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

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Date:   Thu, 17 Mar 2022 09:17:13 +0100
From:   David Hildenbrand <david@...hat.com>
To:     CGEL <cgel.zte@...il.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 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.

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