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From:   Suren Baghdasaryan <surenb@google.com>
Date:   Fri, 10 Feb 2023 15:03:00 -0800
Message-ID: <CAJuCfpHWQ8NV=iR3BN+pt1c8FynCnRqyyriHb1gLxFgiNVrwjA@mail.gmail.com>
Subject: Re: [PATCH] psi: reduce min window size to 50ms
To:     Sudarshan Rajagopalan <quic_sudaraja@quicinc.com>
Cc:     David Hildenbrand <david@redhat.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Mike Rapoport <rppt@kernel.org>,
        Oscar Salvador <osalvador@suse.de>,
        Anshuman Khandual <anshuman.khandual@arm.com>,
        mark.rutland@arm.com, will@kernel.org,
        virtualization@lists.linux-foundation.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        linux-arm-msm@vger.kernel.org,
        Trilok Soni <quic_tsoni@quicinc.com>,
        Sukadev Bhattiprolu <quic_sukadev@quicinc.com>,
        Srivatsa Vaddagiri <quic_svaddagi@quicinc.com>,
        Patrick Daly <quic_pdaly@quicinc.com>
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On Fri, Feb 10, 2023 at 2:31 PM Sudarshan Rajagopalan
<quic_sudaraja@quicinc.com> wrote:
>
> The PSI mechanism is useful tool to monitor pressure stall
> information in the system. Currently, the minimum window size
> is set to 500ms. May we know what is the rationale for this?

The limit was set to avoid regressions in performance and power
consumption if the window is set too small and the system ends up
polling too frequently. That said, the limit was chosen based on
results of specific experiments which might not represent all
usecases. If you want to change this limit, you would need to describe
why the new limit is inherently better than the current one (why not
higher, why not lower).
Thanks,
Suren.

>
> For lightweight systems such as Linux Embedded Systems, PSI
> can be used to monitor and track memory pressure building up
> in the system and respond quickly to such memory demands.
> Example, the Linux Embedded Systems could be a secondary VM
> system which requests for memory from Primary host. With 500ms
> window size, the sampling period is 50ms (one-tenth of windwo
> size). So the minimum amount of time the process needs to stall,
> so that a PSI event can be generated and actions can be done
> is 50ms. This reaction time can be much reduced by reducing the
> sampling time (by reducing window size), so that responses to
> such memory pressures in system can be serviced much quicker.
>
> Please let us know your thoughts on reducing window size to 50ms.
>
> Sudarshan Rajagopalan (1):
>   psi: reduce min window size to 50ms
>
>  kernel/sched/psi.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> --
> 2.7.4
>