linux-kernel - Re: Default zone_reclaim_mode = 1 on NUMA kernel is bad for file/email/web servers

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Message-ID: <alpine.DEB.2.00.1009170916130.11900@router.home>
Date:	Fri, 17 Sep 2010 09:22:00 -0500 (CDT)
From:	Christoph Lameter <cl@...ux.com>
To:	Bron Gondwana <brong@...tmail.fm>
cc:	Robert Mueller <robm@...tmail.fm>,
	Shaohua Li <shaohua.li@...el.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>, Mel Gorman <mel@....ul.ie>
Subject: Re: Default zone_reclaim_mode = 1 on NUMA kernel is bad for
 file/email/web servers

On Sat, 18 Sep 2010, Bron Gondwana wrote:

> > From the first look that seems to be the problem. You do not need to be
> > bound to a particular cpu, the scheduler will just leave a single process
> > on the same cpu by default. If you then allocate all memory only from this
> > process then you get the scenario that you described.
>
> Huh?  Which bit of forking server makes you think one process is allocating
> lots of memory?  They're opening and reading from files.  Unless you're
> calling the kernel a "single process".

I have no idea what your app does. The data that I glanced over looks as
if most allocations happen for a particular memory node and since the
memory is optimized to be local to that node other memory is not used
intensively. This can occur because of allocations through one process /
thread that is always running on the same cpu and therefore always
allocates from the memory node local to that cpu.

It can also happen f.e. if a driver always allocates memory local to the
I/O bus that it is using.

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