Re: [PATCH 0/4 v6] sched,time: reduce nohz_full syscall overhead 40%

[Thomas Gleixner]

On Wed, 10 Feb 2016, riel@xxxxxxxxxx wrote:

> (v6: make VIRT_CPU_ACCOUNTING_GEN jiffy granularity)
> 
> Running with nohz_full introduces a fair amount of overhead.
> Specifically, various things that are usually done from the
> timer interrupt are now done at syscall, irq, and guest
> entry and exit times.
> 
> However, some of the code that is called every single time
> has only ever worked at jiffy resolution. The code in
> __acct_update_integrals was also doing some unnecessary
> calculations.
> 
> Getting rid of the unnecessary calculations, without
> changing any of the functionality in __acct_update_integrals
> gets us about an 11% win.
> 
> Not calling the time statistics updating code more than
> once per jiffy, like is done on housekeeping CPUs and on
> all the CPUs of a non-nohz_full system, shaves off a
> further 30%.

Reviewed-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>