Date: Fri, 19 Aug 2016 23:18:40 +0300 From: Slawa Olhovchenkov <slw@zxy.spb.ru> To: Karl Denninger <karl@denninger.net>, freebsd-fs@freebsd.org Subject: Re: ZFS ARC under memory pressure Message-ID: <20160819201840.GA12519@zxy.spb.ru> In-Reply-To: <c3bc6c5a-961c-e3a4-2302-f0f7417bc34f@denninger.net> References: <20160816193416.GM8192@zxy.spb.ru> <8dbf2a3a-da64-f7f8-5463-bfa23462446e@FreeBSD.org> <20160818202657.GS8192@zxy.spb.ru> <c3bc6c5a-961c-e3a4-2302-f0f7417bc34f@denninger.net>
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On Thu, Aug 18, 2016 at 03:31:26PM -0500, Karl Denninger wrote: > > On 8/18/2016 15:26, Slawa Olhovchenkov wrote: > > 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. > > _______________________________________________ > > freebsd-fs at freebsd.org mailing list > > https://lists.freebsd.org/mailman/listinfo/freebsd-fs > > To unsubscribe, send any mail to "freebsd-fs-unsubscribe at freebsd.org" > > You might want to look at the code contained in here: > > https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=187594 In may case arc.c issuse cused by revision r286625 in HEAD (and r288562 in STABLE) -- all in 2015, not touch in 2014. > There are some ugly interactions with the VM system you can run into if > you're not careful; I've chased this issue before and while I haven't > yet done the work to integrate it into 11.x (and the underlying code > *has* changed since the 10.x patches I developed) if you wind up driving > the VM system to evict pages to swap rather than pare back ARC you're > probably making the wrong choice. > > In addition UMA can come into the picture too and (at least previously) > was a severe contributor to pathological behavior. I am only do less aggresive (and more controlled) shrink of ARC size. Now ARC just collapsed. Pointed PR is realy BIG. I am can't read and understund all of this. r286625 change behaivor of interaction between ARC and VM. You problem still exist? Can you explain (in list)? -- Slawa Olhovchenkov
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