Date: Wed, 7 Oct 2015 16:02:39 -0500 From: Eric van Gyzen <vangyzen@FreeBSD.org> To: freebsd-arch@freebsd.org Subject: RFC: Automatically Reloading /etc/resolv.conf Message-ID: <5615886F.3060601@FreeBSD.org>
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I would like to change the libc resolver to automatically reload /etc/resolv.conf when the latter changes. I would like to hear opinions about the implementation. Broadly, I see two approaches. == "stat" == When loading the file, record the mod time. Before each query, stat() the file to see if it has changed. Advantage: It uses no extra persistently allocated objects. Disadvantage: It incurs a stat() on every query. I don't see this as a major disadvantage, since the resolver already does a lot of work on every query. (For example, it creates and destroys a kqueue and a socket.) OpenBSD uses this approach. It also uses clock_gettime(CLOCK_MONOTONIC) to rate-limit the stat() calls to one per several seconds. == "kqueue" == When loading the file, open a kqueue and register for the appropriate events. Before each query, check for kevents. Advantage: The per-query overhead is fairly small. Disadvantage: This would persistently allocate an open file and a kqueue for every thread that ever uses the resolver, for the life of the thread. This seems fairly expensive. NetBSD uses this approach. It mitigates most of the space-cost by using a shared pool of res_state objects, instead of one per thread [that uses the resolver]. On each query, a thread allocates/borrows a res_state from the pool, uses it, and returns it. So, the number of objects is only the high water mark of the number of threads _concurrently_ issuing resolver queries. There are probably several variations on each theme, of course. I would appreciate your thoughts on these approaches and others I missed, as well as variations and details. FYI, I'm leaning toward the "stat" approach. Cheers, Eric
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