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2005-07-25

inactive_dirty and inactive_clean are considered harmful


Found interesting discussion on linux-mm (Sept. 2002):

AKPM and Rik („inactive_dirty list“)

[AKPM proposed separate inactive_dirty and inactive_clean lists to avoid excessive scanning of pages that cannot be reclaimed (due to then-new non-blocking VM scanner]:

    Riel:
    > AKPM:
    > > - inactive_dirty holds pages which are dirty or under writeback.
    > 
    > > - everywhere where we add a page to the inactive list will now
    > >   add it to either inactive_clean or inactive_dirty, based on
    > >   its PageDirty || PageWriteback state.
    > 
    > If I had veto power I'd use it here ;)
    > 
    > We did this in early 2.4 kernels and it was a disaster. The
    > reason it was a disaster was that in many workloads we'd
    > always have some clean pages and we'd end up always reclaiming
    > those before even starting writeout on any of the dirty pages.
    > 
    > It also meant we could have dirty (or formerly dirty) inactive
    > pages eating up memory and never being recycled for more active
    > data.
And later in the same thread:
    AKPM:
    > You're proposing that we get that IO underway sooner if there
    > is page reclaim pressure, and that one way to do that is to
    > write one page for every reclaimed one.  Guess that makes
    > sense as much as anything else ;)

(decrease dirty cache balancing rules when hitting memory pressure, so that balance_dirty_pages() and pdflush do write-out in scanner's stead)

And further more:

    Daniel Phillips
    > On Saturday 07 September 2002 01:34, Andrew Morton wrote:
    > > You're proposing that we get that IO underway sooner if there
    > > is page reclaim pressure, and that one way to do that is to
    > > write one page for every reclaimed one.  Guess that makes
    > > sense as much as anything else ;)
    > 
    > Not really.  The correct formula will incorporate the allocation rate
    > and the inactive dirty/clean balance.  The reclaim rate is not
    > relevant, it is a time-delayed consequence of the above.  Relying on
    > it in a control loop is simply asking for oscillation.
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