Erlang -- Non-blocking code loading

# 6 Non-blocking code loading

###
6.1 Introduction

Before OTP R16 when an Erlang code module was loaded, all other execution in the VM were halted while the load operation was carried out in single threaded mode. This might not be a big problem for initial loading of modules during VM boot, but it can be a severe problem for availability when upgrading modules or adding new code on a VM with running payload. This problem grows with the number of cores as both the time it takes to wait for all schedulers to stop increases as well as the potential amount of halted ongoing work.

In OTP R16, modules are loaded without blocking the VM. Erlang processes may continue executing undisturbed in parallel during the entire load operation. The code loading is carried out by a normal Erlang process that is scheduled like all the others. The load operation is completed by making the loaded code visible to all processes in a consistent way with one single atomic instruction. Non-blocking code loading will improve real-time characteristics when modules are loaded/upgraded on a running SMP system.

###
6.2 The Load Phases

The loading of a module is divided into two phases; a prepare phase and a finishing phase. The prepare phase contains reading the BEAM file format and all the preparations of the loaded code that can easily be done without interference with the running code. The finishing phase will make the loaded (and prepared) code accessible from the running code. Old module versions (replaced or deleted) will also be made inaccessible by the finishing phase.

The prepare phase is designed to allow several "loader" processes to prepare separate modules in parallel while the finishing phase can only be done by one loader process at a time. A second loader process trying to enter finishing phase will be suspended until the first loader is done. This will only block the process, the scheduler is free to schedule other work while the second loader is waiting. (See erts_try_seize_code_write_permission and erts_release_code_write_permission).

The ability to prepare several modules in parallel is not currently used as almost all code loading is serialized by the code_server process. The BIF interface is however prepared for this.

      erlang:prepare_loading(Module, Code) -> LoaderState
      erlang:finish_loading([LoaderState])

The idea is that prepare_loading could be called in parallel for different modules and returns a "magic binary" containing the internal state of each prepared module. Function finish_loading could take a list of such states and do the finishing of all of them in one go.

Currently we use the legacy BIF erlang:load_module which is now implemented in Erlang by calling the above two functions in sequence. Function finish_loading is limited to only accepts a list with one module state as we do not yet use the multi module loading feature.

###
6.3 The Finishing Sequence

During VM execution, code is accessed through a number of data structures. These code access structures are

- Export table. One entry for every exported function.
- Module table. One entry for each loaded module.
- "beam_catches". Identifies jump destinations for catch instructions.
- "beam_ranges". Map code address to function and line in source file.

The most frequently used of these structures is the export table that is accessed in run time for every executed external function call to get the address of the callee. For performance reasons, we want to access all these structures without any overhead from thread synchronization. Earlier this was solved with an emergency break. Stop the entire VM to mutate these code access structures, otherwise treat them as read-only.

The solution in R16 is instead to replicate the code access structures. We have one set of active structures read by the running code. When new code is loaded the active structures are copied, the copy is updated to include the newly loaded module and then a switch is made to make the updated copy the new active set. The active set is identified by a single global atomic variable the_active_code_index. The switch can thus be made by a single atomic write operation. The running code have to read this atomic variable when using the active access structures, which means one atomic read operation per external function call for example. The performance penalty from this extra atomic read is however very small as it can be done without any memory barriers at all (as described below). With this solution we also preserve the transactional feature of a load operation. Running code will never see the intermediate result of a half loaded module.

The finishing phase is carried out in the following sequence by the BIF erlang:finish_loading:

1. Seize exclusive code write permission (suspend process if needed until we get it).

2. Make a full copy of all the active access structures. This copy is called the staging area and is identified by the global atomic variable the_staging_code_index.

3. Update all access structures in the staging area to include the newly prepared module.

4. Schedule a thread progress event. That is a time in the future when all schedulers have yielded and executed a full memory barrier.

5. Suspend the loader process.

6. After thread progress, commit the staging area by assigning the_staging_code_index to the_active_code_index.

7. Release the code write permission allowing other processes to stage new code.

8. Resume the loader process allowing it to return from erlang:finish_loading.

####
Thread Progress

The waiting for thread progress in 4-6 is necessary in order for processes to read the_active_code_index atomic during normal execution without any expensive memory barriers. When we write a new value into the_active_code_index in step 6, we know that all schedulers will see an updated and consistent view of all the new active access structures once they become reachable through the_active_code_index.

The total lack of memory barrier when reading the_active_code_index has one interesting consequence however. Different processes may see the new code at different point in time depending on when different cores happen to refresh their hardware caches. This may sound unsafe but it actually does not matter. The only property we must guarantee is that the ability to see the new code must spread with process communication. After receiving a message that was triggered by new code, the receiver must be guaranteed to also see the new code. This will be guaranteed as all types of process communication involves memory barriers in order for the receiver to be sure to read what the sender has written. This implicit memory barrier will then also make sure that the receiver reads the new value of the_active_code_index and thereby also sees the new code. This is true for all kinds of inter process communication (TCP, ETS, process name registering, tracing, drivers, NIFs, etc) not just Erlang messages.

####
Code Index Reuse

To optimize the copy operation in step 2, code access structures are reused. In current solution we have three sets of code access structures, identified by a code index of 0, 1 and 2. These indexes are used in a round robin fashion. Instead of having to initialize a completely new copy of all access structures for every load operation we just have to update with the changes that have happened since the last two code load operations. We could get by with only two code indexes (0 and 1), but that would require yet another round of waiting for thread progress before step 2 in the finish_loading sequence. We cannot start reusing a code index as staging area until we know that no lingering scheduler thread is still using it as the active code index. With three generations of code indexes, the waiting for thread progress in step 4-6 will give this guarantee for us. Thread progress will wait for all running schedulers to reschedule at least one time. No ongoing execution reading code access structures reached from an old value of the_active_code_index can exist after a second round of thread progress.

The design choice between two or three generations of code access structures is a trade-off between memory consumption and code loading latency.

####
A Consistent Code View

Some native BIFs may need to get a consistent snapshot view of the active code. To do this it is important to only read the_active_code_index one time and then use that index value for all code accessing during the BIF. If a load operation is executed in parallel, reading the_active_code_index a second time might result in a different value, and thereby a different view of the code.