From owner-freebsd-fs@freebsd.org  Thu Aug 18 20:01:33 2016
Return-Path: <owner-freebsd-fs@freebsd.org>
Delivered-To: freebsd-fs@mailman.ysv.freebsd.org
Received: from mx1.freebsd.org (mx1.freebsd.org
 [IPv6:2001:1900:2254:206a::19:1])
 by mailman.ysv.freebsd.org (Postfix) with ESMTP id B1E90BBE785
 for <freebsd-fs@mailman.ysv.freebsd.org>; Thu, 18 Aug 2016 20:01:33 +0000 (UTC)
 (envelope-from avg@FreeBSD.org)
Received: from citapm.icyb.net.ua (citapm.icyb.net.ua [212.40.38.140])
 by mx1.freebsd.org (Postfix) with ESMTP id DD1D11845;
 Thu, 18 Aug 2016 20:01:32 +0000 (UTC) (envelope-from avg@FreeBSD.org)
Received: from porto.starpoint.kiev.ua (porto-e.starpoint.kiev.ua
 [212.40.38.100])
 by citapm.icyb.net.ua (8.8.8p3/ICyb-2.3exp) with ESMTP id XAA16268;
 Thu, 18 Aug 2016 23:01:24 +0300 (EEST)
 (envelope-from avg@FreeBSD.org)
Received: from localhost ([127.0.0.1])
 by porto.starpoint.kiev.ua with esmtp (Exim 4.34 (FreeBSD))
 id 1baTVE-000JoD-HQ; Thu, 18 Aug 2016 23:01:24 +0300
Subject: Re: ZFS ARC under memory pressure
To: Slawa Olhovchenkov <slw@zxy.spb.ru>, freebsd-fs@FreeBSD.org,
 Alexander Motin <mav@FreeBSD.org>
References: <20160816193416.GM8192@zxy.spb.ru>
From: Andriy Gapon <avg@FreeBSD.org>
Message-ID: <8dbf2a3a-da64-f7f8-5463-bfa23462446e@FreeBSD.org>
Date: Thu, 18 Aug 2016 23:00:28 +0300
User-Agent: Mozilla/5.0 (X11; FreeBSD amd64; rv:45.0) Gecko/20100101
 Thunderbird/45.2.0
MIME-Version: 1.0
In-Reply-To: <20160816193416.GM8192@zxy.spb.ru>
Content-Type: text/plain; charset=windows-1252
Content-Transfer-Encoding: 7bit
X-BeenThere: freebsd-fs@freebsd.org
X-Mailman-Version: 2.1.22
Precedence: list
List-Id: Filesystems <freebsd-fs.freebsd.org>
List-Unsubscribe: <https://lists.freebsd.org/mailman/options/freebsd-fs>,
 <mailto:freebsd-fs-request@freebsd.org?subject=unsubscribe>
List-Archive: <http://lists.freebsd.org/pipermail/freebsd-fs/>
List-Post: <mailto:freebsd-fs@freebsd.org>
List-Help: <mailto:freebsd-fs-request@freebsd.org?subject=help>
List-Subscribe: <https://lists.freebsd.org/mailman/listinfo/freebsd-fs>,
 <mailto:freebsd-fs-request@freebsd.org?subject=subscribe>
X-List-Received-Date: Thu, 18 Aug 2016 20:01:33 -0000

On 16/08/2016 22:34, Slawa Olhovchenkov wrote:
> I see issuses with ZFS ARC inder memory pressure.
> ZFS ARC size can be dramaticaly reduced, up to arc_min.
> 
> As I see memory pressure event cause call arc_lowmem and set needfree:
> 
> arc.c:arc_lowmem
> 
>         needfree = btoc(arc_c >> arc_shrink_shift);
> 
> After this, arc_available_memory return negative vaules (PAGESIZE *
> (-needfree)) until needfree is zero. Independent how too much memory
> freed. needfree set to 0 in arc_reclaim_thread(), when arc_size <=
> arc_c. Until arc_size don't drop below arc_c (arc_c deceased at every
> loop interation).
> 
> arc_c droped to minimum value if arc_size fast enough droped.
> 
> No control current to initial memory allocation.
> 
> As result, I can see needless arc reclaim, from 10x to 100x times.
> 
> Can some one check me and comment this?

You might have found a real problem here, but I am short of time right now to
properly analyze the issue.  I think that on illumos 'needfree' is a variable
that's managed by the virtual memory system and it is akin to our
vm_pageout_deficit.  But during the porting it became an artificial value and
its handling might be sub-optimal.

-- 
Andriy Gapon