linux-kernel - Re: [dm-devel] [RFC PATCH 2/2] mm, mempool: do not throttle PF_LESS

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Date:	Thu, 28 Jul 2016 07:33:19 +1000
From:	NeilBrown <neilb@...e.com>
To:	Michal Hocko <mhocko@...nel.org>
Cc:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org,
	dm-devel@...hat.com, Mikulas Patocka <mpatocka@...hat.com>,
	Mel Gorman <mgorman@...e.de>,
	David Rientjes <rientjes@...gle.com>,
	Ondrej Kozina <okozina@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [dm-devel] [RFC PATCH 2/2] mm, mempool: do not throttle PF_LESS_THROTTLE tasks

On Thu, Jul 28 2016, Michal Hocko wrote:

> On Wed 27-07-16 13:43:35, NeilBrown wrote:
>> On Mon, Jul 25 2016, Michal Hocko wrote:
>> 
>> > On Sat 23-07-16 10:12:24, NeilBrown wrote:
> [...]
>> So should there be a limit on dirty
>> pages in the swap cache just like there is for dirty pages in any
>> filesystem (the max_dirty_ratio thing) ??
>> Maybe there is?
>
> There is no limit AFAIK. We are relying that the reclaim is throttled
> when necessary.

Is that a bit indirect?  It is hard to tell without a clear big-picture.
Something to keep in mind anyway.

>
>> I think we'd end up with cleaner code if we removed the cute-hacks.  And
>> we'd be able to use 6 more GFP flags!!  (though I do wonder if we really
>> need all those 26).
>
> Well, maybe we are able to remove those hacks, I wouldn't definitely
> be opposed.  But right now I am not even convinced that the mempool
> specific gfp flags is the right way to go.

I'm not suggesting a mempool-specific gfp flag.  I'm suggesting a
transient-allocation gfp flag, which would be quite useful for mempool.

Can you give more details on why using a gfp flag isn't your first choice
for guiding what happens when the system is trying to get a free page
:-?

Thanks,
NeilBrown

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