Is it necessary to acquire a lock on a variable before reading it from multiple threads?
The short answer is: it depends.
The long answer is:
If it is not a shared value, i.e, only one thread can see it (or use it), you don't need any synchronization.
If it is an immutable value, i.e., you set it only once and then only ever read, it is safe to do so without synchronization (as long as you don't start reading before the first write completes).
If it is a "primitive" type of at most 32-bits (e.g. byte, short, int) you can get stale (old) data when reading. If that doesn't bother you, you're set. If stale data is undesirable, making the variable volatile can fix this problem without additional synchronization for reads. But if you have racing writers, you will need to follow the same advice as for longs below.
If it is a "primitive" type longer than 32-bits (e.g. long, decimal, double) you need synchronization, otherwise you could read "half" of one value, "half" of another, and get crazy results. For this the recommended approach is to use the methods in the Interlocked class, for both reads and writes..
If it is a reference type, you will need synchronization to avoid seeing an invalid state (Jeff Lamb's picture example is a good one). The lock statement might be enough for that. Again, you need to lock for both reads and writes.
There are some other points to consider (how long to lock, for example), but I think these are enough to answer your question.
It depends on the type of variable and your platform. For example, reading Int64s is not guaranteed to be atomic on 32 bit machines. Hence, Interlocked.Read.
If the loading of the value is done in 1 assembly instruction, it's not necessary to get a lock. You don't care if the value changed 10 minutes ago or 1 microsecond ago. You just want the value now.
However, if you're loading a HUGE array or picture or something, it'd probably be a good idea to lock it out. In theory, you can get preempted while loading the data and have half of the first item and half of the second item.
If it's a simple variable, though, like a bool or int, it's not necessary.
In adition to the answers below you can also do a read lock using the ReadWriterLockSlim.
That would allow you to do only a read lock when reading and a write lock when modifying your variable. Multiple threads can have a read lock at the same time but as soon as a thread requests a write lock all new request are blocked until it is complete.
This sort of locking would be usefull if you are doing alot of reads and not many writes.
As with most multithreading issues, research it enough to understand if it really fits your problem the ReadWriterLock would not be suitable for every locking situation.
It depends on whether or not is it a local or shared variable, and whether something else may write to it in the meantime, and what you're going to do after reading it.
If you make a decision based on the variable, consider that the next line of code may then be based on data which is now stale.
Answer is it depends. If the value of the variable does not change when the threads are accessing the variable. otherwise, its needed.
Also, You can use Interlocked.XXX series for maintaining atomicity in reading\writing the variable .
Reading does not require a lock; as long as you don't care about the 'correctness' of the read. It is only dangerous if you attempt to write without a lock.
If it is a constant, no.
If it is an updatable value, yes, if you need consistency.
For updatable values where the exact value must be managed, then yes, you should use a lock or other synchronization method for reads and writes; and perhaps block during the entire scope where the value is used.
It is 100% necessary unless you are 100% sure that the variable's value won't change while the reader threads are running.
Necessary? No.
...but if it's possible that another thread could try to write to it during the read (like a collection, etc.) then it might be a good idea.
As long as it doesn't change during others threads execution you don't need to lock it.
If change, you should use it.
If the variable is never written to by someone (at least at the time it is accessible), you don't need to lock it, because there are no possibilities for missed updates. The same goes if you don't care about missed updates (meaning it is not a problem if you get an older value). Otherwise you should use some sort of synchronization
Related
I'm using such configuration:
.NET framework 4.5
Windows Server 2008 R2
HP DL360p Gen8 (2 * Xeon E5-2640, x64)
I have such field somewhere in my program:
protected int HedgeVolume;
I access this field from several threads. I assume that as I have multi-processor system it's possible that this threads are executing on different processors.
What should I do to guarantee that any time I use this field the most recent value is "read"? And to make sure that when I "write" value it become available to all other threads immediately?
What should I do?
just leave field as is.
declare it volatile
use Interlocked class to access the field
use .NET 4.5 Volatile.Read, Volatile.Write methods to access the field
use lock
I only need simplest way to make my program work on this configuration I don't need my program to work on another computers or servers or operation systems. Also I want minimal latency so I'm looking for fastest solution that will always work on this standard configuration (multiprocessor intel x64, .net 4.5).
Your question is missing one key element... How important is the integrity of the data in that field?
volatile gives you performance, but if a thread is currently writing changes to the field, you won't get that data until it's done, so you might access out of date information, and potentially overwrite changes another thread is currently doing. If the data is sensitive, you might get bugs that would get very hard to track. However, if you are doing very quick update, overwrite the value without reading it and don't care that once in a while you get outdated (by a few ms) data, go for it.
lock guaranty that only one thread can access the field at a time. You can put it only on the methods that write the field and leave the reading method alone. The down side is, it is slow, and may block a thread while another is performing its task. However, you are sure your data stay valid.
Interlock exist to shield yourself from the scheduler context switch. My opinion? Don't use it unless you know exactly why you would be using it and exactly how to use it. It gives options, but with great options comes great problematic. It prevents a context switch while a variable is being update. It might not do what you think it does and won't prevent parallel threads from performing their tasks simultaneously.
You want to use Volatile.Read().
As you are running on x86, all writes in C# are the equivalent of Volatile.Write(), you only need to use this for Itanium.
Volatile.Read() will ensure that you get the latest copy regardless of which thread last wrote it.
There is a fantastic write up here, C# Memory Model Explained
Summary of it includes,
On some processors, not only must the compiler avoid certain
optimizations on volatile reads and writes, it also has to use special
instructions. On a multi-core machine, different cores have different
caches. The processors may not bother to keep those caches coherent by
default, and special instructions may be needed to flush and refresh
the caches.
Hopefully that much is obvious, other than the need for volatile to stop the compiler from optimising it, there is the processor as well.
However, in C# all writes are volatile (unlike say in Java),
regardless of whether you write to a volatile or a non-volatile field.
So, the above situation actually never happens in C#. A volatile write
updates the thread’s cache, and then flushes the entire cache to main
memory.
You do not need Volatile.Write(). More authoratitive source here, Joe Duffy CLR Memory Model. However, you may need it to stop the compiler reordering it.
Since all C# writes are volatile, you can think of all writes as going
straight to main memory. A regular, non-volatile read can read the
value from the thread’s cache, rather than from main
You need Volatile.Read()
When you start designing a concurrent program, you should consider these options in order of preference:
1) Isolation: each thread has it's own private data
2) Immutability: threads can see shared state, but it never changes
3) Mutable shared state: protect all access to shared state with locks
If you get to (3), then how fast do you actually need this to be?
Acquiring an uncontested lock takes in the order of 10ns ( 10-8 seconds ) - that's fast enough for most applications and is the easiest way to guarantee correctness.
Using any of the other options you mention takes you into the realm of low-lock programming, which is insanely difficult to get correct.
If you want to learn how to write concurrent software, you should read these:
Intro: Joe Albahari's free e-book - will take about a day to read
Bible: Joe Duffy's "Concurrent Programming on Windows" - will take about a month to read
Depends what you DO. For reading only, volatile is easiest, interlocked allows a little more control. Lock is unnecessary as it is more ganular than the problem you describe. Not sure about Volatile.Read/Write, never used them.
volatile - bad, there are some issues (see Joe Duffy's blog)
if all you do is read the value or unconditionally write a value - use Volatile.Read and Volatile.Write
if you need to read and subsequently write an updated value - use the lock syntax. You can however achieve the same effect without lock using the Interlocked classes functionality, but this is more complex (involves CompareExchange s to ensure that you are updating the read value i.e. has not been modified since the read operation + logic to retry if the value was modified since the read).
From this i can understand that you want to be able to read the last value that it was writtent in a field. Lets make an analogy with the sql concurency problem of the data. If you want to be able to read the last value of a field you must make atomic instructions. If someone is writing a field all of the threads must be locked for reading until that thread finished the writing transaction. After that every read on that thread will be safe. The problem is not with reading as it is with writing. A lock on that field whenever its writtent should be enough if you ask me ...
First have a look here: Volatile vs. Interlocked vs. lock
The volatile modifier shurely is a good option for a multikernel cpu.
But is this enough? It depends on how you calculate the new HedgeVolume value!
If your new HedgeVolume does not depend on current HedgeVolume then your done with volatile.
But if HedgeVolume[x] = f(HedgeVolume[x-1]) then you need some thread synchronisation to guarantee that HedgeVolume doesn't change while you calculate and assign the new value. Both, lock and Interlocked szenarios would be suitable in this case.
I had a similar question and found this article to be extremely helpful. It's a very long read, but I learned a LOT!
I will try to make this question as generic as possible, but I will give a brief introduction to my actual problem -
I am trying to implement a concurrent skiplist for a priority queue. Each 'node', has a value, and an array of 'forward' nodes, where node.forward[i] represents the next node on the i-th level of the skiplist. For write access (i.e. insertions and deletions), I use a Spinlock (still to determine if that is the best lock to use)
My question is essentially, when I need a read access for a traversal,
node = node.forward[i]
What kind of thread safety do I need around something like this? If another thread is modifying node.forward[i] at exactly the same time that I read (with no current locking mechanism for read), what can happen here?
My initial thought is to have a ReaderWriterLockSLim on the getter and setter of the indexer for Forward. Will there be too much unnecessary locking in this scenario?
Edit: Or would it be best to instead use a Interlocked.Exchange for all of my reads?
If another thread is modifying node.forward[i] at exactly the same time that I read (with no current locking mechanism for read), what can happen here?
It really depends on the implementation. It's possible to use Interlocked.Exchange when setting "forward" in a way that can prevent the references from being invalid (as it can make the "set" atomic), but there is no guarantee of which reference you'd get on read. However, with a naive implementation, anything can happen, including getting bad data.
My initial thought is to have a ReaderWriterLockSLim on the getter and setter of the indexer for Forward.
This is likely to be a good place to start. It will be fairly easy to make a properly synchronized collection using a ReaderWriterLockSlim, and functional is always the first priority.
This would likely be a good starting point.
Will there be too much unnecessary locking in this scenario?
There's no way to know without seeing how you implement it, and more importantly, how it's goign to be used. Depending on your usage, you can profile and look for optimization opportunities if necessary at that point.
On a side note - you might want to reconsider using node.Forward[i] as opposed to more of a "linked list" approach here. Any access to Forward[i] is likely to require a fair bit of synchronization to iterate through the skip list i steps, all of which will need some synchronization to prevent errors if there are concurrent writes anywhere between node and i elements beyond node. If you only look ahead one step, you can (potentially) reduce the amount of synchronization required.
I have a 2d Array of memory. I have multiple threads reading and writing to single elements in the array spontaneously, arbitrarily, and concurrently.
What is the fastest way or best practice to construct my memory access code? I don't like the idea of locking because it blocks other threads.
Data integrity is actually not that important, but it should be (mostly) consistent. My code can handle a few memory errors.
It needs to be really, really fast!
Thanks for feedback.
If data integrity is not important, you can just access the data without caring about multithreading at all.
No one can predict the result, though.
I wouldn't call this approach "best practice", however. IMHO best pratice is caring about multithreading, and protecting the data with appropriately-grained mutexes. My opinion is that every application should be first correct, and only then fast. Inconsistent results are just wrong, doesn't matter if they come fast or not.
Use the Interlocked class to CAS (CompareAndExchange) the objects/values in your array. It makes the operation atomic which ensures that the data is not corrupted. That's about the fastest thing you can do (aside from accessing/modifying the data directly without interlocking). However, if you're modifying the size of the 2D array (growing/shrinking) then you will have some serious problems unless you use some kind of locking mechanism on your array.
Declare the array as volatile and ensure it's scoped such that it's visible to all your threads. I generally like to avoid statics, so either pass the array by reference, or set up all your threads to run methods of an instance class that has the array defined as an instance field.
However, I strongly urge you to rethink what "volatile access" means in terms of data integrity. Best practice is NOT to do what you are attempting without good locking mechanics. You may think it's a small problem, but you can find yourself with a very non-deterministic system, so much so that its data isn't reliable in the slightest.
Let's say you have 8 threads running, and all of them will get a value from an index of the array, do some calculation, then add the result back to the index of the array. Thread 1 starts first and gets the value of the index, 0. Then threads 2-7 all start and get the same value. Thread 1 performs its calculation, gets the index again to ensure it has the "latest" value, then tries to update the value. However, other threads are waiting for that memory, and due to some scheduling implementation you know nothing about, in between Thread 1 getting the index (still zero) and writing its result, threads 2-7 have ALL written their values. Then Thread 1 writes its value, overwriting everything the other 7 threads have done. The other 7 threads, in turn, probably had similar "races" with each other such that the value overwritten by Thread 1 probably overwrote the results of half the threads anyway.
I guarantee you that this behavior is NOT what you want, no matter how much you think you can get away with it; it WILL cause data corruption, which WILL affect other areas of the system, and you WILL be forced to implement proper locking.
If you are interested solely in performance, then the way in which you order your memory accesses can play a big role. Spend an hour or so reading through the slides from Lecture 1 of MIT's Performance Engineering class. The other lectures may also be interesting to you (such as Lecture 6).
Basically, you can optimize your use of the cache to greatly improve performance, depending on your read/write patterns, given the workload you are using.
This should not stop you from doing something that is correct, however.
When I said atomic, I meant set of instructions will execute without any context switching to another thread on the same process (other kinds of switches have to be done of course). The only solution I came up with is to suspend all threads except currently executed before part and resume them after it. Any more elegant way?
The reason I want to do that is to collect a coherent state of objects running on multiple threads. However, their code cannot be changed (they're already compiled), so I cannot insert mutexes, semaphores, etc in it. The atomic operation is of course state collecting (i.e. copying some variables).
There are some atomic operations in the Interlocked class but it only provides a few very simple operations. It can't be used to create an entire atomic block of code.
I'd advise using locking carefully to make sure that your code will still work even if the context changes.
Well, you can use locks, but you can't prevent context switching exactly. But if your threads lock on the same object, then the threads waiting obviously won't be running, so there's no context switching involved since there's nothing to run.
You might want to look at this page too.
No. You can surround a block of code with a Monitor to make it thread-safe, but you cannot make general code snippets atomic.
object lck = new object();
lock(lck)
{
// thread safe code goes in here
}
No, that's against multi-tasking.
Unless very simple operations like incrementing ... which are not subject of your question.
It is possible to obtain a global state from a shared memory composed of a collection (array) of atomic one reader/multi writer registers. The solution is simple but not trivial. You can read the algorithm published in the paper "atomic snapshots of shared memory" or you can read the chapter 4 from the art of multiprocesor programming book, there you can get ideas on the implementation on the java language, of course, once you are familiarized with the idea you should be able to transport it to any other language. Sorry if my english is not well enough.
I've been reading Joe Duffy's book on Concurrent programming. I have kind of an academic question about lockless threading.
First: I know that lockless threading is fraught with peril (if you don't believe me, read the sections in the book about memory model)
Nevertheless, I have a question:
suppose I have an class with an int property on it.
The value referenced by this property will be read very frequently by multiple threads
It is extremely rare that the value will change, and when it does it will be a single thread that changes it.
If it does change while another operation that uses it is in flight, no one is going to lose a finger (the first thing anyone using it does is copy it to a local variable)
I could use locks (or a readerwriterlockslim to keep the reads concurrent).
I could mark the variable volatile (lots of examples where this is done)
However, even volatile can impose a performance hit.
What if I use VolatileWrite when it changes, and leave the access normal for reads. Something like this:
public class MyClass
{
private int _TheProperty;
internal int TheProperty
{
get { return _TheProperty; }
set { System.Threading.Thread.VolatileWrite(ref _TheProperty, value); }
}
}
I don't think that I would ever try this in real life, but I'm curious about the answer (more than anything, as a checkpoint of whether I understand the memory model stuff I've been reading).
Marking a variable as "volatile" has two effects.
1) Reads and writes have acquire and release semantics, so that reads and writes of other memory locations will not "move forwards and backwards in time" with respect to reads and writes of this memory location. (This is a simplification, but you take my point.)
2) The code generated by the jitter will not "cache" a value that seems to logically be unchanging.
Whether the former point is relevant in your scenario, I don't know; you've only described one memory location. Whether or not it is important that you have only volatile writes but not volatile reads is something that is up to you to decide.
But it seems to me that the latter point is quite relevant. If you have a spin lock on a non-volatile variable:
while(this.prop == 0) {}
the jitter is within its rights to generate this code as though you'd written
if (this.prop == 0) { while (true) {} }
Whether it actually does so or not, I don't know, but it has the right to. If what you want is for the code to actually re-check the property on each go round the loop, marking it as volatile is the right way to go.
The question is whether the reading thread will ever see the change. It's not just a matter of whether it sees it immediately.
Frankly I've given up on trying to understand volatility - I know it doesn't mean quite what I thought it used to... but I also know that with no kind of memory barrier on the reading thread, you could be reading the same old data forever.
The "performance hit" of volatile is because the compiler now generates code to actually check the value instead of optimizing that away - in other words, you'll have to take that performance hit regardless of what you do.
At the CPU level, yes every processor will eventually see the change to the memory address. Even without locks or memory barriers. Locks and barriers would just ensure that it all happened in a relative ordering (w.r.t other instructions) such that it appeared correct to your program.
The problem isn't cache-coherency (I hope Joe Duffy's book doesn't make that mistake). The caches stay conherent - it is just that this takes time, and the processors don't bother to wait for that to happen - unless you enforce it. So instead, the processor moves on to the next instruction, which may or may not end up happening before the previous one (because each memory read/write make take a different amount of time. Ironically because of the time for the processors to agree on coherency, etc. - this causes some cachelines to be conherent faster than others (ie depending on whether the line was Modified, Exclusive, Shared, or Invalid it takes more or less work to get into the necessary state).)
So a read may appear old or from an out of date cache, but really it just happened earlier than expected (typically because of look-ahead and branch prediction). When it really was read, the cache was coherent, it has just changed since then. So the value wasn't old when you read it, but it is now when you need it. You just read it too soon. :-(
Or equivalently, it was written later than the logic of your code thought it would be written.
Or both.
Anyhow, if this was C/C++, even without locks/barriers, you would eventually get the updated values. (within a few hundred cycles typically, as memory takes about that long). In C/C++ you could use volatile (the weak non-thread volatile) to ensure that the value wasn't read from a register. (Now there's a non-coherent cache! ie the registers)
In C# I don't know enough about CLR to know how long a value could stay in a register, nor how to ensure you get a real re-read from memory. You've lost the 'weak' volatile.
I would suspect as long as the variable access doesn't completely get compiled away, you will eventually run out of registers (x86 doesn't have many to start with) and get your re-read.
But no guarantees that I see. If you could limit your volatile-read to a particular point in your code that was often, but not too often (ie start of next task in a while(things_to_do) loop) then that might be the best you can do.
This is the pattern I use when the 'last writer wins' pattern is applicable to the situation. I had used the volatile keyword, but after seeing this pattern in a code example from Jeffery Richter, I started using it.
For normal things (like memory-mapped devices), the cache-coherency protocols going on within/between the CPU/CPUs is there to ensure that different threads sharing that memory get a consistent view of things (i.e., if I change the value of a memory location in one CPU, it will be seen by other CPUs that have the memory in their caches). In this regard volatile will help to ensure that the optimizer doesn't optimize away memory accesses (which are always going through cache anyway) by, say, reading the value cached in a register. The C# documentation seems pretty clear on this. Again, the application programmer doesn't generally have to deal with cache-coherency themselves.
I highly recommend reading the freely available paper "What Every Programmer Should Know About Memory". A lot of magic goes on under the hood that mostly prevents shooting oneself in the foot.
In C#, the int type is thread-safe.
Since you said that only one thread writes to it, you should never have contention as to what is the proper value, and as long as you are caching a local copy, you should never get dirty data.
You may, however, want to declare it volatile if an OS thread will be doing the update.
Also keep in mind that some operations are not atomic, and can cause problems if you have more than one writer. For example, even though the bool type wont corrupt if you have more than one writer, a statement like this:
a = !a;
is not atomic. If two threads read at the same time, you have a race condition.