# 13 Thread Progress

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13.1 Problems

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Knowing When Threads Have Completed Accesses to a Data Structure

When multiple threads access the same data structure you often need to know when all threads have completed their accesses. For example, in order to know when it is safe to deallocate the data structure. One simple way to accomplish this is to reference count all accesses to the data structure. The problem with this approach is that the cache line where the reference counter is located needs to be communicated between all involved processors. Such communication can become extremely expensive and will scale poorly if the reference counter is frequently accessed. That is, we want to use some other approach of keeping track of threads than reference counting.

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Knowing That Modifications of Memory is Consistently Observed

Different hardware architectures have different memory models. Some architectures allows very aggressive reordering of memory accesses while other architectures only reorder a few specific cases. Common to all modern hardware is, however, that some type of reordering will occur. When using locks to protect all memory accesses made from multiple threads such reorderings will not be visible. The locking primitives will ensure that the memory accesses will be ordered. When using lock free algorithms one do however have to take this reordering made by the hardware into account.

Hardware memory barriers or memory fences are instructions that can be used to enforce order between memory accesses. Different hardware architectures provide different memory barriers. Lock free algorithms need to use memory barriers in order to ensure that memory accesses are not reordered in such ways that the algorithm breaks down. Memory barriers are also expensive instructions, so you typically want to minimize the use of these instructions.

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13.2 Functionality Used to Address These Problems

The "thread progress" functionality in the Erlang VM is used to address these problems. The name "thread progress" was chosen since we want to use it to determine when all threads in a set of threads have made such progress so that two specific events have taken place for all them.

The set of threads that we are interested in we call managed threads. The managed threads are the only threads that we get any information about. These threads have to frequently report progress. Not all threads in the system are able to frequently report progress. Such threads cannot be allowed in the set of managed threads and are called unmanaged threads. An example of unmanaged threads are threads in the async thread pool. Async threads can be blocked for very long times and by this be prevented from frequently reporting progress. Currently only scheduler threads and a couple of other threads are managed threads.

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Thread Progress Events

Any thread in the system may use the thread progress functionality in order to determine when the following events have occurred at least once in all managed threads:

1. The thread has returned from other code to a known state in the thread progress functionality, which is independent of any other code.
2. The thread has executed a full memory barrier.

These events, of course, need to occur ordered to other memory operations. The operation of determining this begins by initiating the thread progress operation. The thread that initiated the thread progress operation after this poll for the completion of the operation. Both of these events must occur at least once after the thread progress operation has been initiated, and at least once before the operation has completed in each managed thread. This is ordered using communication via memory which makes it possible to draw conclusion about the memory state after the thread progress operation has completed. Lets call the progress made from initiation to completion for "thread progress".

Assuming that the thread progress functionality is efficient, a lot of algorithms can both be simplified and made more efficient than using the first approach that comes to mind. A couple of examples follows.

By being able to determine when the first event above has occurred we can easily know when all managed threads have completed accesses to a data structure. This can be determined the following way. We have an implementation of some functionality F using a data structure D. The reference to D is always looked up before D is being accessed, and the references to D is always dropped before we leave the code implementing F. If we remove the possibility to look up D and then wait until the first event has occurred in all managed threads, no managed threads can have any references to the data structure D. This could for example have been achieved by using reference counting, but the cache line containing the reference counter would in this case be ping ponged between all processors accessing D at every access.

By being able to determine when the second event has occurred it is quite easy to do complex modifications of memory that needs to be seen consistently by other threads without having to resort to locking. By doing the modifications, then issuing a full memory barrier, then wait until the second event has occurred in all managed threads, and then publish the modifications, we know that all managed threads reading this memory will get a consistent view of the modifications. Managed threads reading this will not have to issue any extra memory barriers at all.

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13.3 Implementation of the Thread Progress Functionality

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Requirement on the Implementation

In order to be able to determine when all managed threads have reached the states that we are interested in we need to communicate between all involved threads. We of course want to minimize this communication.

We also want threads to be able to determine when thread progress has been made relatively fast. That is we need to have some balance between communication overhead and time to complete the operation.

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API

I will only present the most important functions in the API here.

- ErtsThrPrgrVal erts_thr_progress_later(void) - Initiation of the operation. The thread progress value returned can be used testing for completion of the operation.
- int erts_thr_progress_has_reached(ErtsThrPrgrVal val) - Returns a non zero value when we have reached the thread progress value passed as argument. That is, when a non zero value is returned the operation has completed.

When a thread calls my_val = erts_thr_progress_later() and waits for erts_thr_progress_has_reached(my_val) to return a non zero value it knows that thread progress has been made.

While waiting for erts_thr_progress_has_reached() to return a non zero value we typically do not want to block waiting, but instead want to continue working with other stuff. If we run out of other stuff to work on we typically do want to block waiting until we have reached the thread progress value that we are waiting for. In order to be able to do this we provide functionality for waking up a thread when a certain thread progress value has been reached:

- void erts_thr_progress_wakeup(ErtsSchedulerData *esdp, ErtsThrPrgrVal val) - Request wake up. The calling thread will be woken when thread progress has reached val.

Managed threads frequently need to update their thread progress by calling the following functions:

- int erts_thr_progress_update(ErtsSchedulerData *esdp) - Update thread progress. If a non zero value is returned erts_thr_progress_leader_update() has to be called without any locks held.
- int erts_thr_progress_leader_update(ErtsSchedulerData *esdp) - Leader update thread progress.

Unmanaged threads can delay thread progress being made:

- ErtsThrPrgrDelayHandle erts_thr_progress_unmanaged_delay(void) - Delay thread progress.
- void erts_thr_progress_unmanaged_continue(ErtsThrPrgrDelayHandle handle) - Let thread progress continue.

Scheduler threads can schedule an operation to be executed by the scheduler itself when thread progress has been made:

- void erts_schedule_thr_prgr_later_op(void (*funcp)(void *), void *argp, ErtsThrPrgrLaterOp *memp) - Schedule a call to funcp. The call (*funcp)(argp) will be executed when thread progress has been made since the call to erts_schedule_thr_prgr_later_op() was made.

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Implementation

In order to determine when the events has happened we use a global counter that is incremented when all managed threads have called erts_thr_progress_update() (or erts_thr_progress_leader_update()). This could naively be implemented using a "thread confirmed" counter. This would however cause an explosion of communication where all involved processors would need to communicate with each other at each update.

Instead of confirming at a global location each thread confirms that it accepts in increment of the global counter in its own cache line. These confirmation cache lines are located in sequence in an array, and each confirmation cache line will only be written by one and only one thread. One of the managed threads always have the leader responsibility. This responsibility may jump between threads, but as long as there are some activity in the system always one of them will have the leader responsibility. The thread with the leader responsibility will call erts_thr_progress_leader_update() which will check that all other threads have confirmed an increment of the global counter before doing the increment of the global counter. The leader thread is the only thread reading the confirmation cache lines.

Doing it this way we will get a communication pattern of information going from the leader thread out to all other managed threads and then back from the other threads to the leader thread. This since only the leader thread will write to the global counter and all other threads will only read it, and since each confirmation cache lines will only be written by one specific thread and only read by the leader thread. When each managed thread is distributed over different processors, the communication between processors will be a reflection of this communication pattern between threads.

The value returned from erts_thr_progress_later() equals the, by this thread, latest confirmed value plus two. The global value may be latest confirmed value or latest confirmed value minus one. In order to be certain that all other managed threads actually will call erts_thr_progress_update() at least once before we reach the value returned from erts_thr_progress_later(), the global counter plus one is not enough. This since all other threads may already have confirmed current global value plus one at the time when we call erts_thr_progress_later(). They are however guaranteed not to have confirmed global value plus two at this time.

The above described implementation more or less minimizes the communication needed before we can increment the global counter. The amount of communication in the system due to the thread progress functionality however also depend on the frequency with which managed threads call erts_thr_progress_update(). Today each scheduler thread calls erts_thr_progress_update() more or less each time an Erlang process is scheduled out. One way of further reducing communication due to the thread progress functionality is to only call erts_thr_progress_update() every second, or third time an Erlang process is scheduled out, or even less frequently than that. However, by doing updates of thread progress less frequently all operations depending on the thread progress functionality will also take a longer time.

##### Delay of Thread Progress by Unmanaged Threads

In order to implement delay of thread progress from unmanaged threads we use two reference counters. One being current and one being waiting. When an unmanaged thread wants to delay thread progress it increments current and gets a handle back to the reference counter it incremented. When it later wants to enable continuation of thread progress it uses the handle to decrement the reference counter it previously incremented.

When the leader threads is about to increment the global thread progress counter it verifies that the waiting counter is zero before doing so. If not zero, the leader isn't allowed to increment the global counter, and needs to wait before it can do this. When it is zero, it swaps the waiting and current counters before increasing the global counter. From now on the new waiting counter will decrease, so that it eventually will reach zero, making it possible to increment the global counter the next time. If we only used one reference counter it would potentially be held above zero for ever by different unmanaged threads.

When an unmanaged thread increment the current counter it will not prevent the next increment of the global counter, but instead the increment after that. This is sufficient since the global counter needs to be incremented two times before thread progress has been made. It is also desirable not to prevent the first increment, since the likelihood increases that the delay is withdrawn before any increment of the global counter is delayed. That is, the operation will cause as little disruption as possible.

However, this feature of delaying thread progress from unmanaged threads should preferably be used as little as possible, since heavy use of it will cause contention on the reference counter cache lines. The functionality is however very useful in code which normally only executes in managed threads, but which may under some infrequent circumstances be executed in other threads.

##### Overhead

The overhead caused by the thread progress functionality is more or less fixed using the same amount of schedulers regardless of the number of uses of the functionality. Already today quite a lot of functionality use it, and we plan to use it even more. When rewriting old implementations of ERTS internal functionality to use the thread progress functionality, this implies removing communication in the old implementation. Otherwise it is simply no point rewriting the old implementation to use the thread progress functionality. Since the thread progress overhead is more or less fixed, the rewrite will cause a reduction of the total communication in the system.

##### An Example

The main structure of an ETS table was originally managed using reference counting. Already a long time ago we replaced this strategy since the reference counter caused contention on each access of the table. The solution used was to schedule "confirm deletion" jobs on each scheduler in order to know when it was safe to deallocate the table structure of a removed table. These confirm deletion jobs needed to be allocated. That is, we had to allocate and deallocate as many blocks as schedulers in order to deallocate one block. This of course was a quite an expensive operation, but we only needed to do this once when removing a table. It was more important to get rid of the contention on the reference counter which was present on every operation on the table.

When the thread progress functionality had been introduced, we could remove the code implementing the "confirm deletion" jobs, and then just schedule a thread progress later operation which deallocates the structure. Besides simplifying the code a lot, we got an increase of more than 10% of the number of transactions per second handled on a mnesia tpcb benchmark executing on a quad core machine.