linux-kernel - Re: [PATCH 08/10] psi: pressure stall information for CPU, memory, and IO

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Message-ID: <CA+55aFw7t++BzEy-XsatNcauw3Wn22SSXfd3iTYECi4fJ97CCg@mail.gmail.com>
Date:   Thu, 19 Jul 2018 08:08:20 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Ingo Molnar <mingo@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Tejun Heo <tj@...nel.org>, surenb@...gle.com,
        Vinayak Menon <vinmenon@...eaurora.org>,
        Christoph Lameter <cl@...ux.com>,
        Mike Galbraith <efault@....de>, shakeelb@...gle.com,
        linux-mm <linux-mm@...ck.org>, cgroups <cgroups@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        kernel-team <kernel-team@...com>
Subject: Re: [PATCH 08/10] psi: pressure stall information for CPU, memory,
 and IO

On Wed, Jul 18, 2018 at 5:03 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> And as said before, we can compress the state from 12 bytes, to 6 bits
> (or 1 byte), giving another 11 bytes for 59 bytes free.
>
> Leaving us just 5 bytes short of needing a single cacheline :/

Do you actually need 64 bits for the times?

That's the big cost. And it seems ridiculous, if you actually care about size.

You already have a 64-bit start time. Everything else is some
cumulative relative time. Do those really need 64-bit and nanosecond
resolution?

Maybe a 32-bit microsecond would be ok - would you ever account more
than 35 minutes of anything without starting anew?

             Linus