On Wed, Jul 23, 2014 at 02:36:24AM +0000, Dylan Davis wrote: > Interestingly, the reported delay under > load is 12us less than when unloaded.
Hm.. > These results are unintuitive, and I can't come up with a solid explanation > for > them. Assuming the reported offsets are accurate (which may be a poor > assumption), my best guess is that somehow the 'stress' workload is forcing > ptp4l to be scheduled more consistently, but that's only speculation. Can > anybody shed some light on what's happening here? The effects that you see are indeed unintuitive, and I have also seen improved timing behavior under high load. There are at least three different ways that a load might have this effect. 1. Cache Effects. The high load might cause more cache misses in the time stamping path in the driver, resulting in a longer duration between the packet arrival time and the SW time stamp, but with more consistent timing. 2. Driver and networking stack. Depending on how your MAC hardware works and how the driver is written, there can be long and variable delays between the time stamp point and the actual transmission or reception time. The load can cause more consistent run time behavior, but the details of how this happens can be hard to understand. 3. NTP. Due to bugs in the kernel's NTP servo, high load can improve the clock regulation. I have seen this with nohz=on, but there might be other problems in that code as well. This is an area in the kernel that is still wanting improvement. The 12 us smaller delay is a sign that the load is making the driver or the stack time stamps more consistent. Thanks, Richard ------------------------------------------------------------------------------ Want fast and easy access to all the code in your enterprise? Index and search up to 200,000 lines of code with a free copy of Black Duck Code Sight - the same software that powers the world's largest code search on Ohloh, the Black Duck Open Hub! Try it now. http://p.sf.net/sfu/bds _______________________________________________ Linuxptp-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/linuxptp-users
