Hi Mike,
Le 20 juin 2019 à 15:24, Tobias Gierke <tobias.gie...@voipfuture.com> a écrit :
Hi everyone,
I'm facing a NTP-relateded challenge at work and hope that somebody on this
list has maybe solved a similar problem already:
- Our product is a system consisting of multiple components that may or may not
run on the same host and all parts require time synchronization at all times
because we're processing timestamped measurement data
- Our application does a lot of heavy computation triggered at the start of
every minute (by crond)
- Since a lot of different application versions are deployed in the field and
customers are slow to upgrade, we need to test many different versions of our
application in our lab
- To ease hardware requirements for testing we run all those different versions
of our application inside virtual machines, currently all synced to the same
NTP server
- Since all those VMs are synchronized to the same server, all cron jobs on
those VMs kick in at the same time, overloading the VM host periodically
I have also hit a similar issue, so I just modified the cron jobs to drop in a
sleep command before running the compute command. Problem fixed. Since you are
happy with offsetting your clocks, offsetting your post-processing startup
shouldn't hurt.
Unfortunately this is not possible since the bulk of the workload is
generated through ~150 SQL stored procedures triggered by pg_cron (a
PostgreSQL extension running inside PostgreSQL) so we'd need to have
configurable pg_sleep() calls in all those stored procedures.
Obviously we could introduce this offset as an application feature but
then we'd still have to deal with the associated measurement/capturing
systems (different VMs) that need to run with the same time as the
database VMs. So not really feasible I'm afraid.
Cheers,
Tobias
I already looked into a simple solution (like being able to pass some magic
"--offset XX seconds" option to ntpd) but it seems there is no such thing.
So my current idea is to
1.) have 4 different time servers that each are offset from all the others by
15 seconds (so server #1 would have offset 00 seconds, server #2 would have
offset 15 seconds, etc.)
2.) configure those servers with ULC w/ PPS and hook-up the same PPS to all 4
servers OR have one PPS source for each server and have all of them sync to a
common source (like a DCF77 time signal)
My problems are with the second step: I don't have a PPS source, I only have a Meinberg DCF600 USB
(https://www.meinbergglobal.com/english/products/usb-dcf77-clock.htm), but I couldn't find any
documentation on how to use it as a PPS source only (and ignore the actual "time" part
of the synchronization). Furthermore I don't really want to buy 3 more of those clocks if I can
help it (especially considering the fact that we might need even more different "time
domains" as the number of VMs increases).
Any ideas?
Thanks in advance,
Tobias
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Tobias Gierke
Software Developer
Voipfuture GmbH Wendenstr. 4 20097 Hamburg Germany
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