linux-kernel - Re: [PATCH] mm: filemap: Avoid unnecessary barries and waitqueue lookups in unlock

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Date:	Thu, 22 May 2014 10:47:22 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Mel Gorman <mgorman@...e.de>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Vlastimil Babka <vbabka@...e.cz>, Jan Kara <jack@...e.cz>,
	Michal Hocko <mhocko@...e.cz>, Hugh Dickins <hughd@...gle.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>,
	Linux-FSDevel <linux-fsdevel@...r.kernel.org>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	David Howells <dhowells@...hat.com>
Subject: Re: [PATCH] mm: filemap: Avoid unnecessary barries and waitqueue
 lookups in unlock_page fastpath v5

On Thu, 22 May 2014 09:46:43 +0100 Mel Gorman <mgorman@...e.de> wrote:

> > > If I'm still on track here, what happens if we switch to wake-all so we
> > > can avoid the dangling flag?  I doubt if there are many collisions on
> > > that hash table?
> > 
> > Wake-all will be ugly and loose a herd of waiters, all racing to
> > acquire, all but one of whoem will loose the race. It also looses the
> > fairness, its currently a FIFO queue. Wake-all will allow starvation.
> > 
> 
> And the cost of the thundering herd of waiters may offset any benefit of
> reducing the number of calls to page_waitqueue and waker functions.

Well, none of this has been demonstrated.

As I speculated earlier, hash chain collisions will probably be rare,
except for the case where a bunch of processes are waiting on the same
page.  And in this case, perhaps wake-all is the desired behavior.

Take a look at do_read_cache_page().  It does lock_page(), but it
doesn't actually *need* to.  It checks ->mapping and PG_uptodate and
then...  unlocks the page!  We could have used wait_on_page_locked()
there and permitted concurrent threads to run concurrently.

btw, I'm struggling a bit to understand why we bother checking
->mapping there as we're about to unlock the page anyway...

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