Re: [RFC PATCH] treewide: remove GFP_TEMPORARY allocation flag

From: Pavel Machek <pavel@ucw.cz>
To: Michal Hocko <mhocko@kernel.org>
Cc: NeilBrown <neilb@suse.com>,
	linux-mm@kvack.org, Mel Gorman <mgorman@suse.de>,
	Matthew Wilcox <willy@infradead.org>,
	Vlastimil Babka <vbabka@suse.cz>, Neil Brown <neilb@suse.de>,
	Theodore Ts'o <tytso@mit.edu>,
	Andrew Morton <akpm@linux-foundation.org>,
	LKML <linux-kernel@vger.kernel.org>
Subject: Re: [RFC PATCH] treewide: remove GFP_TEMPORARY allocation flag
Date: Thu, 31 Aug 2017 11:07:22 +0200	[thread overview]
Message-ID: <20170831090722.GA12920@amd> (raw)
In-Reply-To: <20170828123657.GK17097@dhcp22.suse.cz>

[-- Attachment #1: Type: text/plain, Size: 2056 bytes --]

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

[-- Attachment #2: Digital signature --]
[-- Type: application/pgp-signature, Size: 181 bytes --]