I'm confused about how Concurrent Dictionaries lock their resources.
For example, if I run a method that iterates over every item in the Dictionary and edits its value in a thread and I try to read the value of a key from another thread:
Will the second thread be accessing a snapshot of the Dictionary?
If not, will it access the updated entry if that one has already been updated?
Concurrent Dictionaries are thread-safe that can be accessed by multiple threads concurrently. Read operations on the dictionary are performed in a lock-free manner whereas write is protected with lock. For implementation details, please check ConcurrentDictionary.
The second thread will be accessing the updated dictionary. All a ConcurrentDictionary does is provide put a lock on the collection on any add or remove operations. Read operations are unaffected.
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I am using a ConcurrentDictionary<string, List<CustomObjects>> in a multi threaded environment.
Multiple threads are accessing the same dictionary for retrieving and modifying the item with unique key only. I mean although there are multiple threads accessing the dictionary each thread is accessing and modifying the element at unique key only.
At any point of time no situation will come that more than one thread are trying to access and modify the element at same key location.
Now my question is even in this case the internal locking happens? Or since the different threads are accessing different element at a time on same dictionary no internal locking or any concurrency mechanism execute?
I believe if more than one thread are trying to modify the element at same key location, by default locks will be executed and concurrency will be ensured. But if the different threads are accessing the different items within the dictionary, will it be done in lock free manner internally?
If I understand your question correctly, you have a number of threads, and each thread interacts with an exclusive subset of the ConcurrentDictionary<K,V>. As a minimal example, you have two threads 1 and 2, and a dictionary with two keys "A" and "B". The thread 1 reads and modifies only the key "A", and the thread 2 reads and modifies only the key "B". And you are asking if locking is happening internally in the dictionary.
Reading the value of a key in a ConcurrentDictionary<K,V> is look-free (source code¹).
Updating the value of a key in a ConcurrentDictionary<K,V> requires acquiring a lock (source code).
So locking is happening whenever any thread updates any key. Even if it's a single thread and the dictionary has a single key. But what is more interesting to know, is whether contention happens whenever a thread tries to acquire a lock. Acquiring an uncontended lock is very cheap², but if the lock is already acquired by another thread then the current thread might get blocked, which is costly. During a TryUpdate operation a thread has to acquire a lock and then compare the searched key with existing keys stored in the buckets of the dictionary, and a key comparison can be competitively intensive. For example case-insensitive comparison of extremely long strings. So blocking threads and causing thread-switching is a real concern.
Fortunately the ConcurrentDictionary<K,V> doesn't use the same locker object for all lock operations. It maintains internally a large number of lockers, up to around a thousand, and selects one based on the hashcode of the key. So it's very unlikely that two threads will block each other, contending for the same locker, but it's not mathematically impossible.
¹ .NET 7
² "You can expect to acquire and release a lock in as little as 20 nanoseconds on a 2010-era computer if the lock is uncontended."
I have a dictionary only supports add and modify operations and can be concurrently operated, but always for different keys. Keys are int and values are a reference type. Also modify means change some properties of a value.
My questions are:
Do I need to use ConcurrentDictionary in this scenario? If needed, how does it help?
If concurrent modification can happen on the same key, will ConcurrentDictionary help to ensure thread safty? My understanding is no, is that correct?
Thanks!
Do I need to use ConcurrentDictionary in this scenario? If needed, how
does it help?
Yes, the standard dictionary will not behave correctly if more than one thread adds or removes entries at the same time. (although it is safe for multiple threads to read from it at the same time if no others are modifying it).
If concurrent modification can happen on the same key, will
ConcurrentDictionary help to ensure thread safety? My understanding is
no, is that correct?
If you are asking "Will the concurrent dictionary prevent multiple threads from accessing the values inside the dictionary at the same time?", then no, it will not.
If you want to prevent multiple threads from accessing the same value at the same time you will need to use some sort of concurrency control, such as locking.
Best way to find this out is check MSDN documentation.
For ConcurrentDictionary the page is http://msdn.microsoft.com/en-us/library/dd287191.aspx
Under thread safety section, it is stated "All public and protected members of ConcurrentDictionary<TKey, TValue> are thread-safe and may be used concurrently from multiple threads."
Many threads have access to summary. Each thread will have an unique key for accessing the dictionary;
Dictionary<string, List<Result>> summary;
Do I need locking for following operations?
summary[key] = new List<Result>()
summary[key].Add(new Result());
It seems that I don't need locking because each thread will access dictionary with different key, but won't the (1) be problematic because of adding concurrently new record to dictionary with other treads?
Yes, you need to use locking.
Dictionary is not thread safe for add operations.
If you are on .NET 4 you may consider switching to ConcurrentDictionary. Otherwise you should create your own thread safe collection (such as this).
Consider using a ReaderWriterLockSlim for synchronizing access to your collection (in case you won't use ConcurrentDictionary).
All write accesses to your dictionnary must be locked. There is no guarantee that accessing different keys is thread safe, and, in fact, it isn't.
From MSDN:
A Dictionary can support multiple readers concurrently, as long as the collection is not modified. Even so, enumerating through a collection is intrinsically not a thread-safe procedure. In the rare case where an enumeration contends with write accesses, the collection must be locked during the entire enumeration. To allow the collection to be accessed by multiple threads for reading and writing, you must implement your own synchronization.
By default Dictionary is not thread safe. It does not matter what are you going to add. And the most important: you cannot control concurrent additions from different threads. So you need locks for sure. Or switch to a thread safe collection (i.e. CocnurrentDictionary for .NET 4+)
The MSDN documentation for MemoryCache.Set unfortunately doesn’t state explicitly whether it is thread-safe or not.
Is it safe to use .Get() and .Set() from several threads without an explicit lock?
Yes, the MemoryCache class is thread safe:
System.Runtime.Caching.MemoryCache is threadsafe. Multiple concurrent
threads can read and write a MemoryCache instance. Internally
thread-safety is automatically handled to ensure the cache is updated
in a consistent manner.
What this might be referring to is that data stored within the cache
may itself not be threadsafe. For example if a List is placed in
the cache, and two separate threads both get a reference to the cached
List, the two threads will end up stepping on each other if they
both attempt to update the list simultaneously.
This being said the Get and Set methods are thread safe but if the data structure you might be storing into this cache is not thread safe you might get into trouble. Imagine for example that you stored a dictionary inside this cache. Then while thread1 uses Get to fetch the dictionary and starts reading from it, thread2 uses Get to fetch this same dictionary and tries to write to it. While the Get operation will be thread safe what will happen next could be pretty nasty.
The documentation for MemoryCache states:
This type is thread safe.
lock(dictionaryX)
{
dictionaryX.TryGetValue(key, out value);
}
is locking necessary while doing lookups to a Dictionary ?
THe program is multithreaded, and while adding key/value to dict. dict is being locked.
As mentioned here:
Using TryGetValue() without locking is not safe. The dictionary is temporarily in a state that makes it unsuitable for reading while another thread is writing the dictionary. A dictionary will reorganize itself from time to time as the number of entries it contains grows. When you read at the exact time this re-organization takes place, you'll run the risk of finding the wrong value for the key when the buckets got updated but not yet the value entries.
UPDATE:
take a look at "Thread Safety" part of this page too.
As with many subtle questions in programming, the answer is: Not necessarily.
If you only add values as an initialization, then the subsequent reading does not need to be synchronized. But, on the other hand, if you're going to be reading and writing at all times, then absolutely you need to protect that resource.
However, a full-blown lock may not be the best way, depending on the amount of traffic your Dictionary gets. Try a ReaderWriterLockSlim if you are using .NET 3.5 or greater.
If you have multiple threads accessing the Dictionary, then you do need to lock on updates and on lookup. The reason you need to lock on lookup is that there could be an update taking place at the same time you're doing the lookup, and the Dictionary could be in an inconsistent state during the update. For example, imagine that your have one thread doing this:
if (myDictionary.TryGetValue(key, out value))
{
}
and a separate thread is doing this:
myDictionary.Remove(key);
What could happen is that the thread doing the TryGetValue determines that the item is in the dictionary, but before it can retrieve the item, the other thread removes it. The result would be that the thread doing the lookup would either throw an exception or TryGetValue would return true but the value would be null or possibly an object that does not match the key.
That's only one thing that can happen. Something similarly disastrous can happen if you're doing a lookup on one thread and another thread does an add of the value that you're trying to look up.
Locking is only needed when you are synchronizing access to a resource between threads. As long as there are not mulitple threads involved then locking is not needed here.
In the context of updating and reading the value from multiple threads, yes a lock is absolutely necessary. In fact if you're using 4.0 you should consider switching to one of the collections specifically designed for concurrent access.
Use the new ConcurrentDictionary<TKey, TValue> object and you can forget about having to do any locks.
Yes you need to lock the dictionary for access in a multithreaded environment. The writing to a dictionary is not atomic, so it could add the key, but not the value. In that case when you access it, you could get an exception.
If you are on .Net 4, you can replace with ConcurrentDictionary to do this safely. There are other similar collections, preferred when you need multithreaded access, in the System.Collection.Concurrent namespace.
Don't use roll-your-own locking if this is an option for you.
Yes, you should lock if this dictionary is a shared resource between more than one thread. This ensures that you get the correct value and other thread doesn't happen to change value mid-way during your Lookup call.
Yes, you have to lock if you have multithreaded updates on that dictionary. Check this great post for details: “Thread safe” Dictionary(TKey,TValue)
But since ConcurrentDictionary<> introduced you can use it either via .NET 4 or by using Rx in 3.5 (it contains System.Threading.dll with implementation for new thread-safe collections)