quote from Jon Skeet before he gets here:
As part of Parellel Extensions in .Net 4, there are several new collections in a new
System.Collections.Concurrent
namespace. These are designed to be safe in the face of concurrent operations from multiple threads, with relatively little locking.
These include IProducerConsumerCollection<T>, BlockingCollection<T>, ConcurrentBag<T>, ConcurrentQueue<T>, ConcurrentStack<T>, and ConcurrentDictionary<TKey, TValue>
among others.
As alternative to the answer of Andrey:
items.AsParallel().Select(item => SomeProcessingFunc(item)).ToList();
You could also write
items.AsParallel().ForAll(item => SomeProcessingFunc(item));
This makes the query that is behind it even more efficient because no merge is required, MSDN.
Make sure the SomeProcessingFunc
function is thread-safe.
And I think, but didn't test it, that you still need a lock if the list can be modified in an other thread (adding or removing) elements.
Using ConcurrentBag of type Something
var bag = new ConcurrentBag<List<Something>>;
var items = GetAllItemsINeed();
Parallel.For(items,i =>
{
bag.Add(i.DoSomethingInEachI());
});
reading is thread safe, but adding is not. You need a reader/writer lock setup as adding may cause the internal array to resize which would mess up a concurrent read.
If you can guarantee the array won't resize on add, you may be safe to add while reading, but don't quote me on that.
But really, a list is just an interface to an array.