I don't remember the exact numbers (this was 2011), but the overhead of using e.g. CLOCK_MONOTONIC were substantial. Under Xen, the cost of reading CLOCK_MONOTO

I don't remember the exact numbers (this was 2011), but the overhead of using e.g. CLOCK_MONOTONIC were substantial. Under Xen, the cost of reading CLOCK_MONOTONIC was a few orders of magnitude higher than reading the TSC. I think on Xen PV it was like 500ns per read, while on HVM it was at about 2000ns-3000ns or something like that. I remember with 8 disks that should have been able to do 60K 4K IOPS each (early SSD models), we were capping out at 90K IOPS with all disks in parallel at a queue depth of 32 while reading from CLOCK_MONOTONIC. When we switched to TSC I think we ended up getting around 320K IOPS. Still not perfect, but we were also capped by the particular HBA we chose (which didn't have multiqueue support).