ZFS ARC under memory pressure

[Matthias Gamsjager]

On 16 August 2016 at 21:34, Slawa Olhovchenkov <slw at zxy.spb.ru> 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?
> _______________________________________________
>


What version are you on?