Re: [RFC 0/7] Postphone reclaim laundry to write at high water marks

[Rik van Riel <riel@redhat.com>]

Christoph Lameter wrote:
> On Tue, 21 Aug 2007, Rik van Riel wrote:
> 
>> Christoph Lameter wrote:
>>
>>> 1. First reclaiming non dirty pages. Dirty pages are deferred until reclaim
>>>    has reestablished the high marks. Then all the dirty pages (the laundry)
>>>    is written out.
>> That sounds like a horrendously bad idea.  While one process
>> is busy freeing all the non dirty pages, other processes can
>> allocate those pages, leaving you with no memory to free up
>> the dirty pages!
> 
> What is preventing that from occurring right now? If the dirty pags are 
> aligned in the right way you can have the exact same situation.

For one, dirty page writeout is done even when free memory
is low.  The kernel will dig into the PF_MEMALLOC reserves,
instead of deciding not to do writeout unless there is lots
of free memory.

Secondly, why would you want to recreate this worst case on
purpose every time the pageout code runs?

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