From owner-freebsd-fs@freebsd.org  Tue Aug 16 19:34:20 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 D68C4BBCCFB
 for <freebsd-fs@mailman.ysv.freebsd.org>; Tue, 16 Aug 2016 19:34:20 +0000 (UTC)
 (envelope-from slw@zxy.spb.ru)
Received: from zxy.spb.ru (zxy.spb.ru [195.70.199.98])
 (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits))
 (Client did not present a certificate)
 by mx1.freebsd.org (Postfix) with ESMTPS id 9E30E1F3C
 for <freebsd-fs@freebsd.org>; Tue, 16 Aug 2016 19:34:20 +0000 (UTC)
 (envelope-from slw@zxy.spb.ru)
Received: from slw by zxy.spb.ru with local (Exim 4.86 (FreeBSD))
 (envelope-from <slw@zxy.spb.ru>) id 1bZk7s-000P7Q-88
 for freebsd-fs@freebsd.org; Tue, 16 Aug 2016 22:34:16 +0300
Date: Tue, 16 Aug 2016 22:34:16 +0300
From: Slawa Olhovchenkov <slw@zxy.spb.ru>
To: freebsd-fs@freebsd.org
Subject: ZFS ARC under memory pressure
Message-ID: <20160816193416.GM8192@zxy.spb.ru>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
User-Agent: Mutt/1.5.24 (2015-08-30)
X-SA-Exim-Connect-IP: <locally generated>
X-SA-Exim-Mail-From: slw@zxy.spb.ru
X-SA-Exim-Scanned: No (on zxy.spb.ru); SAEximRunCond expanded to false
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: Tue, 16 Aug 2016 19:34:20 -0000

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?