Re: ntpd segfaults on start

On Sat, 2019-09-07 at 09:28 -0700, Cy Schubert wrote:
> In message <20190907161749.GJ2559_at_kib.kiev.ua>, Konstantin Belousov writes:
> > On Sat, Sep 07, 2019 at 08:45:21AM -0700, Cy Schubert wrote:
> > > I've been able to set the memlock rlimit as low as 20 MB. The issue is 
> > > letting it default to 0 which allows ntp to mlockall() anything it wants. 
> > > ntpd on my sandbox is currently using 18 MB.
> > 
> > Default stack size on amd64 is 512M, and default stack gap percentage is
> > 3%. This means that the gap can be as large as ~17MB. If 3MB is enough
> > for the stack of the main thread of ntpd, then fine.
> 
> The default stack is 200K, which is also tuneable in ntp.conf.
> 
> [...]

I haven't seen anyone ask what I consider to be the crucial question
yet:  why are we locking ntpd into memory by default at all?

I have seen two rationales for ntpd using mlockall() and setrlimit(): 

   - There are claims that it improves timing performance.

   - Because ntpd is a daemon that can run for months at a time,
   setting limits on memory and stack growth can help detect and
   mitigate against memory leak problems in the daemon.

I am *highly* skeptical of claims that locking ntpd in memory will
improve timing performance on freebsd (to the point where I'm inclined
to dismiss them out of hand, but I'd be willing to look at any actual
evidence).

The second point has at least some validity, but would be better
implemented (IMO) by decoupling the address space limit from the act of
locking down memory, and allowing configuration of RLIMIT_AS separately
from RLIMIT_MEMLOCK.  If there's any interest in this, I could try to
put together a patchset and submit it upstream for that.

I would propose that for freebsd, the autoconf-generated value for
DFLT_RLIMIT_MEMLOCK should be -1 to avoid calling setrlimit() and
mlockall() by default.  Then in the ntp.conf we distribute we have a
section like:

   # To lock ntpd into memory, uncomment the following rlimit line.
   # This should not be necessary on most systems, but may improve
   # performance on heavily-loaded servers experiencing enough
   # memory pressure to cause ntpd to be paged out.
   # rlimit memlock <something> stacksize <something>

Then we would need to come up with reasonable values for <something>.

-- Ian