Date: Thu, 18 Aug 2016 23:26:57 +0300 From: Slawa Olhovchenkov <slw@zxy.spb.ru> To: Andriy Gapon <avg@FreeBSD.org> Cc: freebsd-fs@FreeBSD.org, Alexander Motin <mav@FreeBSD.org> Subject: Re: ZFS ARC under memory pressure Message-ID: <20160818202657.GS8192@zxy.spb.ru> In-Reply-To: <8dbf2a3a-da64-f7f8-5463-bfa23462446e@FreeBSD.org> References: <20160816193416.GM8192@zxy.spb.ru> <8dbf2a3a-da64-f7f8-5463-bfa23462446e@FreeBSD.org>
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On Thu, Aug 18, 2016 at 11:00:28PM +0300, Andriy Gapon wrote: > 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. As I see, totaly not optimal. I am create some patch for sub-optimal handling and now test it.
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