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What is Eventfd in Linux?
eventfd() creates an “eventfd object” that can be used as an event wait/notify mechanism by user-space applications, and by the kernel to notify user-space applications of events. The object contains an unsigned 64-bit integer (uint64_t) counter that is maintained by the kernel.
What are open file descriptors?
A file descriptor is a number that uniquely identifies an open file in a computer’s operating system. When a program asks to open a file — or another data resource, like a network socket — the kernel: Grants access.
What is Epoll in Linux?
epoll is a Linux kernel system call for a scalable I/O event notification mechanism, first introduced in version 2.5. Its function is to monitor multiple file descriptors to see whether I/O is possible on any of them.
Is Eventfd thread safe?
1 Answer. Writing to an eventfd is thread safe, i.e., two threads (or even processes) are allowed to write to the same fd at the same time, and they will be properly serialized.
What is Epollout?
This system call is used to add, modify, or remove entries in the interest list of the epoll(7) instance referred to by the file descriptor epfd. It requests that the operation op be performed for the target file descriptor, fd. EPOLLOUT The associated file is available for write(2) operations.
What is O_nonblock?
O_NONBLOCK is a property of the open file description, not of the file descriptor, nor of the underlying file. Yes, you could have separate file descriptors open for the same file, one of which is blocking and the other of which is non-blocking.
What is difference between $$ and $!?
$$ gives the process id of the currently executing process whereas $! Shows the process id of the process that recently went into the background.
When do functions need to be marked parallel unsafe?
Functions and aggregates must be marked PARALLEL UNSAFE if they write to the database, access sequences, change the transaction state even temporarily (e.g., a PL/pgSQL function which establishes an EXCEPTION block to catch errors), or make persistent changes to settings.
Is the write to eventfd a syscall or a write?
Despite writing to eventfd is just a write to a memory, the write itself is a syscall. So you CPU has to switch a context, jump into the kernel, do a quick write and then switch back to your userspace application. The context switch costs a lot, much more that just a single 64 bit write.
What does parallel safe mean in PostgreSQL planner?
Parallel Safety. The planner classifies operations involved in a query as either parallel safe, parallel restricted, or parallel unsafe. A parallel safe operation is one which does not conflict with the use of parallel query.
Can a Planner automatically determine if a function is parallel safe?
The planner cannot automatically determine whether a user-defined function or aggregate is parallel safe, parallel restricted, or parallel unsafe, because this would require predicting every operation which the function could possibly perform. In general, this is equivalent to the Halting Problem and therefore impossible.