svn commit: r351673 - in head: lib/libmemstat share/man/man9 sys/cddl/compat/opensolaris/kern sys/kern sys/vm

[Andriy Gapon]

On 02/09/2019 01:22, Mark Johnston wrote:
> Author: markj
> Date: Sun Sep  1 22:22:43 2019
> New Revision: 351673
> URL: https://svnweb.freebsd.org/changeset/base/351673
> 
> Log:
>   Extend uma_reclaim() to permit different reclamation targets.
>   
>   The page daemon periodically invokes uma_reclaim() to reclaim cached
>   items from each zone when the system is under memory pressure.  This
>   is important since the size of these caches is unbounded by default.
>   However it also results in bursts of high latency when allocating from
>   heavily used zones as threads miss in the per-CPU caches and must
>   access the keg in order to allocate new items.
>   
>   With r340405 we maintain an estimate of each zone's usage of its
>   (per-NUMA domain) cache of full buckets.  Start making use of this
>   estimate to avoid reclaiming the entire cache when under memory
>   pressure.  In particular, introduce TRIM, DRAIN and DRAIN_CPU
>   verbs for uma_reclaim() and uma_zone_reclaim().  When trimming, only
>   items in excess of the estimate are reclaimed.  Draining a zone
>   reclaims all of the cached full buckets (the previous behaviour of
>   uma_reclaim()), and may further drain the per-CPU caches in extreme
>   cases.
>   
>   Now, when under memory pressure, the page daemon will trim zones
>   rather than draining them.  As a result, heavily used zones do not incur
>   bursts of bucket cache misses following reclamation, but large, unused
>   caches will be reclaimed as before.

Mark,

have you considered running UMA_RECLAIM_TRIM periodically, even without a memory
pressure?
I think that with such a periodic trimming there will be less need to invoke
vm_lowmem().

Also, I think that we would be able to retire (or re-purpose) lowmem_period.
E.g., the trimming would be done every lowmem_period, but vm_lowmem() would not
be throttled.

One example of the throttling of vm_lowmem being bad is its interaction with the
ZFS ARC.  When there is a spike in memory usage we want the ARC to adapt as
quickly as possible.  But at present the lowmem_period logic interferes with that.

>   Reviewed by:	jeff
>   Tested by:	pho (an earlier version)
>   MFC after:	2 months
>   Sponsored by:	Netflix
>   Differential Revision:	https://reviews.freebsd.org/D16667


-- 
Andriy Gapon