ZFS ARC under memory pressure

[Andriy Gapon]

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