Re: [RFC 0/3] mm: Discard lazily freed pages when migrating

[Huang\, Ying]

Hi, Matthew,

Matthew Wilcox <willy@xxxxxxxxxxxxx> writes:

> On Fri, Feb 28, 2020 at 11:38:16AM +0800, Huang, Ying wrote:
>> MADV_FREE is a lazy free mechanism in Linux.  According to the manpage
>> of mavise(2), the semantics of MADV_FREE is,
>> 
>>   The application no longer requires the pages in the range specified
>>   by addr and len.  The kernel can thus free these pages, but the
>>   freeing could be delayed until memory pressure occurs. ...
>> 
>> Originally, the pages freed lazily by MADV_FREE will only be freed
>> really by page reclaiming when there is memory pressure or when
>> unmapping the address range.  In addition to that, there's another
>> opportunity to free these pages really, when we try to migrate them.
>> 
>> The main value to do that is to avoid to create the new memory
>> pressure immediately if possible.  Instead, even if the pages are
>> required again, they will be allocated gradually on demand.  That is,
>> the memory will be allocated lazily when necessary.  This follows the
>> common philosophy in the Linux kernel, allocate resources lazily on
>> demand.
>
> Do you have an example program which does this (and so benefits)?

Sorry, what do you mean exactly for "this" here?  Call
madvise(,,MADV_FREE)?  Or migrate pages?

> If so, can you quantify the benefit at all?

The question is what is the right workload?  For example, I can build a
scenario as below to show benefit.

- run program A in node 0 with many lazily freed pages

- run program B in node 1, so that the free memory on node 1 is low

- migrate the program A from node 0 to node 1, so that the program B is
  influenced by the memory pressure created by migrating lazily freed
  pages.

Best Regards,
Huang, Ying