Use of pool servers reveals unacceptable crash rate in async DNS
Eric S. Raymond
esr at thyrsus.com
Mon Aug 29 19:03:40 UTC 2016
Processing old mail...
Hal Murray <hmurray at megapathdsl.net>:
> > I believe you're right that these platforms don't have it. The question is,
> > how important is that fact? Is the performance hit from synchronous DNS
> > really a showstopper? I don't know the answer.
> There are two cases I know of where ntpd does a DNS lookup after it gets
> One is the try again when DNS for the normal server case doesn't work during
> initialization. It will try again occasionally until it gets an answer.
> (which might be negative)
> The main one is the pool code trying for a new server. I think we should be
> extending this rather than dropping it. There are several possibles in this
> area. The main one would be to verify that a server you are using is still
> in the pool. (There isn't a way to do that yet - the pool doesn't have any
> DNS support for that.) The other would be to try replacing the poorest
> server rather than only replacing dead servers.
> DNS lookups can take a LONG time. I think I've seen 40 seconds on a failing
> If we get the recv time stamp from the OS, I think the DNS delays won't
> introduce any lies on the normal path. We could test that by putting a sleep
> in the main loop. (There is a filter to reject packets that take too long,
> but I think that's time-in-flight and excludes time sitting on the server.)
> There are two cases I can think of where a pause in ntpd would cause
> troubles. One is that it would mess up refclocks. The other is that packets
> will get dropped if too many of them arrive.
> I think that means we could use the pool command on a system without
> refclocks. That covers end nodes and maybe lightly loaded servers.
> It's worth checking out the input buffering side of things. There may be
> some code there that we don't need. I think there is a pool of buffers.
> Where can a buffer sit other than on the free queue. Why do we need a pool?
The project has more important priorities than chasing this down. But: I have
edited this text, adding a few details I have learned since, into a new
section for the internals tour (devel/tour.txt). That will give somebody
a better-than-nothing place to start if we ever again try something like
the cAres replacement.
<a href="http://www.catb.org/~esr/">Eric S. Raymond</a>
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