linux-kernel - Re: [PATCH] delayacct: track delays from ksm cow

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Message-ID: <62330402.1c69fb81.d2ba6.0538@mx.google.com>
Date:   Thu, 17 Mar 2022 09:48:49 +0000
From:   CGEL <cgel.zte@...il.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     bsingharora@...il.com, akpm@...ux-foundation.org,
        yang.yang29@....com.cn, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH] delayacct: track delays from ksm cow

On Thu, Mar 17, 2022 at 09:17:13AM +0100, David Hildenbrand wrote:
> On 17.03.22 03:03, CGEL wrote:
> > On Wed, Mar 16, 2022 at 03:56:23PM +0100, David Hildenbrand wrote:
> >> On 16.03.22 14:34, cgel.zte@...il.com wrote:
> >>> From: Yang Yang <yang.yang29@....com.cn>
> >>>
> >>> Delay accounting does not track the delay of ksm cow.  When tasks
> >>> have many ksm pages, it may spend a amount of time waiting for ksm
> >>> cow.
> >>>
> >>> To get the impact of tasks in ksm cow, measure the delay when ksm
> >>> cow happens. This could help users to decide whether to user ksm
> >>> or not.
> >>>
> >>> Also update tools/accounting/getdelays.c:
> >>>
> >>>     / # ./getdelays -dl -p 231
> >>>     print delayacct stats ON
> >>>     listen forever
> >>>     PID     231
> >>>
> >>>     CPU             count     real total  virtual total    delay total  delay average
> >>>                      6247     1859000000     2154070021     1674255063          0.268ms
> >>>     IO              count    delay total  delay average
> >>>                         0              0              0ms
> >>>     SWAP            count    delay total  delay average
> >>>                         0              0              0ms
> >>>     RECLAIM         count    delay total  delay average
> >>>                         0              0              0ms
> >>>     THRASHING       count    delay total  delay average
> >>>                         0              0              0ms
> >>>     KSM             count    delay total  delay average
> >>>                      3635      271567604              0ms
> >>>
> >>
> >> TBH I'm not sure how particularly helpful this is and if we want this.
> >>
> > Thanks for replying.
> > 
> > Users may use ksm by calling madvise(, , MADV_MERGEABLE) when they want
> > save memory, it's a tradeoff by suffering delay on ksm cow. Users can
> > get to know how much memory ksm saved by reading
> > /sys/kernel/mm/ksm/pages_sharing, but they don't know what the costs of
> > ksm cow delay, and this is important of some delay sensitive tasks. If
> > users know both saved memory and ksm cow delay, they could better use
> > madvise(, , MADV_MERGEABLE).
> 
> But that happens after the effects, no?
> 
> IOW a user already called madvise(, , MADV_MERGEABLE) and then gets the
> results.
>
Image user are developing or porting their applications on experiment
machine, they could takes those benchmark as feedback to adjust whether
to use madvise(, , MADV_MERGEABLE) or it's range.

> So how is this interface useful except for somebody writing an
> application and simply being able to benchmark it with vs. without
> MADV_MERGEABLE?
>
> -- 
> Thanks,
> 
> David / dhildenb