- background
- overview of precison time protocol PTP
about ptp on linux
Event ordering is very important
- for incident analysis, performance analysis and so on
- Event ordering is based on timestamps
- Timestamps are collected from multiple servers
- If prescisions and accuracy of clock synchronization are bad, event ordering can reverse against actual time.
NTP is not enough
NTP provides millisecond level synchronization
- Maybe enough for remote machine,s, but not enough for locally cooperating machines
Precision Time Protocol(PTP)
- standardized protocol IEEE1588
この時間同期技術の輪郭をつかむ：期待を背負うIEEE 1588-2008 (1/2) - EDN Japan
Term explanation
- grandmaster clock(ordinary clock) original time source for the ptp network
- boundary clock typically it's switch
slave clock(ordinary clock) synchronize its clock to a master
time offset between master and slave clocks is calculated based on timestamps at packet sending and receiving
- packet timestamp accuracy important for PTP
Timestamp timing
ideally, we want timestamps of the time just sending (or receiving) packet
Type of timestamping
- software timestamping (Application or OS layer)
- hardware timestamping (hardware assisted timestamp at PHY or MAC layer)
minimize error
Linux kernel assists PTP
The protocol itself is implemented on user-land
kernel features for PTP Socket option SO_TIMESTAMPING
The Linux PTP Project
Richard Cochran
Linuxptp Applications
ptp4l(Implementation of PTP)
Ordinary / Boundary colock
phc2sys (Synchronize two clocks (typically PHC and system clock)
How about software timestamping
When you are using Software timestamping ptp4l directly
Typical usage of ptp4l
- start as a slave node
- Use eth0
observe synchronization of ptp4l
- Log is handed over to syslog
- or, you can print if into stdout by using -m option
Typical usage of phc2sys
# phc2sys -s eth0 -c CLOCK_ReALTIME -w
pmc (PTP Management Client)
- send ptp management messages oto PTP nodes
- ptp management messages are specified in IEEE1588
- Many PTP devices have not supported management messages yet.
Typical usage of pmc
- send a message to localhost's node
- Get Values of CURRENT_DATA_SET
# pmc -u -b 0 'GET CURRENT_DATA_SET'
what's happening
- GM sends a timestamp including huge error
- GM's next timestamp does not include so much error
- The offsets between PHC and system clock observed by phc2sys
ptp4l has servo mechanism
ptp4l has PI(proportional-integral) controller servo
tuning sensitivity
/etc/ptp4l.conf in fedora
two diffrent configurations
improve system clock stability
- Dynamic ticks make system clock stability worse
- you can disable dynamic tics
Attempt to fix the issue on upstream
Miroslav Lichvar and John Stultz are working to fix the issue
FYI Running linuxptp on qemu-kvm
- Run two virtual machines for GM and Slave and use recent Fedora, RHEL 6.5 7.0
- Use virtual NIC emulationg e10000 (supports software timestamping)
- don't forget to define appropriate firewall rules to allow multi-cast
- dont' expect high precision and accuracy
