linux-kernel - Re: [RFC][PATCH 2/5] [PATCH 2/5] proc: introduce /proc/PID/idle

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Message-ID: <71597e5c-8a7c-3a74-5b6d-6293f07f9a34@intel.com>
Date:   Thu, 6 Sep 2018 07:12:39 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Fengguang Wu <fengguang.wu@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Linux Memory Management List <linux-mm@...ck.org>,
        Huang Ying <ying.huang@...el.com>,
        Brendan Gregg <bgregg@...flix.com>,
        Peng DongX <dongx.peng@...el.com>,
        Liu Jingqi <jingqi.liu@...el.com>,
        Dong Eddie <eddie.dong@...el.com>, kvm@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC][PATCH 2/5] [PATCH 2/5] proc: introduce
 /proc/PID/idle_bitmap

On 09/01/2018 04:28 AM, Fengguang Wu wrote:
> To walk 1TB memory of 4k active pages, it costs 2s vs 15s system
> time to scan the per-task/global idle bitmaps.

To me, that says this interface simply won't work on large systems.  2s
and 15s are both simply unacceptably long.

> OTOH, the per-task idle bitmap is not suitable in some situations:
> 
> - not accurate for shared pages
> - don't work with non-mapped file pages
> - don't perform well for sparse page tables (pointed out by Huang Ying)

OK, so we have a new ABI that doesn't work on large systems, consumes
lots of time and resources to query and isn't suitable in quite a few
situations.