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
