Thanks for your insights. I even wonder if the change of the daylight time saving could not trigger that bug I'm thinking about. Especially if the server time is in local time .... That's an horrible vision :( I'll work on that next week, I have to complete some work before switching to that. Cheers, ----- Mail original ----- De: "Sage Weil" <sweil@xxxxxxxxxx> À: "Erwan Velu" <evelu@xxxxxxxxxx> Cc: "Adam C. Emerson" <aemerson@xxxxxxxxxx>, "The Sacred Order of the Squid Cybernetic" <ceph-devel@xxxxxxxxxxxxxxx> Envoyé: Mercredi 13 Janvier 2016 14:48:00 Objet: Re: About ceph_clock_now() On Wed, 13 Jan 2016, Erwan Velu wrote: > >One concern with monotonic clocks, even for elapsed time, is that their epoch is > >unspecified. You cannot send a monotonic time over the network to another > >machine nor can you save it to a file and re-read it later. Anything to be > >serialized must be a real time. > > While speaking about ceph_clock_now(), I was more likely targeting lots of local code like > start = ceph_clock_now(); > ... do_stuff ... > elapse_time = ceph_clock_now() - start > > > I'm thinking of code like this in void OSD::tick_without_osd_lock(): > Starting at https://github.com/ceph/ceph/blob/master/src/osd/OSD.cc#L4009. > > This code is trying to estimate if a mon is still alive by computing a difference of system time by > doing a "if (now - last_pg_stats_sent > max) {" where now & last_pg_stats_sent are the output of "ceph_clock_now()" > > As ceph_clock_now() is using CLOCK_REALTIME, we could have cases where a simple drift caused by a NTP trigger that code. > As we are only trying to compute a difference of absolute time (and not system time), using a MONOTONIC_* seems much more accurate at my taste. > > I know that's pretty picky but that could exists. That's a perfect example of where the monotonic time should be used. There are a zillion instances of this pattern that could be cleaned up... :) > >I find it interesting. If you want to change more subsystems to use ceph_time > >and use monotonic and coarse clocks in appropriate places I think that would be > >wonderful. It might be worthwhile to focus on the places where we use double to > >represent a time value. > >Please read through ceph_time.h and osdc to get a feel for it. If you think it > >could use improvement, I'd be happy to hear your ideas. > > You mean we could switch some of ceph_clock_now() calls to ceph_time ? > We could use coarse_mono_clock to perform that. > > If you agree that switching code like the one I'm speaking is valuable, > I can work on it. That would be great! sage -- To unsubscribe from this list: send the line "unsubscribe ceph-devel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
