On Wed, Oct 30, 2013 at 12:29:18PM -0700, Dave Taht wrote: > > - the ping6 result is surprising, as two groups of points clearly > > appear, separated by about 200 µs. It seems that interrupts might > > be responsible for this, but it's still somewhat unclear. > > It could be caused by napi. There is also some parameter to ethtool > (?) to tell it to return pings more immediately (that I recall was > specific to the e1000e network card). I'm pretty sure it was the > --coalesce some_option
Ok, I'll give it a try. > You might find it easier to verify your measurement with owamp and gps > synced clocks. > > I'm delighted to see this measurement as collected by babel as my > sekret plan was to be able to measure heavy traffic benchmarks vs > various qdiscs like the new "fq" and older fq_codel ones... The new code should be finalised and merged in babeld by the end of the week. > > - Babel sees a RTT that is 400 µs higher than ping6. The babel-rtt > > implementation timestamps outgoing message as late as possible, and > > timestamps incoming messages as early as possible, but it's not > > perfect. > > Did you try hardware timestamping on rx? (this is a kernel compile option) Nope, nor kernel timestamping. This has low priority, I'd like to finalise and merge the current version first. > > Context-switching and syscalls can probably account for > > these 400 µs: a quick test, on the slower laptop, showed that > > calling the gettimeofday() syscall takes about 100 µs. Err, I mixed figures up. My basic measurement (running gettimeofday() many times in a loop) indicates each gettimeofday() takes 100 ns, not 100 µs. > Kernel version? This was sped up in some fairly recent kernel version (?) Here are the actual results on various hardware: http://paste.debian.net/63080/ The code used is there: http://paste.debian.net/63079/ There doesn't seem to be any big difference between the tested kernel version. However, the measured system time is 0 in all cases, except with the VM. There may be some optimisations preventing the actual syscall to happen.
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