Matthew Ahrens 026e529cb3 Remove skc_reclaim, hdr_recl, kmem_cache shrinker ... The SPL kmem_cache implementation provides a mechanism, `skc_reclaim`, whereby individual caches can register a callback to be invoked when there is memory pressure. This mechanism is used in only one place: the ARC registers the `hdr_recl()` reclaim function. This function wakes up the `arc_reap_zthr`, whose job is to call `kmem_cache_reap()` and `arc_reduce_target_size()`. The `skc_reclaim` callbacks are invoked only by shrinker callbacks and `arc_reap_zthr`, and only callback only wakes up `arc_reap_zthr`. When called from `arc_reap_zthr`, waking `arc_reap_zthr` is a no-op. When called from shrinker callbacks, we are already aware of memory pressure and responding to it. Therefore there is little benefit to ever calling the `hdr_recl()` `skc_reclaim` callback. The `arc_reap_zthr` also wakes once a second, and if memory is low when allocating an ARC buffer. Therefore, additionally waking it from the shrinker calbacks has little benefit. The shrinker callbacks can be invoked very frequently, e.g. 10,000 times per second. Additionally, for invocation of the shrinker callback, skc_reclaim is invoked many times. Therefore, this mechanism consumes significant amounts of CPU time. The kmem_cache shrinker calls `spl_kmem_cache_reap_now()`, which, in addition to invoking `skc_reclaim()`, does two things to attempt to free pages for use by the system: 1. Return free objects from the magazine layer to the slab layer 2. Return entirely-free slabs to the page layer (i.e. free pages) These actions apply only to caches implemented by the SPL, not those that use the underlying kernel SLAB/SLUB caches. The SPL caches are used for objects >=32KB, which are primarily linear ABD's cached in the DBUF cache. These actions (freeing objects from the magazine layer and returning entirely-free slabs) are also taken whenever a `kmem_cache_free()` call finds a full magazine. So there would typically be zero entirely-free slabs, and the number of objects in magazines is limited (typically no more than 64 objects per magazine, and there's one magazine per CPU). Therefore the benefit of `spl_kmem_cache_reap_now()`, while nonzero, is modest. We also call `spl_kmem_cache_reap_now()` from the `arc_reap_zthr`, when memory pressure is detected. Therefore, calling `spl_kmem_cache_reap_now()` from the kmem_cache shrinker is not needed. This commit removes the `skc_reclaim` mechanism, its only callback `hdr_recl()`, and the kmem_cache shrinker callback. Reviewed-By: Brian Behlendorf <behlendorf1@llnl.gov> Reviewed-by: George Wilson <gwilson@delphix.com> Reviewed-by: Pavel Zakharov <pavel.zakharov@delphix.com> Signed-off-by: Matthew Ahrens <mahrens@delphix.com> Closes #10576		2020-07-19 09:58:30 -07:00
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rpc	Drop unnecessary srcdir paths	2020-06-24 18:20:18 -07:00
sys	Remove skc_reclaim, hdr_recl, kmem_cache shrinker	2020-07-19 09:58:30 -07:00
Makefile.am	OpenZFS restructuring - move platform specific headers	2019-09-05 09:34:54 -07:00