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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 |
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OpenZFS is an advanced file system and volume manager which was originally developed for Solaris and is now maintained by the OpenZFS community. This repository contains the code for running OpenZFS on Linux and FreeBSD.
Official Resources
- Documentation - for using and developing this repo
- ZoL Site - Linux release info & links
- Mailing lists
- OpenZFS site - for conference videos and info on other platforms (illumos, OSX, Windows, etc)
Installation
Full documentation for installing OpenZFS on your favorite Linux distribution can be found at the ZoL Site.
Contribute & Develop
We have a separate document with contribution guidelines.
We have a Code of Conduct.
Release
OpenZFS is released under a CDDL license.
For more details see the NOTICE, LICENSE and COPYRIGHT files; UCRL-CODE-235197
Supported Kernels
- The
METAfile contains the officially recognized supported Linux kernel versions. - Supported FreeBSD versions are 12-STABLE and 13-CURRENT.