Date: Mon, 09 Sep 2019 12:21:38 -0700 From: Cy Schubert <Cy.Schubert@cschubert.com> To: Ian Lepore <ian@freebsd.org> Cc: Cy Schubert <Cy.Schubert@cschubert.com>, Konstantin Belousov <kostikbel@gmail.com>, Harlan Stenn <stenn@nwtime.org>, Vladimir Zakharov <zakharov.vv@gmail.com>, freebsd-current@freebsd.org Subject: Re: ntpd segfaults on start Message-ID: <201909091921.x89JLcBv051522@slippy.cwsent.com> In-Reply-To: <66f80012757134b6317b673f9eeb24db66c996a2.camel@freebsd.org> References: <201909051307.x85D7MGs034053@slippy.cwsent.com> <20190905142817.GB2559@kib.kiev.ua> <201909060355.x863tRhP089169@slippy.cwsent.com> <201909060639.x866dJ7f090176@slippy.cwsent.com> <201909062356.x86NuKdk003780@slippy.cwsent.com> <156d1e7c-0dbb-8707-90b3-13ae97c87449@nwtime.org> <20190907075619.GG2559@kib.kiev.ua> <201909071309.x87D9GxZ089964@slippy.cwsent.com> <20190907153226.GI2559@kib.kiev.ua> <201909071545.x87FjLS6004448@slippy.cwsent.com> <20190907161749.GJ2559@kib.kiev.ua> <201909071628.x87GSFPV005827@slippy.cwsent.com> <66f80012757134b6317b673f9eeb24db66c996a2.camel@freebsd.org>
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In message <66f80012757134b6317b673f9eeb24db66c996a2.camel@freebsd.org>, Ian Le pore writes: > On Sat, 2019-09-07 at 09:28 -0700, Cy Schubert wrote: > > In message <20190907161749.GJ2559@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 want > s. > > > > 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. Our upstream is already cc'd on this thread. diff --git a/usr.sbin/ntp/config.h b/usr.sbin/ntp/config.h index 56dbfedba6e..b26dd417885 100644 --- a/usr.sbin/ntp/config.h +++ b/usr.sbin/ntp/config.h @@ -287,7 +287,7 @@ #define DEFAULT_HZ 100 /* Default number of megabytes for RLIMIT_MEMLOCK */ -#define DFLT_RLIMIT_MEMLOCK 32 +#define DFLT_RLIMIT_MEMLOCK -1 /* Default number of 4k pages for RLIMIT_STACK */ #define DFLT_RLIMIT_STACK 50 But I'd wait for Harlan to weigh in on this. I agree with kib@ that this may introduce unwanted jitter. I'd also want to understand why this defaults to -1 for Linux only. > > I would propose that for freebsd, the autoconf-generated value for For the port but in base we control this directly. > 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>. I haven't had a chance to look at this in depth yet but I suspect that <something> isn't in fact 32 as specified in config.h. It behaves as if this is set to 0 to mlockall() all it asks for. > > -- Ian -- Cheers, Cy Schubert <Cy.Schubert@cschubert.com> FreeBSD UNIX: <cy@FreeBSD.org> Web: http://www.FreeBSD.org The need of the many outweighs the greed of the few.
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