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authorDavid Herrmann <dh.herrmann@gmail.com>2014-07-10 00:47:23 +0200
committerDavid Herrmann <dh.herrmann@gmail.com>2014-07-11 16:43:53 +0200
commitb63c8d4f0364457b0ead8793504012bb7113974f (patch)
treefc5ec3e4181b72d7f9bee68d953679d2858ce024 /catalog
parent1ca5fd003f5d3ffb32db581cb8d82d0721b5b12d (diff)
sd-event: always call epoll_ctl() on mask-updates if edge-triggered
A call to sd_event_source_set_io_events() skipps calling into the kernel if the new event-mask matches the old one. This is safe for level-triggered sources as the kernel moves them onto the ready-list automatically if events change. However, edge-triggered sources might not be on the ready-list even though events are present. A call to sd_event_source_set_io_events() with EPOLLET set might thus be used to just move the io-source onto the ready-list so the next poll will return it again. This is very useful to avoid starvation in priority-based event queues. Imagine a read() loop on an edge-triggered fd. If we cannot read data fast enough to drain the receive queue, we might decide to skip reading for now and schedule it for later. On edge-triggered io-sources we have to make sure it's put on the ready-list so the next dispatch-round will return it again if it's still the highest priority task. We could make sd-event handle edge-triggered sources directly and allow marking them ready again. However, it's much simpler to let the kernel do that for now via EPOLL_CTL_MOD.
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