Re: [PATCH] delayacct: track delays from ksm cow

David Hildenbrand
2022-03-17T08:17:19Z

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@gmail.com wrote:
>>> From: Yang Yang <yang.yang29@zte.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