ZFS ARC under memory pressure
Slawa Olhovchenkov
slw at zxy.spb.ru
Thu Aug 18 20:27:00 UTC 2016
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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