libwebsockets/READMEs/README.lws_sul.md

lws_sul scheduler api

Since v3.2 lws no longer requires periodic checking for timeouts and other events. A new system was refactored in where future events are scheduled on to a single, unified, sorted linked-list in time order, with everything at us resolution.

This makes it very cheap to know when the next scheduled event is coming and restrict the poll wait to match, or for event libraries set a timer to wake at the earliest event when returning to the event loop.

Everything that was checked periodically was converted to use lws_sul and schedule its own later event. The end result is when lws is idle, it will stay asleep in the poll wait until a network event or the next scheduled lws_sul event happens, which is optimal for power.

Side effect for older code

If your older code uses lws_service_fd(), it used to be necessary to call this with a NULL pollfd periodically to indicate you wanted to let the background checks happen. lws_sul eliminates the whole concept of periodic checking and NULL is no longer a valid pollfd value for this and related apis.

Using lws_sul in user code

See minimal-http-client-multi for an example of using the lws_sul scheduler from your own code; it uses it to spread out connection attempts so they are staggered in time. You must create an lws_sorted_usec_list_t object somewhere, eg, in you own existing object.

static lws_sorted_usec_list_t sul_stagger;

Create your own callback for the event... the argument points to the sul object used when the callback was scheduled. You can use pointer arithmetic to translate that to your own struct (when the lws_sorted_usec_list_t was a member of the some struct) by using lws_container_of(sul, container_struct_type, field_name).

typedef struct my_connection_data {
	...
	lws_sorted_usec_list_t sul_stagger;
	...
} my_connection_data_t;

static void
stagger_cb(lws_sorted_usec_list_t *sul)
{
	my_connection_data_t* my_data = lws_container_of(sul, my_connection_data_t, sul_stagger);
	...
}

Important note: make sure, that lws_sorted_usec_list_t data initiallized by zeros (memset(&sul_stagger, 0, sizeof(lws_sorted_usec_list_t)). This struct contains pointers, so them must initially pointing to NULL!

When you want to schedule the callback, use lws_sul_schedule()... this will call it 10ms in the future

	lws_sul_schedule(context, 0, &sul_stagger, stagger_cb, 10 * LWS_US_PER_MS);

In the case you destroy your object and need to cancel the scheduled callback, use

	lws_sul_schedule(context, 0, &sul_stagger, NULL, LWS_SET_TIMER_USEC_CANCEL);

lws_sul2 and system suspend

In v4.1, alongside the existing lws_sul apis there is a refactor and additional functionality aimed at negotiating system suspend, while remaining completely backwards-compatible with v3.2+ lws_sul apis.

Devicewide suspend is basically the withdrawal of CPU availability for an unbounded amount of time, so what may have been scheduled by the user code may miss its time slot because the cpu was down and nothing is getting serviced. Whether that is actively desirable, OK, a big disaster, or a failure that will be corrected at other layers at the cost of, eg, some additional latency, depends on the required device behaviours and the function of the user code that was scheduled, and its meaning to the system.

Before v4.1, lws just offers the same scheduling service for everything both internal and arranged by user code, and has no way to know what is critical for the device to operate as intended, and so must force wake from suspend, or if for that scheduled event 'failure [to get the event] is an option'.

For example locally-initiated periodic keepalive pings not happening may allow persistently dead (ie, no longer passing data) connections to remain unrenewed, but eventually when suspend ends for another reason, the locally-initiated PING probes will resume and it will be discovered and if the connectivity allows, corrected.

If the device's function can handle the latency of there being no connectivity in suspend under those conditions until it wakes for another reason, it's OK for these kind of timeouts to be suppressed during suspend and basically take the power saving instead. If for a particular device it's intolerable to ever have a silently dead connection for more than a very short time compared to suspend durations, then these kind of timeouts must have the priority to wake the whole device from suspend so they continue to operate unimpeded.

That is just one example, lws offers generic scheduler services the user code can exploit for any purpose, including mission-critical ones. The changes give the user code a way to tell lws if a particular scheduled event is important enough to the system operation to wake the system from devicewide suspend.