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As far as I understand, the wa % stat displayed by top is the I/O wait (i.e. hard drive).

Is there a way to get similar wait stats, but for when the CPU is waiting for memory?

My server is pretty much always 0.0 wa, because I've got an NVMe SSD, and a large RAM. But I would be interested in finding out how much CPU time is spent on waiting for the RAM (and measure the impact of RAM speed on my workload).

Is there any way to find this out?

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To get a precise number, you need to tap into CPU's performance counter. You can use oprofile and/or perf to get an inside view of how your CPU/memory/application combo performs.

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  • Thanks, so this is not available system-wise, I guess because of the overhead it would add?
    – BenMorel
    Nov 29, 2017 at 0:50
  • It is available system-wide, via an appropriate profiling utility as the two suggested above. However, as these performance counters are CPU and platform specific, no "standard" Unix tool exists to accurately measure them (note: top and other standard utilities can give approximate and useful results via minor page faults count)
    – shodanshok
    Nov 29, 2017 at 11:15

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