linux-kernel - Re: [PATCHv7 00/33] kernel: Introduce Time Namespace

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Date:   Thu, 17 Oct 2019 11:24:45 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Dmitry Safonov <dima@...sta.com>
cc:     linux-kernel@...r.kernel.org,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Adrian Reber <adrian@...as.de>,
        Andrei Vagin <avagin@...nvz.org>,
        Andy Lutomirski <luto@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Cyrill Gorcunov <gorcunov@...nvz.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Jann Horn <jannh@...gle.com>, Jeff Dike <jdike@...toit.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Pavel Emelyanov <xemul@...tuozzo.com>,
        Shuah Khan <shuah@...nel.org>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        containers@...ts.linux-foundation.org, criu@...nvz.org,
        linux-api@...r.kernel.org, x86@...nel.org,
        Andrei Vagin <avagin@...il.com>
Subject: Re: [PATCHv7 00/33] kernel: Introduce Time Namespace

On Fri, 11 Oct 2019, Dmitry Safonov wrote:
> We wrote two small benchmarks. The first one gettime_perf.c calls
> clock_gettime() in a loop for 3 seconds. It shows us performance with
> a hot CPU cache (more clock_gettime() cycles - the better):
> 
>         | before    | CONFIG_TIME_NS=n | host      | inside timens
> --------------------------------------------------------------
>         | 153242367 | 153567617        | 150933203 | 139310914
>         | 153324800 | 153115132        | 150919828 | 139299761
>         | 153125401 | 153686868        | 150930471 | 139273917
>         | 153399355 | 153694866        | 151083410 | 139286081
>         | 153489417 | 153739716        | 150997262 | 139146403
>         | 153494270 | 153724332        | 150035651 | 138835612
> -----------------------------------------------------------
> avg     | 153345935 | 153588088        | 150816637 | 139192114
> diff %  | 100       | 100.1            | 98.3      | 90.7


That host 98.3% number is weird and does not match the tests I did with the
fallback code I provided you. On my limited testing that fallback hidden in
the slowpath did not show any difference to the TIME_NS=n case when not
inside a time namespace.

Thanks,

	tglx