Re: Differences in cpu utilization reported by sar, emon

[Solio Sarabia]

Thanks Andi, Stephen, for your help/insights.

TICK_CPU_ACCOUNTING (default option) does not account for cpu util on
cores handling irqs and softriqs.

IRQ_TIME_ACCOUNTING or VIRT_CPU_ACCOUTING_GEN helps to reduce the util
gap. With either option, there is still a difference, for example, up to
8% in terms of sar/emon ratio (sar shows lesser util). This is an
improvement to the default case though.


This is a brief description of the Kbuild options:

-> General setup
  -> CPU/Task time and stats accounting
    -> Cputime accounting
TICK_CPU_ACCOUNTING
    Simple/basic tick based cpu accounting--maintains statistics about
    user, system and idle time spent on per jiffies granularity.
VIRT_CPU_ACCOUNTING_NATIVE (not available on my kernel)
    Deterministic task and cpu time accounting--more accurate task
    and cpu time accounting. Kernel reads a cpu counter on each kernel
    entry and exit, and on transitions within the kernel between
    system, softirq, and hardirq state, so there is a small performance
    impact.
VIRT_CPU_ACCOUTING_GEN
    Full dynticks cpu time accounting--enable task and cpu time
    accounting on full dynticks systems. Kernel watches every
    kernel-user boundaries using the context tracking subsystem.
    There is significant overhead. For now only useful if you are
    working on the full dynticks subsystem development.
IRQ_TIME_ACCOUNTING
    Fine granularity task level irq time accounting--kernel reads a
    timestamp on each transition between softirq and hardirq state,
    so there can be a performance impact.

-Solio


On Thu, Jun 14, 2018 at 08:41:33PM -0700, Solio Sarabia wrote:
> Hello --
> 
> I'm running into an issue where sar, mpstat, top, and other tools show
> less cpu utilization compared to emon [1]. Sar uses /proc/stat as its
> source, and was configured to collect in 1s intervals. Emon reads
> hardware counter MSRs in the PMU in timer intervals, 0.1s for this
> scenario.
> 
> The platform is based on Xeon E5-2699 v3 (Haswell) 2.3GHz, 2_sockets,
> 18_cores/socket, 36_cores in total, running Ubuntu 16.04, Linux
> 4.4.0-128-generic. A network micro workload, ntttcp-for-linux [2],
> sends packets from client to server, through a 40GbE direct link.
> Numbers below are from server side.
> 
>                  total %util
>            CPU11    CPU21    CPU22    CPU25
> emon       99.99    15.90    36.22    36.82
> sar        99.99     0.06     0.36     0.35
> 
>                  interrupts/sec
>            CPU11    CPU21    CPU22    CPU25
> intrs/sec    846    28923    12844     6304
>     Contributors to /proc/interrupts:
>     CPU11: Local timer interrupts and Rescheduling interrupts
>     CPU21-CPU25: PCI MSI vector from network driver
> 
>                  softirqs/sec
>            CPU11    CPU21    CPU22    CPU25
> TIMER        198        1        2        1
> NET_RX         1    28889    23553    18546
> TASKLET        0    28889    11676     6249
> 
> 
> Somehow hardware irqs and softirqs do not have an effect on the core's
> utilization. Another observation is that as more cores are used to
> process packets, the emon/sar gap increases.
> 
> Kernels used default HZ=250. I also tried HZ=1000, which helped improve
> throughput, but difference in util is still there. Same for newer
> kernels 4.13, 4.15. I would appreciate pointers to debug this, or
> insights as what could cause this behavior.
> 
> [1] https://software.intel.com/en-us/download/emon-users-guide
> [2] https://github.com/simonxiaoss/ntttcp-for-linux
> 
> Thanks,
> -Solio