linux-kernel - Re: [RFC PATCH] treewide: remove GFP

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Date:   Thu, 31 Aug 2017 11:07:22 +0200
From:   Pavel Machek <pavel@....cz>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     NeilBrown <neilb@...e.com>, linux-mm@...ck.org,
        Mel Gorman <mgorman@...e.de>,
        Matthew Wilcox <willy@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>, Neil Brown <neilb@...e.de>,
        Theodore Ts'o <tytso@....edu>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] treewide: remove GFP_TEMPORARY allocation flag

Hi!

> > > "This allocation is temporary. It lasts milliseconds, not hours."
> > 
> > It isn't sufficient to give a rule for when GFP_TEMPORARY will be used,
> > you also need to explain (at least in general terms) how the information
> > will be used.  Also you need to give guidelines on whether the flag
> > should be set for allocation that will last seconds or minutes.
> > 
> > If we have a flag that doesn't have a well defined meaning that actually
> > affects behavior, it will not be used consistently, and if we ever
> > change exactly how it behaves we can expect things to break.  So it is
> > better not to have a flag, than to have a poorly defined flag.
> 
> Absolutely agreed!
> 
> > My current thoughts is that the important criteria is not how long the
> > allocation will be used for, but whether it is reclaimable.  Allocations
> > that will only last 5 msecs are reclaimable by calling "usleep(5000)".
> > Other allocations might be reclaimable in other ways.  Allocations that
> > are not reclaimable may well be directed to a more restricted pool of
> > memory, and might be more likely to fail.  If we grew a strong
> > "reclaimable" concept, this 'temporary' concept that you want to hold on
> > to would become a burden.
> 
> ... and here again. The whole motivation for the flag was to gather
> these objects together and reduce chances of internal fragmentation
> due to long lived objects mixed with short term ones. Without an
> explicit way to reclaim those objects or having a clear checkpoint to
> wait for it is not really helping us to reach desired outcome (less
> fragmented memory).

Really?

If you group allocations that last << 1 second, and ones that last >>
1 second, I'm pretty sure it reduces fragmentation... "reclaimable" or
not.

Fragmentation is just statistical property, so getting it "mostly
right" helps...
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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