So I have just discovered that libuv is a fairly small library as far as C libraries go (compare to FFmpeg). I have spent the past 6 hours reading through the source code to get a feel for the event loop at a deeper level. But still not seeing where the "nonblockingness" is implemented. Where some event interrupt signal or whatnot is being invoked in the codebase.
I have been using Node.js for over 8 years so I am familar with how to use an async non-blocking event loop, but I never actually looked into the implementation.
My question is twofold:
Where exactly is the "looping" occuring within libuv?
What are the key steps in each iteration of the loop that make it non-blocking and async.
So we start with a hello world example. All that is required is this:
#include <stdio.h>
#include <stdlib.h>
#include <uv.h>
int main() {
uv_loop_t *loop = malloc(sizeof(uv_loop_t));
uv_loop_init(loop); // initialize datastructures.
uv_run(loop, UV_RUN_DEFAULT); // infinite loop as long as queue is full?
uv_loop_close(loop);
free(loop);
return 0;
}
The key function which I have been exploring is uv_run. The uv_loop_init function essentially initializes data structures, so not too much fancness there I don't think. But the real magic seems to happen with uv_run, somewhere. A high level set of code snippets from the libuv repo is in this gist, showing what the uv_run function calls.
Essentially it seems to boil down to this:
while (NOT_STOPPED) {
uv__update_time(loop)
uv__run_timers(loop)
uv__run_pending(loop)
uv__run_idle(loop)
uv__run_prepare(loop)
uv__io_poll(loop, timeout)
uv__run_check(loop)
uv__run_closing_handles(loop)
// ... cleanup
}
Those functions are in the gist.
uv__run_timers: runs timer callbacks? loops with for (;;) {.
uv__run_pending: runs regular callbacks? loops through queue with while (!QUEUE_EMPTY(&pq)) {.
uv__run_idle: no source code
uv__run_prepare: no source code
uv__io_poll: does io polling? (can't quite tell what this means tho). Has 2 loops: while (!QUEUE_EMPTY(&loop->watcher_queue)) {, and for (;;) {,
And then we're done. And the program exists, because there is no "work" to be done.
So I think I have answered the first part of my question after all this digging, and the looping is specifically in these 3 functions:
uv__run_timers
uv__run_pending
uv__io_poll
But not having implemented anything with kqueue or multithreading and having dealt relatively little with file descriptors, I am not quite following the code. This will probably help out others along the path to learning this too.
So the second part of the question is what are the key steps in these 3 functions that implement the nonblockingness? Assuming this is where all the looping exists.
Not being a C expert, does for (;;) { "block" the event loop? Or can that run indefinitely and somehow other parts of the code are jumped to from OS system events or something like that?
So uv__io_poll calls poll(...) in that endless loop. I don't think is non-blocking, is that correct? That seems to be all it mainly does.
Looking into kqueue.c there is also a uv__io_poll, so I assume the poll implementation is a fallback and kqueue on Mac is used, which is non-blocking?
So is that it? Is it just looping in uv__io_poll and each iteration you can add to the queue, and as long as there's stuff in the queue it will run? I still don't see how it's non-blocking and async.
Can one outline similar to this how it is async and non-blocking, and which parts of the code to take a look at? Basically, I would like to see where the "free processor idleness" exists in libuv. Where is the processor ever free in the call to our initial uv_run? If it is free, how does it get reinvoked, like an event handler? (Like a browser event handler from the mouse, an interrupt). I feel like I'm looking for an interrupt but not seeing one.
I ask this because I want to implement an MVP event loop in C, but just don't understand how nonblockingness actually is implemented. Where the rubber meets the road.
I think that trying to understand libuv is getting in your way of understanding how reactors (event loops) are implemented in C, and it is this that you need to understand, as opposed to the exact implementation details behind libuv.
(Note that when I say "in C", what I really means is "at or near the system call interface, where userland meets the kernel".)
All of the different backends (select, poll, epoll, etc) are, more-or-less, variations on the same theme. They block the current process or thread until there is work to be done, like servicing a timer, reading from a socket, writing to a socket, or handling a socket error.
When the current process is blocked, it literally is not getting any CPU cycles assigned to it by the OS scheduler.
Part of the issue behind understanding this stuff IMO is the poor terminology: async, sync in JS-land, which don't really describe what these things are. Really, in C, we're talking about non-blocking vs blocking I/O.
When we read from a blocking file descriptor, the process (or thread) is blocked -- prevented from running -- until the kernel has something for it to read; when we write to a blocking file descriptor, the process is blocked until the kernel accepts the entire buffer.
In non-blocking I/O, it's exactly the same, except the kernel won't stop the process from running when there is nothing to do: instead, when you read or write, it tells you how much you read or wrote (or if there was an error).
The select system call (and friends) prevent the C developer from having to try and read from a non-blocking file descriptor over and over again -- select() is, in effect, a blocking system call that unblocks when any of the descriptors or timers you are watching are ready. This lets the developer build a loop around select, servicing any events it reports, like an expired timeout or a file descriptor that can be read. This is the event loop.
So, at its very core, what happens at the C-end of a JS event loop is roughly this algorithm:
while(true) {
select(open fds, timeout);
did_the_timeout_expire(run_js_timers());
for (each error fd)
run_js_error_handler(fdJSObjects[fd]);
for (each read-ready fd)
emit_data_events(fdJSObjects[fd], read_as_much_as_I_can(fd));
for (each write-ready fd) {
if (!pendingData(fd))
break;
write_as_much_as_I_can(fd);
pendingData = whatever_was_leftover_that_couldnt_write;
}
}
FWIW - I have actually written an event loop for v8 based around select(): it really is this simple.
It's important also to remember that JS always runs to completion. So, when you call a JS function (via the v8 api) from C, your C program doesn't do anything until the JS code returns.
NodeJS uses some optimizations like handling pending writes in a separate pthreads, but these all happen in "C space" and you shouldn't think/worry about them when trying to understand this pattern, because they're not relevant.
You might also be fooled into the thinking that JS isn't run to completion when dealing with things like async functions -- but it absolutely is, 100% of the time -- if you're not up to speed on this, do some reading with respect to the event loop and the micro task queue. Async functions are basically a syntax trick, and their "completion" involves returning a Promise.
I just took a dive into libuv's source code, and found at first that it seems like it does a lot of setup, and not much actual event handling.
Nonetheless, a look into src/unix/kqueue.c reveals some of the inner mechanics of event handling:
int uv__io_check_fd(uv_loop_t* loop, int fd) {
struct kevent ev;
int rc;
rc = 0;
EV_SET(&ev, fd, EVFILT_READ, EV_ADD, 0, 0, 0);
if (kevent(loop->backend_fd, &ev, 1, NULL, 0, NULL))
rc = UV__ERR(errno);
EV_SET(&ev, fd, EVFILT_READ, EV_DELETE, 0, 0, 0);
if (rc == 0)
if (kevent(loop->backend_fd, &ev, 1, NULL, 0, NULL))
abort();
return rc;
}
The file descriptor polling is done here, "setting" the event with EV_SET (similar to how you use FD_SET before checking with select()), and the handling is done via the kevent handler.
This is specific to the kqueue style events (mainly used on BSD-likes a la MacOS), and there are many other implementations for different Unices, but they all use the same function name to do nonblocking IO checks. See here for another implementation using epoll.
To answer your questions:
1) Where exactly is the "looping" occuring within libuv?
The QUEUE data structure is used for storing and processing events. This queue is filled by the platform- and IO- specific event types you register to listen for. Internally, it uses a clever linked-list using only an array of two void * pointers (see here):
typedef void *QUEUE[2];
I'm not going to get into the details of this list, all you need to know is it implements a queue-like structure for adding and popping elements.
Once you have file descriptors in the queue that are generating data, the asynchronous I/O code mentioned earlier will pick it up. The backend_fd within the uv_loop_t structure is the generator of data for each type of I/O.
2) What are the key steps in each iteration of the loop that make it non-blocking and async?
libuv is essentially a wrapper (with a nice API) around the real workhorses here, namely kqueue, epoll, select, etc. To answer this question completely, you'd need a fair bit of background in kernel-level file descriptor implementation, and I'm not sure if that's what you want based on the question.
The short answer is that the underlying operating systems all have built-in facilities for non-blocking (and therefore async) I/O. How each system works is a little outside the scope of this answer, I think, but I'll leave some reading for the curious:
https://www.quora.com/Network-Programming-How-is-select-implemented?share=1
The first thing to keep in mind is that work must be added to libuv's queues using its API; one cannot just load up libuv, start its main loop, and then code up some I/O and get async I/O.
The queues maintained by libuv are managed by looping. The infinite loop in uv__run_timers isn't actually infinite; notice that the first check verifies that a soonest-expiring timer exists (presumably, if the list is empty, this is NULL), and if not, breaks the loop and the function returns. The next check breaks the loop if the current (soonest-expiring) timer hasn't expired. If neither of those conditions breaks the loop, the code continues: it restarts the timer, calls its timeout handler, and then loops again to check more timers. Most times when this code runs, it's going to break the loop and exit, allowing the other loops to run.
What makes all this non-blocking is the caller/user following the guidelines and API of libuv: adding your work to queues, and allowing libuv to perform its work on those queues. Processing-intensive work may block these loops and other work from running, so it's important to break your work into chunks.
btw, uv__run_idle, uv__run_check, uv__run_prepare 's source code is defined on src/unix/loop-watcher.c
So, I have a MMO game server listening for connections (up to 1100, hardcoded limit because of game design) and needing to log everything without blocking my main thread (which handles all the virtual-world logic), including but not limited to: what NPCs give to players, when a monster/player dies, every single chat message, when players level up, when players drop an item to the ground, etc.
I get a thread from a pool for each connection, and each thread can log to a file at any time (it may actually never happen, but the server must be prepared enough to handle it appropriately).
I thought about an independent logging thread constantly checking an order queue, until it is not empty.
while Game is Running
Check the queue, if it's empty, sleep.
File IO. Open plain-text file, log and gracefully close it.
Repeat.
Is a linked list enough for organizing the queue and just protect it with a mutex like this?
LogThreadOrder Queue[1024];
LogThreadOrder *FirstThreadOrder;
int NumberOfOrders;
struct LogThreadOrder {
char FileName[4096];
char LogMessage[4096];
LogThreadOrder* NextOrder;
};
void Log(const char* FileName, const char* LogMessage) {
LogMutex.lock();
ProtocolThreadOrder* Order = Queue[NumberOfOrders++];
Order->NextOrder = FirstThreadOrder;
FirstThreadOrder = Order;
LogMutex.release();
}
int LogThreadLoop() {
// Loop through Queue until its empty.
}
So, my question is:
Is this a smart design?
Is it thread-safe?
Is there any better way of accomplishing what I'm trying to do?
Would it be better/more-efficient to have multiple logger threads and multiple queues?
Your pseudocode could be made to work, but it's difficult in C and mutex synchronization is inefficient. In my experience it's always better to use existing, well-established, asynchronous inter-process communication mechanisms between threads like named pipes. Even then there are many pitfalls and when dependencies aren't a problem using something like ZeroMQ might be the best idea. Specifically something like this. I don't think having multiple threads is a good idea, as file you want to write to will be a contention point.
I need to write a Terminal to communicate with a COM - port and I need to be able to send commands from the COM-Port, as well as from the Console at the same time.
(I want to get access to a computer via two sensor nodes, that are communicating with each other wirelessly, so I still need a way to send something from the node to the Computer)
Now, I have implemented a Non Overlapped Serial Communication, but I am not sure, how to implement the "Send and Receive at the same time"-Part and I only have around 4 days to solve the problem. There is not really that much information out there, so I would welcome any pointers on how to implement that fastest or easiest way.
Overlapped I/O-Communication is not exactly very time friendly as far as I can see.
Is it possible to do this with multi-threading (only overlapped)?
I am guessing in that case I would have to read the buffer every few ms and make an own thread for the input?
Whether to use overlapped I/O or not isn't really the issue: overlapping just frees up some time for your program. I have written many programs like this and the conclusion is always to use a thread to handle all COM routines. Whether this thread calls overlapped or synchronous methods is less relevant, as long as the thread lies idle doing WaitForMultipleObjects().
The way I have written my most recent COM terminal is this (pseudo):
thread()
{
while not kill the thread event
{
WaitForMultipleObjects (open port, close port, kill the thread event)
if (open port)
{
send();
receive();
wait_for_send_and_receive();
}
}
}
send()
{
take COM_port mutex
if(there is something to send)
{
copy send_data to local_data, protect this with mutex
WriteFileEx(COM_port,
local_data,
size,
some_overlapped_struct_stuff);
handle errors
}
release COM_port mutex
}
receive()
{
take COM_port mutex
ReadFileEx(COM_port, ...);
handle errors
release COM_port mutex
}
wait_for_send_and_receive()
{
WaitForMultipleObjects (open port,
close port,
kill the thread event,
send done event from send callback routine (overlapped I/O),
receive done event from receive callback routine (overlapped I/O)
);
}
Naturally this is quite an over-simplification since you need various functions for open/close COM port, data shuffling etc. Several mutices are likely needed.
I'd share the real, working production code if it wasn't corporate property :( 4 days seems a bit optimistic, judging from my project log, it took me several months to develop a working COM port terminal to production quality level. The COM port driver alone is around 1k loc, with a lot of Win API calls all over.
I'm trying to implement a UDP-based server that maintains two sockets, one for controlling(ctrl_sock) and the other for data transmission(data_sock). The thing is, ctrl_sock is always uplink and data_sock is downlink. That is, clients will request data transmission/stop via the ctrl_sock and data will be sent to them via data_sock.
Now the problem is, since the model is connection-less, the server will have to maintain a list of registered clients' information( I call it peers_context) such that it can "blindly" push data to them until they ask to stop. During this blind transmission, clients may send controlling messages to the server via the ctrl_sock asynchronously. These information, besides initial Request and Stop, can also be, for example, preferences of file parts. Therefore, the peers_context has to be updated asynchronously. However, the transmission over the data_sock relies on this peers_context structure, hence raises a synchronization problem between ctrl_sock and data_sock. My question is, what can I do to safely maintain these two socks and the peers_context structure such that the asynchronous update of peers_context won't cause a havoc. By the way, the update of peers_context wouldn't be very frequent, that is why I need to avoid the request-reply model.
My initial consideration of the implementation is, to maintain ctrl_sock in the main thread(listener thread), and transmission over data_sock is maintained in the other thread(worker thread). However, I found it is difficult to synchronize in this case. For example, if I use mutexes in peers_context, whenever the worker thread locks peers_context, the listener thread wouldn't have access to it anymore when it needs to modify peers_context, because the worker thread works endlessly. On the other hand, if the listener thread holds the peers_context and writes to it, the worker thread would fail to read peers_context and terminates. Can anybody give me some suggestions?
By the way, the implementation is done in Linux environment in C. Only the listener thread would need to modify peers_context occasionally, the worker thread only needs to read. Thanks sincerely!
If there is strong contention for your peers_context then you need to need to shorten your critical sections. You talked about using a mutex. I assume you've already considered changing to a reader+writer lock and rejected it because you don't want the constant readers to starve a writer. How about this?
Make a very small structure that is an indirect reference to a peers_context like this:
struct peers_context_handle {
pthread_mutex_t ref_lock;
struct peers_context *the_actual_context;
pthread_mutex_t write_lock;
};
Packet senders (readers) and control request processors (writers) always access the peers_mutex through this indirection.
Assumption: the packet senders never modify the peers_context, nor do they ever free it.
Packer senders briefly lock the handle, obtain the current version of the peers_context and unlock it:
pthread_mutex_lock(&(handle->ref_lock));
peers_context = handle->the_actual_context;
pthread_mutex_unlock(&(handle->ref_lock));
(In practice, you can even do away with the lock if you introduce memory barriers, because a pointer dereference is atomic on all platforms that Linux supports, but I wouldn't recommend it since you would have to start delving into memory barriers and other low-level stuff, and neither C nor POSIX guarantees that it will work anyway.)
Request processors don't update the peers_context, they make a copy and completely replace it. That's how they keep their critical section small. They do use write_lock to serialize updates, but updates are infrequent so that's not a problem.
pthread_mutex_lock(&(handle->write_lock));
/* Short CS to get the old version */
pthread_mutex_lock(&(handle->ref_lock));
old_peers_context = handle->the_actual_context;
pthread_mutex_unlock(&(handle->ref_lock));
new_peers_context = allocate_new_structure();
*new_peers_context = *old_peers_context;
/* Now make the changes that are requested */
new_peers_context->foo = 42;
new_peers_context->bar = 52;
/* Short CS to replace the context */
pthread_mutex_lock(&(handle->ref_lock));
handle->the_actual_context = new_peers_context;
pthread_mutex_unlock(&(handle->ref_lock));
pthread_mutex_unlock(&(handle->write_lock));
magic(old_peers_context);
What's the catch? It's the magic in the last line of code. You have to free the old copy of the peers_context to avoid a memory leak but you can't do it because there might be packet senders still using that copy.
The solution is similar to RCU, as used inside the Linux kernel. You have to wait for all of the packet sender threads to have entered a quiescent state. I'm leaving the implementation of this as an exercise for you :-) but here are the guidelines:
The magic() function adds old_peers_context so a to-be-freed queue (which has to be protected by a mutex).
One dedicated thread frees this list in a loop:
It locks the to-be-freed list
It obtains a pointer to the list
It replaced the list with a new empty list
It unlocks the to-be-freed list
It clears a mark associated with each worker thread
It waits for all marks to be set again
It frees each item in its previously obtained copy of the to-be-freed list
Meanwhile, each worker thread sets its own mark at an idle point in its event loop (i.e. a point when it is not busy sending any packets or holding any peer_contexts.
I'm using libnetfilter_queue for userspace modification of
incoming/outgoing packets. I have been using a single threaded model.
But i've found out that from 2.6.31 kernel, its possible to have
different queues for different connection. so i was wandering if its
possible to manage each queues in different threads.
normally i set up the queue handling like bellow:
struct nfq_handle * h = nfq_open();
nfq_unbind_pf(h, AF_NET);
nfq_bind_pf(h,m AF_NET);
struct nfq_q_handle(h, 0, &cb, NULL);
nfq_set_mode(qh, NFQNL_COPY_PACKET, 0xffff);
now if i want to manage like 100 queues, i will pack h,qh and que_num in
a structure and loop over it to initialize.
now my question is :
if i initialize above in the main thread and want to run the callbacks
in separate threads, is it enough to run the even loop in a function which will
be given to pthread_create()? Will it run the callbacks in threads?
i'm not sure, but my understanding tells me, a packet is popped from the
queues when nfq_set_verdict is returned. so i need to run
nfq_set_verdict in separate threads so that i packets can be popped from
queues parallel.
EDIT: i provide my code if someone needs them to understand my problem. pasting all codes here seems unreasonable as it creates visual noise.
I know this is a bit of necroposting, but in case anyone stumbles on this problem.
If you use --queue-balance for multithreading and netfilter seems to put all packets in one queue, one should add --queue-cpu-fanout, which forces iptables to share packets based on cpuid instead of flow.