Date: Thu, 18 Aug 2016 23:00:28 +0300 From: Andriy Gapon <avg@FreeBSD.org> To: Slawa Olhovchenkov <slw@zxy.spb.ru>, freebsd-fs@FreeBSD.org, Alexander Motin <mav@FreeBSD.org> Subject: Re: ZFS ARC under memory pressure Message-ID: <8dbf2a3a-da64-f7f8-5463-bfa23462446e@FreeBSD.org> In-Reply-To: <20160816193416.GM8192@zxy.spb.ru> References: <20160816193416.GM8192@zxy.spb.ru>
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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
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