linux-kernel - Re: [PATCH] mm: disallow direct reclaim page writeback

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Date:	Thu, 15 Apr 2010 11:43:48 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	kosaki.motohiro@...fujitsu.com, Mel Gorman <mel@....ul.ie>,
	Dave Chinner <david@...morbit.com>,
	Chris Mason <chris.mason@...cle.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] mm: disallow direct reclaim page writeback

Hi

> On Wed, Apr 14, 2010 at 09:51:33AM +0100, Mel Gorman wrote:
> > They will need to be tackled in turn then but obviously there should be
> > a focus on the common paths. The reclaim paths do seem particularly
> > heavy and it's down to a lot of temporary variables. I might not get the
> > time today but what I'm going to try do some time this week is
> > 
> > o Look at what temporary variables are copies of other pieces of information
> > o See what variables live for the duration of reclaim but are not needed
> >   for all of it (i.e. uninline parts of it so variables do not persist)
> > o See if it's possible to dynamically allocate scan_control
> > 
> > The last one is the trickiest. Basically, the idea would be to move as much
> > into scan_control as possible. Then, instead of allocating it on the stack,
> > allocate a fixed number of them at boot-time (NR_CPU probably) protected by
> > a semaphore. Limit the number of direct reclaimers that can be active at a
> > time to the number of scan_control variables. kswapd could still allocate
> > its on the stack or with kmalloc.
> > 
> > If it works out, it would have two main benefits. Limits the number of
> > processes in direct reclaim - if there is NR_CPU-worth of proceses in direct
> > reclaim, there is too much going on. It would also shrink the stack usage
> > particularly if some of the stack variables are moved into scan_control.
> > 
> > Maybe someone will beat me to looking at the feasibility of this.
> 
> I already have some patches to remove trivial parts of struct scan_control,
> namely may_unmap, may_swap, all_unreclaimable and isolate_pages.  The rest
> needs a deeper look.

Seems interesting. but scan_control diet is not so effective. How much
bytes can we diet by it?


> A rather big offender in there is the combination of shrink_active_list (360
> bytes here) and shrink_page_list (200 bytes).  I am currently looking at
> breaking out all the accounting stuff from shrink_active_list into a separate
> leaf function so that the stack footprint does not add up.

pagevec. it consume 128bytes per struct. I have removing patch.


> Your idea of per-cpu allocated scan controls reminds me of an idea I have
> had for some time now: moving reclaim into its own threads (per cpu?).
> 
> Not only would it separate the allocator's stack from the writeback stack,
> we could also get rid of that too_many_isolated() workaround and coordinate
> reclaim work better to prevent overreclaim.
> 
> But that is not a quick fix either...

So, I haven't think this way. probably seems good. but I like to do
simple diet at first.



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