Sree Kuchibhotla (sreek@) [May - 2016] (Design input from Craig Tiller and David Klempner)
Status: As of June 2016, this change is implemented and merged.
The document talks about the proposed changes to
epoll-based implementation of pollsets in gRPC. Section-2 gives an overview of the current implementation, Section-3 talks about the problems in the current implementation and finally Section-4 talks about the proposed changes.
Figure 1: Current implementation
A gRPC client or a server can have more than one completion queue. Each completion queue creates a pollset.
The gRPC core library does not create any threads[^1] on its own and relies on the application using the gRPC core library to provide the threads. A thread starts to poll for events by calling the gRPC core surface APIs
grpc_completion_queue_pluck(). More than one thread can call
grpc_completion_queue_next()on the same completion queue[^2].
A file descriptor can be in more than one completion queue. There are examples in the next section that show how this can happen.
When an event of interest happens in a pollset, multiple threads are woken up and there are no guarantees on which thread actually ends up performing the work i.e executing the callbacks associated with that event. The thread that performs the work finally queues a completion event
grpc_cq_completion on the appropriate completion queue and “kicks” (i.e wakes ups) the thread that is actually interested in that event (which can be itself - in which case there is no thread hop)
For example, in Figure 1, if
fd1 becomes readable, any one of the threads i.e Threads 1 to Threads K or Thread P, might be woken up. Let's say Thread P was calling a
grpc_completion_queue_pluck() and was actually interested in the event on
fd1 but Thread 1 woke up. In this case, Thread 1 executes the callbacks and finally kicks Thread P by signalling
event_fd_P. Thread P wakes up, realizes that there is a new completion event for it and returns from
grpc_completion_queue_pluck() to its caller.
If multiple threads concurrently call
epoll_wait(), we are guaranteed that only one thread is woken up if one of the
fds in the set becomes readable/writable. However, in our current implementation, the threads do not directly call a blocking
epoll_wait()[^3]. Instead, they call
poll() on the set containing
, epoll_fd]. (see Figure 1)
Considering the fact that an
fd can be in multiple
pollsets and that each
pollset might have multiple poller threads, it means that whenever an
fd becomes readable/writable, all the threads in all the
pollsets (in which that
fd is present) are woken up.
The performance impact of this would be more conspicuous on the server side. Here are a two examples of thundering herds on the server side.
Example 1: Listening fds on server
This means that for every incoming new channel, all the threads waiting on all the pollsets are woken up.
Example 2: New Incoming-channel fds on server
fd(i.e the socket
fdthat is returned by doing an
accept()on the new incoming channel) is added to all the server completion queues’ pollsets [^5]).
There are other scenarios especially on the client side where an fd can end up being on multiple pollsets which would cause thundering herds on the clients.
The main idea in this proposal is to group ‘related’
fds into a single epoll-based set. This would ensure that only one thread wakes up in case of an event on one of the
fds in the epoll set.
To accomplish this, we introduce a new abstraction called
polling_island which will have an epoll set underneath (See Figure 2 below). A
polling_island contains the following:
epoll_fd: The file descriptor of the underlying epoll set
fd_set: The set of ‘fds’ in the pollset island i.e in the epoll set (The pollset island merging operation described later requires the list of fds in the pollset island and currently there is no API available to enumerate all the fds in an epoll set)
event_fd: A level triggered event fd that is used to wake up all the threads waiting on this epoll set (Note: This
event_fdis added to the underlying epoll set during pollset island creation. This is useful in the pollset island merging operation described later)
merged_to: The polling island into which this one merged. See section 4.2 (case 2) for more details on this. Also note that if
merged_tois set, all the other fields in this polling island are not used anymore
In this new model, only one thread wakes up whenever an event of interest happens in an epoll set.
Figure 2: Proposed changes
fdmay belong to multiple
pollsetsbut belongs to exactly one
pollsetbelongs to exactly one
pollset(s) it belongs to, have same
There are two cases to check here:
pollsetalready belong to the same
pollsetpoint to different
polling_islands: In this case we merge both the polling islands i.e:
fdsfrom the smaller
polling_islandto the larger
polling_islandand update the
merged_topointer on the smaller island to point to the larger island.
epoll_fd(by signalling the
event_fdon that island) and make them now wait on the larger
pollsetto now point to the larger
The new implementation, just like the current implementation, does not provide us any guarantees that the thread that is woken up is the thread that is actually interested in the event. So the thread that woke up executes the callbacks and finally has to ‘kick’ the appropriate polling thread interested in the event.
In the current implementation, every polling thread also had a
event_fd on which it was listening to and hence waking it up was as simple as signalling that
event_fd. However, using an
event_fd also meant that every thread has to use a
epoll_fd) instead of doing an
epoll_wait() and this resulted in the thundering herd problems described above.
The proposal here is to use signals and kicking a thread would just be sending a signal to that thread. Unfortunately there are only a few signals available on posix systems and most of them have pre-determined behavior leaving only a few signals
SIGRTx (SIGRTMIN to SIGRTMAX) for custom use.
The calling application might have registered other signal handlers for these signals. `We will provide a new API where the applications can “give a signal number” to gRPC library to use for this purpose.
void grpc_use_signal(int signal_num)
If the calling application does not provide a signal number, then the gRPC library will relegate to using a model similar to the current implementation (where every thread does a blocking
poll() on its
wakeup_fd and the
epoll_fd). The function
psi_wait()in figure 2 implements this logic.
**>> **(NOTE: Or alternatively, we can implement a turnstile polling (i.e having only one thread calling
epoll_wait() on the epoll set at any time - which all other threads call poll on their
wakeup_fds) in case of not getting a signal number from the applications.
[^1]: Only exception is in case of name-resolution
[^2]: However, a
grpc_completion_queue_pluck() must not be called in parallel on the same completion queue
[^3]: The threads first do a blocking
[wakeup_fd, epoll_fd]. If the
poll() returns due to an event of interest in the epoll set, they then call a non-blocking i.e a zero-timeout
epoll_wait() on the
event_fd is the linux platform specific implementation of
wakeup_fd is used to wake up polling threads typically when the event for which the polling thread is waiting is already completed by some other thread. It is also used to wake up the polling threads in case of shutdowns or to re-evaluate the poller's interest in the fds to poll (the last scenario is only in case of
epoll-based) implementation of