On 2/13/26 3:42 PM, Hannu Krosing wrote:
I haven't looked at the code here yet, but when using plain rdtsc on
modern CPUs one sees much more overhead from just the fact that the
code is there than from calling the rdtsc instruction, and the
overhead can vary by orders of magnitude based on how complex the work
is that is timed.
I discovered this when I timed the (then-)new dead tid lookups in the
Vacuum in Pg 17 and saw significantly larger overhead per lookup when
the lookups themselves were slower, i.e. a case where the lookups were
done in random order (inded was on created on a column filled with
random())
So while just a tight loop of N million rtdsc calls will give you the
lower limit, it is likely not very representative of actual overhead.
Isn't the same issue still there if you call clock_gettime() but that it
is just less noticeable due to the high cost of clock_gettime()?
Andreas
